This document covers Unit 2 of a course on electronic devices and circuits, focusing on transistors including BJT, JFET, MOSFET, and others. It discusses their structures, operational characteristics, biasing, and configurations such as common base, common emitter, and common collector, along with practical examples and biasing methods. It aims to provide foundational knowledge on transistor operation in electronic applications.

1. EC6202 ELECTRONIC DEVICES AND CIRCUITS
Unit 2
Dr Gnanasekaran Thangavel
Professor and Head
Electronics and Instrumentation
Engineering
R M K Engineering College

2. UNIT II TRANSISTORS
BJT, JFET, MOSFET- structure, operation, characteristics
and Biasing UJT, Thyristor and IGBT Structure and
characteristics.
2 Dr Gnanasekaran Thangavel 8/2/2017
1. https://www.youtube.com/watch?v=yOmPCjPlaEg
2. https://www.youtube.com/watch?v=jQb199oIY5U
3. https://www.youtube.com/watch?v=G-BvuL5IDLw
4. https://www.youtube.com/watch?v=IRok_SGrx9Q
5. https://www.youtube.com/watch?v=2LBKwGwGYt4
6. https://www.youtube.com/watch?v=_DZ7baOhNFQ
7. https://www.youtube.com/watch?v=Dd4im8TMAk0

3. Bipolar Junction
transistor
Holes and electrons
determine device characteristics
Three terminal device
Control of two terminal
currents
Amplification and switching through 3rd contact

4. Understanding of BJT
force – voltage/current
water flow – current
- amplification

5. Basic models of BJT
Diode
Diode
Diode
Diode
npn transistor
pnp transistor

6. Transistor Construction
3 layer semiconductor device consisting:
 2 n- and 1 p-type layers of material  npn transistor
 2 p- and 1 n-type layers of material pnp transistor
The term bipolar reflects the fact that holes and
electrons participate in the injection process into the
oppositely polarized material
A single pn junction has two different types of bias:
 forward bias
 reverse bias
 Thus, a two-pn-junction device has four types of bias.

7. Position of the terminals and symbol of BJT.
• Base is located at the middle
and more thin from the level
of collector and emitter
• The emitter and collector
terminals are made of the
same type of semiconductor
material, while the base of the
other type of material

8. Transistor currents
-The arrow is always drawn
on the emitter
-The arrow always point
toward the n-type
-The arrow indicates the
direction of the emitter
current:
pnp:E B
npn: B E
IC=the collector current
IB= the base current
IE= the emitter current

9.  By imaging the analogy of diode, transistor can be construct
like two diodes that connected together.
 It can be conclude that the work of transistor is base on work of
diode.

10. Basic models of BJT

11. Recall p-n junction
P N
W
Vappl > 0
-+
N P
W
Vappl < 0
-+
Forward bias, + on P, - on N
(Shrink W, Vbi)
Allow holes to jump over barrier
into N region as minority carriers
Reverse bias, + on N, - on P
(Expand W, Vbi)
Remove holes and electrons away
from depletion region
I
V
I
V

12. So if we combine these by fusing their
terminals…
P N
W
Vappl > 0
-+
N P
W
Vappl < 0
-+
Holes from P region (“Emitter”) of 1st PN junction
driven by FB of 1st PN junction into central N region (“Base”)
Driven by RB of 2nd PN junction from Base into P region of
2nd junction (“Collector”)
• 1st region FB, 2nd RB
• If we want to worry about holes alone, need P+ on 1st region
• For holes to be removed by collector, base region must be thin

13. Qualitative basic operation of BJTs

14. Bipolar Junction Transistors: Basics
+
- +
-
IE
IB
IC
IE = IB + IC ………(KCL)
VEC = VEB + VBC ……… (KVL)

15. BJT configurations
https://www.youtube.com/watch?v=Ir4sY5Fm-dU
http://ecetutorials.com/analog-electronics/operation-of-bjt/

16. +
- +
-
IE
IB
IC
Bipolar Junction Transistors: Basics
VEB, VBC > 0  VEC >> 0
IE, IC > 0  IB > 0
VEB >-VBC > 0  VEC > 0 but small
IE > -IC > 0  IB > 0
VEB < 0, VBC > 0  VEC > 0
IE < 0, IC > 0  IB > 0 but small

17. Bipolar Junction Transistors: Basics
Bias Mode E-B Junction C-B Junction
Saturation Forward Forward
Active Forward Reverse
Inverted Reverse Forward
Cutoff Reverse Reverse

18. 8/2/2017Dr Gnanasekaran Thangavel18
 Active Region – the transistor operates as an amplifier and Ic = β.Ib
 Saturation – the transistor is “Fully-ON” operating as a switch and Ic
= I(saturation)
 Cut-off – the transistor is “Fully-OFF” operating as a switch and Ic =
0
 Common Base Configuration – has Voltage Gain but no Current
Gain.
 Common Emitter Configuration – has both Current and Voltage
Gain.
 Common Collector Configuration – has Current Gain but no
Voltage Gain.

19. The Common Base (CB) Configuration
8/2/2017Dr Gnanasekaran Thangavel19
 The input current flowing into the
emitter is quite large as its the sum
of both the base current and
collector current respectively
therefore, the collector current
output is less than the emitter
current input resulting in a current
gain for this type of circuit of “1”
(unity) or less, in other words the
common base configuration
“attenuates” the input signal.
 This type of amplifier configuration is a non-inverting voltage amplifier
circuit, in that the signal voltages Vin and Vout are “in-phase”. Where:
Ic/Ie is the current gain, alpha ( α ) and RL/Rin is the resistance gain.

20. The Common Emitter (CE) Configuration
8/2/2017Dr Gnanasekaran Thangavel20
 The common emitter amplifier
configuration produces the highest
current and power gain of all the three
bipolar transistor configurations. This
is mainly because the input
impedance is LOW as it is connected
to a forward biased PN-junction, while
the output impedance is HIGH as it is
taken from a reverse biased PN-
junction.
This type of bipolar transistor configuration has a greater input impedance, current
and power gain than that of the common base configuration but its voltage gain is
much lower. The common emitter configuration is an inverting amplifier circuit.
This means that the resulting output signal is 180o “out-of-phase” with the input
voltage signal.

21. The Common Collector (CC) Configuration
8/2/2017Dr Gnanasekaran Thangavel21
 The common collector, or
emitter follower
configuration is very
useful for impedance
matching applications
because of the very high
input impedance, in the
region of hundreds of
thousands of Ohms while
having a relatively low
output impedance.
The common emitter configuration has a current gain approximately equal to the β
value of the transistor itself. In the common collector configuration the load
resistance is situated in series with the emitter so its current is equal to that of the
emitter current.

22. The Common Collector Current Gain
8/2/2017Dr Gnanasekaran Thangavel22
This type of bipolar transistor configuration
is a non-inverting circuit in that the signal
voltages of Vin and Vout are “in-phase”. It
has a voltage gain that is always less than
“1” (unity).
The load resistance of the common collector
transistor receives both the base and
collector currents giving a large current gain
(as with the common emitter configuration)

23. Bipolar Transistor Summary
8/2/2017Dr Gnanasekaran Thangavel23
Characteristic
Common
Base
Common
Emitter
Common
Collector
Input Impedance Low Medium High
Output Impedance Very High High Low
Phase Angle 0o 180o 0o
Voltage Gain High Medium Low
Current Gain Low Medium High
Power Gain Low Very High Medium

24. Relationship between DC Currents and
Gains
8/2/2017Dr Gnanasekaran Thangavel24

25. NPN Transistor Example
8/2/2017Dr Gnanasekaran Thangavel25
No1: A bipolar NPN transistor has a DC current gain, (Beta) value of 200.
Calculate the base current Ib required to switch a resistive load of 4mA.
Therefore, β = 200, Ic = 4mA and Ib = 20µA.
----------------------------------------------------------------------------------------------------------
------
No2 :An NPN Transistor has a DC base bias voltage, Vb of 10v and an input
base resistor, Rb of 100kΩ. What will be the value of the base current into the
transistor.
Therefore, Ib = 93µA.

26. Single Stage Common Emitter Amplifier
Circuit
8/2/2017Dr Gnanasekaran Thangavel26
 Common Emitter Amplifier
configuration of an NPN
transistor is called a Class A
Amplifier. A “Class A Amplifier”
operation is one where the
transistors Base terminal is
biased in such a way as to
forward bias the Base-emitter
junction. Ohm´s Law, the
current flowing through the load
resistor, ( RL ),

27. Output Characteristics Curves of a Typical Bipolar
Transistor
8/2/2017Dr Gnanasekaran Thangavel27
 Dynamic Load Line of the transistor can
be drawn directly onto the graph of
curves above from the point of
“Saturation” ( A ) when Vce = 0 to the
point of “Cut-off” ( B ) when Ic = 0 thus
giving us the “Operating” or Q-point of
the transistor. These two points are
joined together by a straight line and
any position along this straight line
represents the “Active Region” of the
transistor. The actual position of the
load line on the characteristics curves
can be calculated as follows:

28. Output Characteristics Curves of a Typical Bipolar
Transistor
8/2/2017Dr Gnanasekaran Thangavel28
 Then, the collector or output characteristics curves
for Common Emitter NPN Transistors can be
used to predict the Collector current, Ic, when
given Vce and the Base current, Ib. A Load Line
can also be constructed onto the curves to
determine a suitable Operating or Q-point which
can be set by adjustment of the base current. The
slope of this load line is equal to the reciprocal of
the load resistance which is given as: -1/RL
 Then we can define a NPN Transistor as being
normally “OFF” but a small input current and a
small positive voltage at its Base ( B ) relative to
its Emitter ( E ) will turn it “ON” allowing a much
large Collector-Emitter current to flow. NPN
transistors conduct when Vc is much greater than

29. AM transmitter
8/2/2017Dr Gnanasekaran Thangavel29

30. Single transistor radio receiver
8/2/2017Dr Gnanasekaran Thangavel30

31. 31
TRANSISTOR BIASING CIRCUITS
The term biasing is used for application of dc voltages to establish a fixed level of
current and voltage.
The purpose of biasing a circuit is to establish a proper stable dc operating point
(Q-point). The dc operating point between saturation and cutoff is called the Q-point.
The goal is to set the Q-point such that that it does not go into saturation or cutoff
when an ac signal is applied.
Transistor must be properly biased with dc voltage to operate as a linear amplifier.
If amplifier is not biased with correct dc voltages on input and output, it can go into
saturation or cutoff when the input signal applied.
There are several methods to establish DC operating point.
We will discuss some of the methods used for biasing transistors.
base-bias circuits
voltage-divider bias circuits
emitter-bias circuits
collector-feedback bias circuits

32. 32
RB
RC
Q1
VCC
VB(ac)
IB(ac)
VCE(ac)
IC(ac)
Amplifier

33. 33
A generic dc load line.
IC
VCE
(sat)
CC
C
C
V
I
R

(off )CE CCV V
CC CE
C
C
V V
I
R



34. 34
RB
RC
2 k
Q1
+12 V
VCE
2 4 6 8 10 12
2
4
6
8
IC
IC(sat)
VCE(off)
Plot the dc load line for the circuit shown in Fig. 7.3a.

35. 35
Fig 7.4 Example 7.2.
Plot the dc load line for the circuit shown in Fig. 7.4. Then,
find the values of VCE for IC = 1, 2, 5 mA respectively.
RB
RC
1 k
Q1
+10 V
VCE
2 4 6 8 10
2
4
6
8
IC
10 IC (mA) VCE (V)
1 9
2 8
5 5
CE CC C CV V I R 

36. 36
Fig 7.6-8 Optimum Q-point with amplifier
operation.
βC BI I
CE CC C CV V I R 
VCE
IB = 0 A
IB = 10 A
IB = 20 A
IB = 30 A
IB = 40 A
IB
= 50 A
IC
Q-Point
VCCVCC/2
IC(sat)
IC(sat)/2
IB

37. 37
Base bias (fixed bias).
CC BE
B
B
V V
I
R


βC BI I
CE CC C CV V I R 
RC
RB
+0.7 V
IC
IB
IE
Input
Output
VBE
VCC
Q1
b = dc current gain = hFE

38. 38
Example
RC
2 k
RB
360 k
+0.7 V
IC
IB
IE
VBE
+8 V
hFE = 100
0.7V 8V 0.7V
360kΩ
20.28μA
CC
B
B
V
I
R
 
 

  100 20.28μA
2.028mA
C FE BI h I 

  8V 2.028mA 2kΩ
3.94V
CE CC C CV V I R 
 

The circuit is midpoint biased.

39. 39
Example
Construct the dc load line for the circuit shown in Fig. 7.10, and plot the Q-point
from the values obtained in Example 7.3. Determine whether the circuit is
midpoint biased.
VCE (V)
2 4 6 8 10
1
2
3
4
IC (mA)
Q
(sat)
8V
4mA
2kΩ
CC
C
C
V
I
R
  
 off
8VCCCE
V V 

40. 40
Example - Q-point shift.
The transistor in Fig. 7.12 has values of hFE = 100 when T = 25 °C and hFE = 150
when T = 100 °C. Determine the Q-point values of IC and VCE at both of these
temperatures.
RC
2 k
RB
360 k
+0.7 V
IC
IB
IE
VBE
+8 V
hFE = 100 (T = 25C)
hFE = 150 (T = 100C)
Temp(°C) IB (A) IC (mA) VCE (V)
25 20.28 2.028 3.94
100 20.28 3.04 1.92

41. 41
Base bias characteristics. (1)
RC
RB
+0.7 V
IC
IB
IE
Input
Output
VBE
VCC
Q1 Advantage: Circuit simplicity.
Disadvantage: Q-point shift with temp.
Applications: Switching circuits only.
Circuit recognition: A single resistor (RB) between
the base terminal and VCC. No emitter resistor.

42. 42
Base bias characteristics. (2)
RC
RB
+0.7 V
IC
IB
IE
Input
Output
VBE
VCC
Q1
(sat)
(off )
CC
C
C
CE CC
V
I
R
V V


Load line equations:
Q-point equations:
CC BE
B
B
C FE B
CE CC C C
V V
I
R
I h I
V V I R



 

43. 43
Voltage divider bias
R1
R2 RE
RC
+VCC
Input
Output
I1
I2 IE
IB
IC
Assume that I2 > 10IB.
2
1 2
B CC
R
V V
R R


0.7VE BV V 
E
E
E
V
I
R

Assume that ICQ  IE (or hFE >>
1). Then
 CEQ CC CQ C EV V I R R  

44. 44
Example -1
Determine the values of ICQ and VCEQ for the circuit shown in Fig. 7.15.
R1
18 k
R2
4.7 k
RE
1.1 k
RC
3 k
+10 V
I1
I2
IE
IB
IC
hFE = 50
 
2
1 2
4.7kΩ
10V 2.07V
22.7kΩ
B CC
R
V V
R R


 
0.7V
2.07V 0.7V 1.37V
E BV V 
  
Because ICQ  IE (or hFE >> 1),
1.37V
1.25mA
1.1kΩ
E
CQ
E
V
I
R
  
 
  10V 1.25mA 4.1kΩ 4.87V
CEQ CC CQ C EV V I R R  
  

45. 45
Example -2
Verify that I2 > 10 IB.
R1
18 k
R2
4.7 k
RE
1.1 k
RC
3 k
+10 V
I1
I2
IE
IB
IC
hFE = 50
2
2
2.07V
440.4μA
4.7kΩ
BV
I
R
  
1.25mA
1 50+1
24.51μA
E
B
FE
I
I
h
 


2 10 BI I 

46. 46
Which value of hFE do I use?
Transistor specification sheet may list any combination of the
following hFE: max. hFE, min. hFE, or typ. hFE. Use typical value
if there is one. Otherwise, use
(ave) (min) (max)FE FE FEh h h 

47. 47
Example
A voltage-divider bias circuit has the following values: R1 = 1.5 k, R2 =
680 , RC = 260 , RE = 240  and VCC = 10 V. Assuming the transistor is
a 2N3904, determine the value of IB for the circuit.
 2
1 2
680Ω
10V 3.12V
2180Ω
B CC
R
V V
R R
  

0.7V 3.12V 0.7V 2.42VE BV V    
2.42V
10mA
240Ω
E
CQ E
E
V
I I
R
   
( ) (min) (max) 100 300 173FE ave FE FEh h h    
(ave)
10mA
57.5μA
1 174
E
B
FE
I
I
h
  


48. 48
Stability of Voltage Divider
Bias Circuit
The Q-point of voltage divider bias circuit is less dependent on hFE than
that of the base bias (fixed bias).
For example, if IE is exactly 10 mA, the range of hFE is 100 to 300. Then
10mA
At 100, 100μA and 9.90mA
1 101
E
FE B CQ E B
FE
I
h I I I I
h
      

10mA
At 300, 33μA and 9.97mA
1 301
E
FE B CQ E B
FE
I
h I I I I
h
      

ICQ hardly changes over the entire range of hFE.

49. 49
Load line for voltage divider bias circuit.
2 4 6 8 10 12
5
10
15
20
25
IC (mA)
VCE (V)
(sat)
10V
20mA
260Ω+240Ω
CC
C
C E
V
I
R R
  

(off ) 10VCE CCV V 
Circuit values are from Example 7.9.

50. 50
Base input resistance - 1
R1
R2 RE
RC
VCC
I1
I2
IE
IB
IC
RIN(base)
R1
R2
I1
I2
VCC
0.7 V
IB RIN(base)
( 1)E E E B FE EV I R I h R  
(base) ( 1)E
IN FE E
B
FE E
V
R h R
I
h R
  

May be ignored.

51. 51
Base input resistance -2
IB
R1
R2
I1
I2
VCC
IB RIN(base)
VB
 
 
 
2 (base)
1 2 (base)
2
1 2
21
//
//
//
//
//
IN
B CC
IN
FE E
CC
FE E
EQ
CC
EQ FE EEQ
R R
V V
R R R
R h R
V
R R h R
R
V
R R h RR R







52. 52
Example
 
 
2 //
10kΩ// 50 1.1kΩ 8.46kΩ
EQ FE ER R h R
  
 
1
8.46kΩ
20V 2.21V
68kΩ 8.46kΩ
EQ
B CC
EQ
R
V V
R R


 

0.7V
2.21V 0.7V
1.37mA
1.1kΩ
E B
CQ E
E E
V V
I I
R R

  

 
 
  20V 1.37mA 7.3kΩ 9.99V
CEQ CC CQ C EV V I R R  
  
R1
68k
R2
10k
RE
1.1k
RC
6.2k
VCC=20V
I1
I2
IE
IC
hFE = 50

53. 53
Voltage-divider bias characteristics - 1
R1
R2 RE
RC
+VCC
Input
Output
I1
I2 IE
IB
IC
Circuit recognition: The voltage
divider in the base circuit.
Advantages: The circuit Q-point values
are stable against changes in hFE.
Disadvantages: Requires more
components than most other biasing
circuits.
Applications: Used primarily to bias
linear amplifier.

54. 54
Voltage-divider bias characteristics -2
R1
R2 RE
RC
+VCC
Input
Output
I1
I2 IE
IB
IC
Load line
equations: (sat)
(off )
CC
C
C E
CE CC
V
I
R R
V V



Q-point equations (assume that
hFERE > 10R2):
 
2
1 2
0.7V
B CC
E B
E
CQ E
E
CEQ CC CQ C E
R
V V
R R
V V
V
I I
R
V V I R R


 
 
  

55. 55
Other Transistor Biasing Circuits
 Emitter-bias circuits
 Feedback-bias circuits
 Collector-feedback bias
 Emitter-feedback bias

56. 56
Emitter bias.
Assume that the transistor operation is in
active region.
RC
RE
RB
IC
IE
IB
Q1
Input
Output
+VCC
-VEE
 
0.7V
1
EE
B
B FE E
V
I
R h R


 
C FE BI h I
 1E FE BI h I 
CE CC C C E E EEV V I R I R V   
Assume that hFE >> 1.
 CE CC C C E EEV V I R R V   

57. 57
Example
RC
750
RE
1.5k
RB
100
IC
IE
IB
Q1
Input
Output
+12 V
-12 V
hFE = 200
Determine the values of
ICQ and VCEQ for the
amplifier shown in
Fig.7.27.
12V 0.7V
( 1)
11.3V
37.47μA
100Ω+201 1.5kΩ
B
B FE E
I
R h R


 
 

200 37.47μA
7.49mA
CQ FE BI h I  

 
 
( )
24V 7.49mA 750Ω 1.5kΩ
7.14V
CEQ CC C C E EEV V I R R V    
  


58. 58
Load Line for
Emitter-Bias Circuit
(sat)
( )CC EE CC EE
C
C E C E
V V V V
I
R R R R
  
 
 
 ( )CE off CC EE CC EEV V V V V    
VCE
IC
IC(sat)
VCE(off)

59. 59
Emitter-bias characteristics -1
RC
RE
RB
IC
IE
IB
Q1
Input
Output
+VCC
-VEE
Circuit recognition: A split (dual-polairty) power
supply and the base resistor is connected to
ground.
Advantage: The circuit Q-point values are stable
against changes in hFE.
Disadvantage: Requires the use of dual-polarity
power supply.
Applications: Used primarily to bias linear
amplifiers.

60. 60
Emitter-bias characteristics- 2
RC
RE
RB
IC
IE
IB
Q1
Input
Output
+VCC
-VEE
Load line equations:
(sat)
(off )
CC EE
C
C E
CE CC EE
V V
I
R R
V V V



 
Q-point equations:
 
 
 
1
BE EE
CQ FE
B FE E
CEQ CC CQ C E EE
V V
I h
R h R
V V I R R V
 

 
   

61. 61
Collector-feedback bias.
RB
RC
+VCC
IC
IE
IB
 CC C B C B B BEV I I R I R V   
( 1)
CC BE
B
FE C B
V V
I
h R R


 
CQ FE BI h I
 1CEQ CC FE B C
CC CQ C
V V h I R
V I R
  
 

62. 62
Fig 7.30 Example 7.14.
Determine the values of ICQ and VCEQ for the amplifier shown
in Fig. 7.30.
RB
RC
1.5 k
+10 V
180 k
hFE = 100
 1
10V 0.7V
28.05μA
180kΩ 101 1.5kΩ
CC BE
B
B FE C
V V
I
R h R


 

 
 
100 28.05μA
2.805mA
CQ FE BI h I  

( 1)
10V 101 28.05μA 1.5kΩ
5.75V
CEQ CC FE B CV V h I R  
   


63. 63
Circuit Stability of
Collector-Feedback Bias
RB
RC
+VCC
IC
IE
IB
hFE increases
IC increases (if IB is the same)
VCE decreases
IB decreases
IC does not increase that much.
Good Stability. Less dependent on hFE and
temperature.

64. 64
Collector-Feedback
Characteristics (1)
RB
RC
+VCC
IC
IE
IB
Circuit recognition: The base resistor is
connected between the base and the
collector terminals of the transistor.
Advantage: A simple circuit with relatively
stable Q-point.
Disadvantage: Relatively poor ac
characteristics.
Applications: Used primarily to bias linear
amplifiers.

65. 65
Collector-Feedback
Characteristics (2)
RB
RC
+VCC
IC
IE
IB
Q-point relationships:
( 1)
CC BE
B
FE C B
V V
I
h R R


 
CQ FE BI h I
CEQ CC CQ CV V I R 

66. 66
Emitter-feedback bias.
RB RC
+VCC
RE
IB
IE
IC
 1
CC BE
B
B FE E
V V
I
R h R


 
CQ FE BI h I
 
CEQ CC C C E E
CC CQ C E
V V I R I R
V I R R
  
  
 1E FE BI h I 

67. 67
Fig 7.32 Example 7.15.
RB
680k
RC
6.2k
+VCC
RE
1.6k
hFE = 50
 
16V 0.7V
1 680kΩ 51 1.6kΩ
20.09μA
CC BE
B
B FE E
V V
I
R h R
 
 
   

50 20.09μA 1mACQ FE BI h I   
 
  16V 1mA 7.8kΩ 8.2V
CEQ CC CQ C EV V I R R  
  

68. 68
Circuit Stability of
Emitter-Feedback Bias
hFE increases
IC increases (if IB is the same)
VE increases
IB decreases
IC does not increase that much.
IC is less dependent on hFE and temperature.
RB RC
+VCC
RE
IB
IE
IC

69. 69
Emitter-Feedback Characteristics -1
Circuit recognition: Similar to voltage
divider bias with R2 missing (or base bias
with RE added).
Advantage: A simple circuit with relatively
stable Q-point.
Disadvantage: Requires more components
than collector-feedback bias.
Applications: Used primarily to bias linear
amplifiers.
RB RC
+VCC
RE
IB
IE
IC

70. 70
Emitter-Feedback Characteristics -2
RB RC
+VCC
RE
IB
IE
IC
Q-point relationships:
( 1)
CC BE
B
B FE E
V V
I
R h R


 
CQ FE BI h I
 CEQ CC CQ C EV V I R R  

71. 71
Summary
 DC Biasing and the dc load line
 Base bias circuits
 Voltage-divider bias circuits
 Emitter-bias circuits
 Feedback-bias circuits
 Collector-feedback bias circuits
 Emitter-feedback bias circuits

72. Field-Effect Transistors -FET
8/2/2017Dr Gnanasekaran Thangavel72
 The FET is based around the concept that charge
on a nearby object can attract charges within a
semiconductor channel.
 The FET consists of a semiconductor channel with
electrodes at either end referred to as the drain and
the source.
 A control electrode called the gate is placed in very
close proximity to the channel so that its electric
charge is able to affect the channel
 In this way, the gate of the FET controls the flow of
carriers (electrons or holes) flowing from the source
to drain. It does this by controlling the size and
shape of the conductive channel.
 The semiconductor channel where the current flow
occurs may be either P-type or N-type. This gives
rise to two types or categories of FET known as P-

73. Field Effect Transistor types
8/2/2017Dr Gnanasekaran Thangavel73
 There are many ways to define the
different types of FET that are available.
They may be categorised in a number of
ways, but some of the major types of FET
can be covered in the tree diagram.
 Junction FET(JFET), Insulated Gate
FET(IGFET), Metal Oxide Silicon
FET(MOSFET), Dual Gate
MOSFET(DGMOSFET), MEtal Silicon
FET(MESFET), High Electron Mobility
Transistor (HEMT) , Pseudomorphic High
Electron Mobility Transistor( PHEMT), Fin
Field Effect Transistor (FinFET), vertical
MOS( VMOS)

74. 8/2/2017Dr Gnanasekaran Thangavel74
 The N-channel JFET’s channel is doped with donor impurities meaning that
the flow of current through the channel is negative (hence the term N-
channel) in the form of electrons.
 The P-channel JFET’s channel is doped with acceptor impurities meaning
that the flow of current through the channel is positive (hence the term P-
channel) in the form of holes. N-channel JFET’s have a greater channel
conductivity (lower resistance) than their equivalent P-channel types, since
electrons have a higher mobility through a conductor compared to holes.
This makes the N-channel JFET’s a more efficient conductor compared to
their P-channel counterparts.
Bipolar Transistor
Field Effect
Transistor
Emitter – (E) >> Source –
(S)
Base – (B) >> Gate –
(G)

75. Biasing of an N-channel JFET
8/2/2017Dr Gnanasekaran Thangavel75
 The cross sectional diagram above shows an
N-type semiconductor channel with a P-type
region called the Gate diffused into the N-
type channel forming a reverse biased PN-
junction and it is this junction which forms the
depletion region around the Gate area when
no external voltages are applied. JFETs are
therefore known as depletion mode devices.

76. JFET Channel Pinched-off
8/2/2017Dr Gnanasekaran Thangavel76
 The width of the channel decreases until no
more current flows between the Drain and
the Source and the FET is said to be
“pinched-off” (similar to the cut-off region for
a BJT). The voltage at which the channel
closes is called the “pinch-off voltage”, ( VP ).
In this pinch-off region the Gate voltage, VGS
controls the channel current and VDS has
little or no effect.
 The result is that the FET acts more like a
voltage controlled resistor which has zero
resistance when VGS = 0 and maximum “ON”
resistance ( RDS ) when the Gate voltage is
very negative. Under normal operating
conditions, the JFET gate is always
negatively biased relative to the source.
JFET
Model

77. Output characteristic V-I curves of a typical
junction FET.
8/2/2017Dr Gnanasekaran Thangavel77
 The characteristics curves example shown above,
shows the four different regions of operation for a JFET
and these are given as:
 Ohmic Region – When VGS = 0 the depletion layer of
the channel is very small and the JFET acts like a
voltage controlled resistor.
 Cut-off Region – This is also known as the pinch-off
region were the Gate voltage, VGS is sufficient to
cause the JFET to act as an open circuit as the
channel resistance is at maximum.
 Saturation or Active Region – The JFET becomes a
good conductor and is controlled by the Gate-Source
voltage, ( VGS ) while the Drain-Source voltage, ( VDS
) has little or no effect.
 Breakdown Region – The voltage between the Drain
and the Source, ( VDS ) is high enough to causes the

78. This another type of Field Effect Transistor
available whose Gate input is electrically
insulated from the main current carrying
channel and is therefore called an Insulated
Gate Field Effect Transistor or IGFET.
The most common type of insulated gate FET
which is used in many different types of
electronic circuits is called the Metal Oxide
Semiconductor Field Effect Transistor or
MOSFET for short.
The IGFET or MOSFET is a voltage
controlled field effect transistor that differs
from a JFET in that it has a “Metal Oxide”
Gate electrode which is electrically insulated
Metal Oxide Semiconductor Field Effect-MOSFET

79. Like the previous JFET tutorial, MOSFETs are three terminal devices
with a Gate, Drain and Source and both P-channel (PMOS) and N-
channel (NMOS) MOSFETs are available. The main difference this
time is that MOSFETs are available in two basic forms:
Depletion Type – the transistor requires the Gate-Source voltage,
( VGS ) to switch the device “OFF”. The depletion mode MOSFET is
equivalent to a “Normally Closed” switch.
Enhancement Type – the transistor requires a Gate-Source
voltage, ( VGS ) to switch the device “ON”. The enhancement mode
MOSFET is equivalent to a “Normally Open” switch.

80. The four MOSFET symbols above show an
additional terminal called the Substrate and is
not normally used as either an input or an output
connection but instead it is used for grounding
the substrate. It connects to the main
semiconductive channel through a diode junction
to the body or metal tab of the MOSFET. Usually
in discrete type MOSFETs, this substrate lead is
connected internally to the source terminal.
When this is the case, as in enhancement types
it is omitted from the symbol for clarification.
The line between the drain and source
connections represents the semiconductive
channel. If this is a solid unbroken line then this
represents a “Depletion” (normally-ON) type
MOSFET as drain current can flow with zero
gate potential. If the channel line is shown dotted
or broken it is an “Enhancement” (normally-OFF)
type MOSFET as zero drain current flows with
zero gate potential. The direction of the arrow
indicates whether the conductive channel is a p-

81. Basic MOSFET Structure and Symbol MOSFET
construction
 The construction of the Metal Oxide Semiconductor FET is
very different to that of the Junction FET. Both the
Depletion and Enhancement type MOSFETs use an
electrical field produced by a gate voltage to alter the flow
of charge carriers, electrons for n-channel or holes for P-
channel, through the semiconductive drain-source
channel. The gate electrode is placed on top of a very thin
insulating layer and there are a pair of small n-type regions
just under the drain and source electrodes.
 We saw in the previous tutorial, that the gate of a junction
field effect transistor, JFET must be biased in such a way
as to reverse-bias the pn-junction. With a insulated gate
MOSFET device no such limitations apply so it is possible
to bias the gate of a MOSFET in either polarity, positive
(+ve) or negative (-ve).
 This makes the MOSFET device especially valuable as
electronic switches or to make logic gates because with no
bias they are normally non-conducting and this high gate
input resistance means that very little or no control current

82. Depletion-mode MOSFET
 The Depletion-mode MOSFET, which is less common than the enhancement
mode types is normally switched “ON” (conducting) without the application of a
gate bias voltage. That is the channel conducts when VGS = 0 making it a
“normally-closed” device. The circuit symbol shown above for a depletion MOS
transistor uses a solid channel line to signify a normally closed conductive
channel.
 For the n-channel depletion MOS transistor, a negative gate-source voltage, -
VGS will deplete (hence its name) the conductive channel of its free electrons
switching the transistor “OFF”. Likewise for a p-channel depletion MOS
transistor a positive gate-source voltage, +VGS will deplete the channel of its
free holes turning it “OFF”.
 In other words, for an n-channel depletion mode MOSFET: +VGS means more
electrons and more current. While a -VGS means less electrons and less
current. The opposite is also true for the p-channel types. Then the depletion
mode MOSFET is equivalent to a “normally-closed” switch.

83. Depletion-mode N-Channel MOSFET and circuit
Symbols
 The depletion-mode MOSFET is
constructed in a similar way to their
JFET transistor counterparts were
the drain-source channel is
inherently conductive with the
electrons and holes already present
within the n-type or p-type channel.
This doping of the channel
produces a conducting path of low
resistance between the Drain and
Source with zero Gate bias.

84. Enhancement-mode MOSFET
 The more common Enhancement-mode MOSFET or eMOSFET, is the reverse of the depletion-mode
type. Here the conducting channel is lightly doped or even undoped making it non-conductive. This
results in the device being normally “OFF” (non-conducting) when the gate bias voltage, VGS is equal
to zero. The circuit symbol shown above for an enhancement MOS transistor uses a broken channel
line to signify a normally open non-conducting channel.
 For the n-channel enhancement MOS transistor a drain current will only flow when a gate voltage ( VGS
) is applied to the gate terminal greater than the threshold voltage ( VTH ) level in which conductance
takes place making it a transconductance device.
 The application of a positive (+ve) gate voltage to a n-type eMOSFET attracts more electrons towards
the oxide layer around the gate thereby increasing or enhancing (hence its name) the thickness of the
channel allowing more current to flow. This is why this kind of transistor is called an enhancement mode
device as the application of a gate voltage enhances the channel.
 Increasing this positive gate voltage will cause the channel resistance to decrease further causing an
increase in the drain current, ID through the channel. In other words, for an n-channel enhancement
mode MOSFET: +VGS turns the transistor “ON”, while a zero or -VGS turns the transistor “OFF”. Then,
the enhancement-mode MOSFET is equivalent to a “normally-open” switch.
 The reverse is true for the p-channel enhancement MOS transistor. When VGS = 0 the device is “OFF”
and the channel is open. The application of a negative (-ve) gate voltage to the p-type eMOSFET

85. Enhancement-mode N-Channel MOSFET and Circuit
Symbols
 Enhancement-mode MOSFETs
make excellent electronics
switches due to their low “ON”
resistance and extremely high
“OFF” resistance as well as their
infinitely high input resistance due
to their isolated gate.
Enhancement-mode MOSFETs are
used in integrated circuits to
produce CMOS type Logic Gates
and power switching circuits in the
form of as PMOS (P-channel) and
NMOS (N-channel) gates. CMOS
actually stands for Complementary
MOS meaning that the logic device

86. Enhancement-mode N-Channel MOSFET
Amplifier
 The DC biasing of this
common source (CS)
MOSFET amplifier circuit
is virtually identical to the
JFET amplifier. The
MOSFET circuit is biased
in class A mode by the
voltage divider network
formed by resistors R1
and R2. The AC input
resistance is given as RIN
= RG = 1MΩ.

87. Enhancement-mode N-Channel MOSFET
Amplifier …
 Metal Oxide Semiconductor Field Effect Transistors are three terminal active
devices made from different semiconductor materials that can act as either an
insulator or a conductor by the application of a small signal voltage. The
MOSFETs ability to change between these two states enables it to have two basic
functions: “switching” (digital electronics) or “amplification” (analogue electronics).
Then MOSFETs have the ability to operate within three different regions:
 1. Cut-off Region – with VGS < Vthreshold the gate-source voltage is lower
than the threshold voltage so the MOSFET transistor is switched “fully-OFF” and
IDS = 0, the transistor acts as an open circuit
 2. Linear (Ohmic) Region – with VGS > Vthreshold and VDS < VGS the
transistor is in its constant resistance region and behaves as a voltage-controlled
resistor whose resistive value is determined by the gate voltage, VGS
 3. Saturation Region – with VGS > Vthreshold the transistor is in its constant
current region and is switched “fully-ON”. The current IDS = maximum as the

88. MOSFET Summary
MOSFET type VGS = +ve VGS = 0
VGS = -
ve
N-Channel Depletion ON ON OFF
N-Channel
Enhancement
ON OFF OFF
P-Channel Depletion OFF ON ON
P-Channel
Enhancement
OFF OFF ON

89. Special Semiconductor Devices

90. INTRODUCTION:
 The SCR is the most important special semiconductor device. This device is
popular for its Forward-Conducting and Reverse-blocking
characteristics.
 SCR can be used in high-power devices. For example, in the central
processing unit of the computer, the SCR is used in switch mode power
supply (SMPS).
 The DIAC, a combination of two Shockley Diodes, and the TRIAC, a
combination of two SCRs connected anti-parallelly are important power-
control devices.
 The UJT is also used as an efficient switching device.

91. SILICON-CONTROLLED RECTIFIER (SCR)
 The silicon-controlled rectifier or semiconductor controlled rectifier is
a two-state device used for efficient power control.
 SCR is the parent member of the thyristor family and is used in high-
power electronics. Its constructional features, physical operation and
characteristics are explained in the following sections.
 The SCR is a four-layer structure, either p–n–p–n or n–p–n–p, that
effectively blocks current through two terminals until it is turned ON by a
small-signal at a third terminal.
 The SCR has two states: a high-current low-impedance ON state and a
low-current high-impedance OFF state.
 The basic transistor action in a four-layer p–n–p–n structure is analyzed
first with only two terminals, and then the third control input is introduced.

92. Physical Operation and Characteristics:
 The physical operation of the SCR can be explained clearly with reference to
the current–voltage characteristics.
 The forward-bias condition and reverse-bias condition illustrate the
conducting state and the reverse blocking state respectively. Based on these
two states a typical I –V characteristic of the SCR is shown in Fig. 8-2.

93. SCR in Forward Bias:
 There are two different states in which we can examine the SCR in the forward-
biased condition:
(i) The high- impedance or forward-blocking state
(ii) The low-impedance or forward-conducting state
At a critical peak forward voltage Vp, the SCR switches from the blocking state to
the conducting state, as shown in Fig. 8-2.
 A positive voltage places junction j1 and j3 under forward-bias, and the centre
junction j2 under reverse-bias.
 The for ward voltage in the blocking state appears across the reverse-biased
junction j2 as the applied voltage V is increased. The voltage from the anode A to
cathode C, as shown in Fig. 8-1, is very small after switching to the forward-
conducting state, and all three junctions are forward-biased. The junction j2
switches from reverse-bias to forward-bias..

94. SCR in Reverse Bias:
 In the reverse-blocking state the junctions j1 and j3 are reverse-
biased, and j2 is forward-biased.
 The supply of electrons and holes to junction j2 is restricted, and due
to the thermal generation of electron–hole pairs near junctions j1 and j2 the
device current is a small saturation current.
 In the reverse blocking condition the current remains small until
avalanche breakdown occurs at a large reverse-bias of several thousand volts.
 An SCR p–n–p–n structure is equivalent to one p–n–p transistor and
one n–p–n transistor sharing some common terminals.
 Collector current I C 1 = α1i + I CO 1 having a transfer ratio α 1 for the p–n–p.
 Collector current I C 2 =α2i + I CO 2 having a transfer ratio a2 for the n–p–n.
 ICO1 and ICO 2 stand for the respective collector-saturation currents.
I C 1 = α 1i + I CO 1 = I B 2 ……………….(8-1)

95. SCR in Reverse Bias:

96.  The total current through the SCR is the sum of iC1 and iC2:
 I C 1 + I = i ………………..(8-3)
 Substituting the values of collector current from Eqs. (8-1) and (8-2) in Eq. (8-3) we get:
 i (α1 + α2) + I CO 1 + I CO 2 = i
 i = (I CO 1 + I CO 2 ) /(1- α1 + α2) ………………..(8-4)
 Case I: When (α1 + α2) → 1, then the SCR current i → infinite.
 As the sum of the values of alphas tends to unity, the SCR current i increases rapidly. The
derivation is no
 longer valid as (α1 + α2) equals unity.
 Case II: When (α1 + α2 → 0, i.e., when the summation value of alphas goes to zero, the
SCR resultant current can be expressed as:
 i = I CO 1 + I CO 2 …………………………….(8-5)
 The current, i, passing through the SCR is very small. It is the combined collector-saturation
currents of the two equivalent transistors as long as the sum (α1 + α2) is very small or almost
near zero.
SCR in Reverse Bias:

97. I–V Characteristics of the SCR:
 Forward-Blocking State:
 When the device is biased in the forward-blocking state, as shown in Fig. 8-4(a), the applied
voltage appears primarily across the reverse-biased junction j2. Al though the junctions j1 and j3 are
forward-biased, the current is small.

98.  Forward-Conducting State of the SCR:
As the value of (α1 + α2 ) approaches unity through one of the mechanisms ,many holes
injected at j1 survive to be swept across j2 into p2.
 This process helps feed the recombination in p2 and support the injection of holes into n2. In a
similar manner, the transistor action of electrons injected at j3 and collected at j2 supplies electrons for n1.
 The current through the device can be much larger.
I–V Characteristics of the SCR:

99. Reverse-Blocking State of the SCR:
 The SCR in reverse-biased condition allows almost negligible
current to flow through it. This is shown in Fig. 8-4(c).
 In the reverse-blocking state of the SCR, a small saturation
current flows from anode to cathode. Holes will flow from the gate into p2, the base of the n–p–n transistor,
due to positive gate current.
 The required gate current for turn-on is only a few milli-amperes, therefore, the SCR can be
turned on by a very small amount of power in the gate.

100. I–V Characteristics of the SCR:
 As shown in Fig. 8-5, if the gate current is 0 mA, the
critical voltage is higher, i.e., the SCR requires more
voltage to switch to the conducting state.
 But as the value of gate current increases, the critical
voltage becomes lower, and the SCR switches to
the conducting state at a lower voltage.
 At the higher gate current IG2, the SCR switches
faster than at the lower gate current IG1,
because IG2 > IG1.
I–V Characteristics of the SCR:

101. Semiconductor-controlled switch (SCS):
 Few SCRs have two gate leads, G2
attached to p2 and G1
attached to n1, as shown in Fig. 8-6.
This configuration is called the
semiconductor-controlled switch
(SCS).
 The SCS, biased in the forward-
blocking state, can be switched to the
conducting state by a negative pulse at
the anode gate n1 or by a positive
current pulse applied to the cathode
gate at p2.

102. Simple Applications:
 The SCR is the most important member of the thyristor family. The SCR is a
capable power device as it can handle thousands of amperes and volts.
 Generally the SCR is used in many applications such as in high power
electronics, switches, power-control and conversion mode.
 It is also used as surge protector.
 Static Switch: The SCR is used as a switch for power-switching in various
control circuits.
 Power Control: Since the SCR can be turned on externally, it can be used to
regulate the amount of power delivered to a load.
 Surge Protection: In an SCR circuit, when the voltage rises beyond the
threshold value, the SCR is turned on to dissipate the charge or voltage quickly.
 Power Conversion: The SCR is also used for high-power conversion and
regulation. This includes conversion of power source from ac to ac, ac to dc and

103. TRIODE AC SWITCH (TRIAC):
 The term TRIAC is derived by combining the first three letters of the word
“TRIODE” and the word “AC”.
 A TRIAC is capable of conducting in both the directions. The TRIAC, is thus, a
bidirectional thyristor with three terminals. It is widely used for the control of
power in ac circuits.

104. Constructional Features:
Depending upon the polarity of the gate pulse and the biasing
conditions, the main four-layer structure that turns ON by a
regenerative process could be one of p1 n1, p2 n2, p1 n1 p2 n3, or
p2 n1 p1 n4, as shown in Fig. 8-8.

105. Advantages of the TRIAC:
 The TRIAC has the following advantages:
(i) They can be triggered with positive- or negative-polarity
voltage.
(ii) They need a single heat sink of slightly larger size.
(iii) They need a single fuse for protection, which simplifies their
construction.
(iv) In some dc applications, the SCR has to be connected with a
parallel diode for protection against reverse voltage, whereas a
TRIAC may work without a diode, as safe breakdown in either
direction is possible.

106.  The TRIAC has the following disadvantages:
(i) TRIACs have low dv/dt ratings compared to SCRs.
(ii) Since TRIACs can be triggered in either direction, the trigger circuits with
TRIACs needs careful consideration.
(iii) Reliability of TRIACs is less than that of SCRs.
Disadvantages of the TRIAC:
Simple Applications of the TRIAC:
 The TRIAC as a bidirectional thyristor has various applications. Some of the
popular applications of the
TRIAC are as follows:
(i) In speed control of single-phase ac series or universal motors.
(ii) In food mixers and portable drills.
(iii) In lamp dimming and heating control.
(iv) In zero-voltage switched ac relay.

107. DIODE AC SWITCH (DIAC):
 The DIAC is a combination of two diodes. Diodes being unidirectional
devices, conduct current only in one direction.
 If bidirectional (ac) operation is desired, two Shockley diodes may be joined
in parallel facing different directions to form the DIAC.

108. Constructional Features:
 The construction of DIAC looks like a transistor but there are major differences.
 They are as follows:
(i) All the three layers, p–n–p or n–p–n, are equally doped in the DIAC, whereas
in the BJT there is a gradation of doping. The emitter is highly doped, the collector
is lightly doped, and the base is moderately doped.
 (ii) The DIAC is a two-terminal diode as opposed to the BJT, which is a three-
terminal device.

109. Physical Operation and Characteristics:
 The main characteristics are of the DIAC are as follows:
(i) Break over voltage
(ii) Voltage symmetry
(iii) Break-back voltage
(iv) Break over current
(v) Lower power dissipation
 Although most DIACs have symmetric switching voltages, asymmetric
DIACs are also available. Typical DIACs have a power dissipations
ranging from 1/2 to 1 watt.

110. I-V characteristics of the DIAC:

111. UNIJUNCTION TRANSISTOR (UJT):
 The uni-junction transistor is a three-terminal single-junction device. The switching
voltage of the UJT can be easily varied.
 The UJT is always operated as a switch in oscillators, timing circuits and in
SCR/TRIAC trigger circuits.

112. Constructional Features:
 The UJT structure consists of a lightly doped n-type silicon bar
provided with ohmic contacts on either side.
 The two end connections are called base B1 and base B2. A small
heavily doped p-region is alloyed into one side of the bar. This p-region
is the UJT emitter (E) that forms a p–n junction with the bar.
 Between base B1 and base B2, the resistance of the n-type bar called
inter-base resistance (RB ) and is in the order of a few kilo ohm.
 This inter-base resistance can be broken up into two resistances—the
resistance from B1 to the emitter is RB1 and the resistance from B2 to
the emitter is RB 2.
 Since the emitter is closer to B2 the value of RB1is greater than RB2.
 Total resistance is given by:

113. Equivalent circuit for UJT:
 The VBB source is generally
fixed and provides a constant
voltage from B2 to B1.
 The UJT is normally
operated with both B2 and E
positive biased relative to B1.
 B1 is always the UJT
reference terminal and all
voltages are measured
relative to B1 . VEE is a
variable voltage source.

114. UJT V–I characteristic curves:

115. ON State of the UJT Circuit:
 As VEE increases, the UJT stays in the OFF state until VE approaches the
peak point value V P. As VE approaches VP the p–n junction becomes forward-
biased and begins to conduct in the opposite direction.
 As a result IE becomes positive near the peak point P on the VE - IE curve.
When VE exactly equals VP the emitter current equals IP .
 At this point holes from the heavily doped emitter are injected into the n-type
bar, especially into the B1 region. The bar, which is lightly doped, offers very
little chance for these holes to recombine.
 The lower half of the bar becomes replete with additional current carriers
(holes) and its resistance RB is drastically reduced; the decrease in BB1
causes Vx to drop.
 This drop, in turn, causes the diode to become more forward-biased and IE

116. OFF State of the UJT Circuit:
 When a voltage VBB is applied across the two base terminals B1 and
B2, the potential of point p with respect to B1 is given by:
VP =[VBB/ (RB1 +RB2)]*RB1=η*RB1,
η is called the intrinsic stand off ratio with its typical value lying between
0.5 and 0.8.
 The VEE source is applied to the emitter which is the p-side. Thus, the
emitter diode will be reverse-biased as long as VEE is less than Vx.
This is OFF state and is shown on the VE - IE curve as being a very
low current region.
 In the OFF the UJT has a very high resistance between E and B1, and
IE is usually a negligible reverse leakage current. With no IE, the drop
across RE is zero and the emitter voltage equals the source voltage.

117. UJT Ratings:
 Maximum peak emitter current : This represents the maximum allowable value of a pulse
of emitter current.
 Maximum reverse emitter voltage :This is the maxi mum reverse-bias that the emitter
base junction B2 can tolerate before breakdown occurs.
 Maximum inter base voltage :This limit is caused by the maxi mum power that the n-type
base bar can safely dissipate.
 Emitter leakage current :This is the emitter current which flows when VE is less than Vp
and the UJT is in the OFF state.
 The UJT is very popular today mainly due to its high switching speed.
 A few select applications of the UJT are as follows:
(i) It is used to trigger SCRs and TRIACs
(ii) It is used in non-sinusoidal oscillators
(iii) It is used in phase control and timing circuits
(iv) It is used in saw tooth generators
(v) It is used in oscillator circuit design
Applications:

118. INSULATED-GATE BIPOLAR TRANSISTOR
(IGBT):

119.  The insulated-gate bipolar transistor is a recent model of a power-switching
device that combines the advantages of a power BJT and a power MOSFET.
 Both power MOSFET and IGBT are the continuously controllable voltage-
controlled switch.
 Constructional Features:
 The structure of an IGBT cell is shown in Fig. 8-19.
 The p region acts as a substrate which forms the anode region, i.e., the
collector region of the IGBT. Then there is a buffer layer of n region and a
bipolar-base drift region.
 The p-region contains two n regions and acts as a MOSFET source. An
inversion layer can be formed by applying proper gate voltage.
 The cathode, i.e., the IGBT emitter is formed on the n source region.
INSULATED-GATE BIPOLAR TRANSISTOR
(IGBT):

120. Physical Operation:
 The principle behind the operation of an
IGBT is similar to that of a power MOSFET.
 The IGBT operates in two modes:
(i) The blocking or non-conducting
mode
(ii) The ON or conducting mode.
 The circuit symbol for the IGBT is shown in
Fig. 8-20.
 It is similar to the symbol for an n–p–n
bipolar-junction power transistor with the

121. REAL-LIFE APPLICATIONS:
 The IGBT is mostly used in high-speed switching devices. They have
switching speeds greater than those of bipolar power transistors.
 The turn-on time is nearly the same as in the case of a power
MOSFET, but the turn-off time is longer.
 Thus, the maximum converter switching frequency of the IGBT is
intermediate between that of a bipolar power transistor and a power
MOSFET.

122. POINTS TO REMEMBER:
 1. A thyristor is a multilayer p–n terminal electronic device used for bi-stable
switching.
 2. The SCR has two states:
(a) High-current low-impedance ON state
(b) Low-current OFF state
 3. Latching current is defined as a minimum value of anode current which is a
must in order to attain the turn-on process required to maintain conduction
when the gate signal is removed.
 4. Holding current is defined as a minimum value of anode current below which
it must fall for turning off the thyristor..
 5. The TRIAC is a bidirectional thyristor with three terminals. It is used
extensively for the control of power in ac circuits.

123.  7. Applications of the UJT:
(a) As trigger mechanism in the SCR and the TRIAC
(b) As non-sinusoidal oscillators
(c) In saw-tooth generators
(d) In phase control and timing circuits
 8. The UJT operation can be stated as follows:
(a) When the emitter diode is reverse-biased, only a very small emitter
current flows. Under this condition RB1 is at its normal high-value. This is the OFF
state of the UJT.
(b) When the emitter diode becomes forward-biased RB1 drops to a very low
value so that the total resistance between E and B1 becomes very low, allowing
emitter current to flow readily. This is the ON state.
 9. The IGBT is mostly used in high-speed switching Devices.
POINTS TO REMEMBER:

124. References
1. David A. Bell ,”Electronic Devices and Circuits”, Prentice Hall of India,.
2. www.ee.ic.ac.uk/fobelets/EE2BJT_1_Q.ppt
3. www.ohio.edu/people/starzykj/network/Class/.../Lecture11%20BJT%20Transistor.ppt
4. https://www.calvin.edu/~pribeiro/courses/engr311/Lecture%20Notes/Chap5.ppt
5. http://www.electronics-tutorials.ws/transistor/tran_1.html
6. http://www.electronics-tutorials.ws/transistor/tran_5.html
7. http://www.electronics-tutorials.ws/transistor/tran_6.html
8. http://www.electronic-circuits-diagrams.com/powerful-am-transmitter-circuit/
9. http://www.circuitstoday.com/single-transistor-radio
10. http://www.radio-electronics.com/info/data/semicond/fet-field-effect-transistor/fet-overview-
types.php
11. http://www.electronics-tutorials.ws/power/unijunction-transistor.html
12. users.prf.jcu.cz/klee/UAI609/documentation/transistor%20biasing.ppt
13. wps.pearsoned.com/wps/media/objects/11427/11702257/Chapter%2B8.ppt
124 Dr Gnanasekaran Thangavel 8/2/2017

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Dr Gnanasekaran Thangavel 8/2/2017