Regions and Operations of BJT and Mosfet

1. Regions of Operation of
BJT and MOSFET
Assoc Prof Chang Chip Hong email: echchang@ntu.edu.sg
EE2002 Analog Electronics

2. EE2002 Analog Electronics
Lesson Objectives
Regions of Operation of BJT and MOSFET 2
At the end of this lesson, you should be able to:
• Identify the symbols used to represent transistors in circuit schematics
• Identify the three different operation regions of BJT and MOSFET
• Describe the criteria for the different operation regions of BJT and MOSFET
• Discuss the i-v characteristics of MOSFET
• Analyse circuits used to bias BJT and MOSFET transistors into various
regions of operation

3. EE2002 Analog Electronics
NPN and PNP BJTs
Regions of Operation of BJT and MOSFET 3
n
p
n
Collector (C)
Emitter (E)
Base (B) B
C
E
p
n
p
Collector (C)
Emitter (E)
Base (B) B
C
E
npn
pnp
IB
-
VBC
+
+
VBE
-
Collector (C)
Emitter (E)
Base (B)
+
VCE
-
IC
IE
p
n
n
-
VCB
+
+
VEB
-
Collector (C)
Emitter (E)
Base (B)
+
VEC
-
IB
IC
IEp
p
n

4. EE2002 Analog Electronics
Operation Regions of BJT
Cutoff region
BEJ (npn) reverse biased
BCJ (npn) reverse biased
IC = 0
 Open Switch
Note: The junctions refer to EBJ and CBJ for pnp transistor.
Regions of Operation of BJT and MOSFET 4
E
-
VBC
+
+
VBE
-
C
B B
C
E
+
VB < VE
VB < VC
-
-
+
E
+
VEB
-
-
VCB
+
C
B B
C
E
+
VB > VC
VB > VE
-
-
+
n
p
n
p
p
n

5. EE2002 Analog Electronics Regions of Operation of BJT and MOSFET 5
Forward-active region
BEJ (npn) forward biased
BCJ (npn) reversed biased
VBE  0.7 V
IC =  IB = a IE
 Good amplifier
a =  /( + 1)
E
+
VEB
-
-
VCB
+
C
B B
C
E
-
VB > VC
VB < VE
+
-
+
E
-
VBC
+
+
VBE
-
C
B B
C
E
+
VB > VE
VB < VC
-
+
-
Note: The junctions refer to EBJ and CBJ for pnp transistor.
Operation Regions of BJT

6. EE2002 Analog Electronics
Saturation region
BEJ (npn) forward biased
BCJ (npn) forward biased
 Closed switch
VBE  0.7 V
VBC = 0.4~ 0.5 V
VCE(SAT) = 0.2~0.3 V
Regions of Operation of BJT and MOSFET 6
E
-
VBC
+
+
VBE
-
C
B B
C
E
-
VB >VE
VB > VC
+
+
-
E
+
VEB
-
-
VCB
+
C
B B
C
E
-
VB < VC
VB < VE
+
+
-
Note: The junctions refer to EBJ and CBJ for pnp transistor.
Operation Regions of BJT

7. EE2002 Analog Electronics
Reverse-active region
BEJ (npn) reverse biased
BCJ (npn) forward biased
 Weak amplifier
 Normally not use
Regions of Operation of BJT and MOSFET 7
B
C
E
-
VB < VE
VB > VC
+
-
+
E
-
VBC
+
+
VBE
-
C
B
E
+
VEB
-
-
VCB
+
C
B B
C
E
+
VB < VC
VB >VE
-
+
-
Note: The junctions refer to EBJ and CBJ for pnp transistor.
Operation Regions of BJT

8. EE2002 Analog Electronics
BJT Bias Analysis: Active Mode
BEJ is forward biased by 0.7 V
BCJ is reversed biased by 3 V
Q in active mode and can be used as linear amplifier.
Regions of Operation of BJT and MOSFET 8
3 2.3
0.7 V
BE B EV V V -
 -

3 6
3 V
BC B CV V V -
 -
 -
C
E
+
+
–
–
3 V
0.7 V
BVB
= 3 V
Q
VC = 6 V
VE = 2.3 V
VCC

9. EE2002 Analog Electronics
BEJ is forward biased by 0.3 V but
inadequate to turn on BEJ
BCJ is reversed biased by 3 V
Q is cutoff.
Regions of Operation of BJT and MOSFET 9
3 2.7
0.3 V
BE B EV V V -
 -

3 6
3 V
BC B CV V V -
 -
 -C
E
+
+
–
–
3 V
0.3 V
BVB
= 3 V
Q
VC = 6 V
VE = 2.7 V
VCC
BJT Bias Analysis: Cutoff Mode

10. EE2002 Analog Electronics
BEJ is forward biased by 0.8 V
BCJ is forward biased by 0.5 V
Q is in Saturation.
Regions of Operation of BJT and MOSFET 10
C
E
+
+
–
–
0.5 V
0.8 V
BVB
= 6 V
Q
VC = 5.5 V
VE = 5.2 V
VCC 6 5.2
0.8 V
BE B EV V V -
 -

6 5.5
0.5 V
BC B CV V V -
 -

0.5 0.8
0.3 V
CE CB BEV V V +
 - +

BJT Bias Analysis: Saturation Mode

11. EE2002 Analog Electronics
BJT Bias Analysis: Determine DC
Node Voltages and Branch Currents
Since  is very large,
Regions of Operation of BJT and MOSFET 11
6 0.7
5.3 V
E B BEV V V -
 -

5.3
3.3
1.6 mA
EI 

Assume active-mode
operation,
1 1.6 mAC EI Ia    
10 1.6 4.7
2.48 V
CV  - 

6 2.48
3.52 V
BC B CV V V -
 -

=> Wrong assumption! Q is in saturation mode.
10 V
4.7 kW
3
2
IE
 > 200
4VCn
n
p
VE 1
3.3 kW
6 V
+
-
IC

12. EE2002 Analog Electronics
=> In saturation, Ic   IB.
Regions of Operation of BJT and MOSFET 12
In saturation region,
VCE  0.2 to 0.3.
Assume VCE(SAT) = 0.2 V,
6 0.7
5.3 V
EV  -

5.3
3.3
1.6 mA
EI 

( )
5.3 0.2
5.5 V
C E CE SATV V V +
 +

10 5.5
4.7
0.96 mA
CI
-


1.6 0.96 0.64 mAB E CI I I -  - 
forced
0.96
1.5
0.64
C
B
I
I
   
and
10 V
4.7 kW
3
2
IE
 > 200
4
VCn
n
p
VE 1
3.3 kW
6 V
+
-
IC
(Cont.)
BJT Bias Analysis: Determine DC
Node Voltages and Branch Currents

13. EE2002 Analog Electronics Regions of Operation of BJT and MOSFET 13
nMOS Transistor Structure and
I-V Characteristics
0 V 2 V 4 V 6 V 8 V 10 V 12 V
1.0 mA
2.0 mA
3.0 mA
4.0 mA
Drain-source voltage, vDS
Draincurrent,iD
0 A
vGS  VTNCutoff, ID = 0
Triode
Region
vDS < vGS – VTN
or vGD > VTN
VGS = 4.5 V
VGS = 4.0 V
VGS = 3.5 V
VGS = 3.0 V
VGS = 2.5 V
VGS = 2.0 V
Saturation region
vDS  vGS – VTN or vGD  VTN
Pinch-off locus
vDS = vGS – VTN or vGD = VTN
D
S
G
vDS
+
-
iD
+
- vGS
VTN : Threshold Voltage of NMOS; VTN = 1 V for this graph
Metal
(or polysilicon) Silicon dioxide
(SiO2)
S
G
D

14. EE2002 Analog Electronics Regions of Operation of BJT and MOSFET 14
Region NMOS PMOS
Cutoff
VGS < VTN
ID = 0
|VGS| < |VTP|
ID = 0
Triode
VGS  VTN and VDS < VGS – VTN |VGS |  |VTP | and |VDS| < |VGS| – |VTP|
Saturation
VGS  VTN and VDS  VGS – VTN |VGS |  |VTP | and |VDS|  |VGS| – |VTP|
2
DS
D n GS TN DS
V
I K V V V
 
 - - 
  2
DS
D p GS TP DS
V
I K V V V
 
 - - 
 
 
2
2
n
D GS TN
K
I V V -  
2
2
p
D GS TP
K
I V V -
+
VDS
-VGS
+
-
G
D
S
-
VDS
+
VGS
+
-
G
S
D
VTN > 0 VTP < 0
MOSFET Biasing for Different
Regions of Operation

15. EE2002 Analog Electronics
MOSFET Bias Analysis: Triode Region
Kn = 250 mA/V2
VTN = 1 V
VGS = VDD = 4 V.
Assume transistor is saturated,
Saturation region
assumption is incorrect.
Regions of Operation of BJT and MOSFET 15
+
-
RD 1.6 kW
ID
+
VDS
-
IG = 0
VGS
+
-
4V VDD
 
2
2
2
250 μ
(4 1)
2
1.13 mA
n
D GS TN
K
I V V -
 -

VDD = ID RD + VDS
4 1.6 1.13
2.19 V
DSV  - 

But VDS = 2.19 < VGS - VTN = 3

16. EE2002 Analog Electronics
Using triode region equation,
VDS = 8.7 (infeasible) or 2.3 V (< VGS - VTN = 3)
Regions of Operation of BJT and MOSFET 16
Kn = 250 mA/V2
VTN = 1 V
+
-
RD 1.6 kW
ID
+
VDS
-
IG = 0
VGS
+
-
4V VDD
254 1 0 1
2
600 4D
DS
DS SVV
V
m  
- - 
 
 - 
Hence, VDS = 2.3 V and ID = 1.06 mA
2
DS
n G ND S T DS
V
K V VI V
 
- - 
 

2
0.2 2.2 4 0DS DSV V- + 
(Cont…)
MOSFET Bias Analysis: Triode Region

17. EE2002 Analog Electronics
MOSFET Bias Analysis:
nMOS Two-Resistor Biasing
Kn = 260 mA/V2
VTN = 1 V
+
VDS
-VGS
+
-
RD 10 kW
3.3 VID
IG = 0
2 MW
RG
+
-
VDD
Since IG = 0, VDS = VGS.
Transistor is saturated because VDS > VGS – 1
VDS = VDD – ID RD
Regions of Operation of BJT and MOSFET 17
 
2260 μ
3.3 1 10000
2
GS GSV V - - 
 
 
2
2
2
260 μ
1
2
n
D GS TN
GS
K
I V V
V
 -
 -

18. EE2002 Analog Electronics
VGS = –0.77 V implies MOSFET is cutoff and
contradicts the observation.
Regions of Operation of BJT and MOSFET 18
2
1.3 1.6 2 0GS GSV V- - 
2
1.6 1.6 4 1.3 ( 2)
2 1.3
0.77 V or 2 V
GSV
 -   -


 -
Kn = 260 mA/V2
VTN = 1 V
VGS = 2 V and VDS = VGS = 2 V.
(Cont.)
+
VDS
-VGS
+
-
RD 10 kW
3.3 VID
IG = 0
2 MW
RG
+
-
VDD
ID = 130 m ×(2 – 1)2 = 130 mA
MOSFET Bias Analysis:
nMOS Two-Resistor Biasing

19. EE2002 Analog Electronics Regions of Operation of BJT and MOSFET 19
Kp = 50 mA/V2
VTP = -2 V
Since IG = 0, VSG = VSD.
Transistor is saturated because |VDS | > |VGS| – |-2|
|VDS| = VDD – ID RD
 
 
32
2
6
15 220 10
15 5.5 2
25 10 2GS
GS
GSVV
V
-
 -  
 - -
 -
 
 
2
2
2
50 μ
2
2
GS
p
D GS TP
K
I V
V
V-
 -

RD 220 kW
15 V
+
VSD
-
VSG
+
-
470 kW
RG +
-
VDD
ID
IG = 0
MOSFET Bias Analysis:
pMOS Two-Resistor Biasing

20. EE2002 Analog Electronics Regions of Operation of BJT and MOSFET 20
|VGS| = 0.37 V < |VTP| = 2 V,
2
5.5 21 7 0GS GSV V- + 
2
21 21 4 5.5 7
2 5.5
0.37 V or 3.45 V
GSV
 -  



|VGS | = 3.45 V or VSG = 3.45 V.
(Cont.)
ID = 25 m ×(3.45 – 2)2 = 52.5 mA
Kp = 50 mA/V2
VTP = -2 V
RD 220 kW
15 V
+
VSD
-
VSG
+
-
470 kW
RG +
-
VDD
ID
IG = 0
VDS = VGS = -3.45 V.
MOSFET Bias Analysis:
pMOS Two-Resistor Biasing