Field Effect Transistor is a transistor that is voltage controlled devices. It has higher input impedance and less sensitive to temperature variations.
7. CONSTRUCTION AND CHARACTERISTICS OF
JFET
๏ Is a three-terminal device with one terminal capable
of controlling the current between the other two.
3 terminals are:
1. DRAIN (D)
2. SOURCE (S) โ connected to n-channel
3. GATE (G) โ connected to p-channel
8. Two types of JFET
1.n-channel
2.p-channel
Note:
๏ n-channel is more widely used.
Drain
Gate
Source
Drain
Gate
Source
13. ๏JFET is always operated with the gate-source PN
junction reversed biased.
๏Reverse biasing of the gate source junction with the
negative voltage produces a depletion region along the
PN junction which extends into the n-channel and thus
increases its resistance by restricting the channel width
as shown in the preceding figure.
14. ๐ฝ ๐ฎ๐บ = ๐, ๐ฝ ๐ซ๐บ SOME POSITIVE VALUE
When ๐ ๐๐ = ๐ ๐๐ง๐ ๐ ๐๐ is increased from
0 to a more positive voltage.
๏ The depletion region between p-gate
and n-channel increases
๏ Increasing the depletion region,
decreases the size of the n-channel
which increases the resistance of the
n-channel.
๏ Even though the n-channel resistance
is increasing, the current (ID) from
source to drain through the n-channel
is increasing. This is because VDS is
increasing.
Recall from DIODE discussion:
- The greater the applied reverse bias, the wider is the depletion region.
IG = 0
15.
16. REGIONS OF JFET ACTION
1. Ohmic Region โ linear region
๏ JFET behaves like an ordinary resistor
2. Pinch Off Region
๏ Saturation or Amplifier Region
๏ JFET operates as a constant current device because Id
is relatively independent of Vds
๏ Idss โ drain current with gate shorted to source.
3. Breakdown Region
๏ If Vds is increased beyond its value corresponding to Va
โ avalanche breakdown voltage.
๏ JFET enters the breakdown region where Id increases to
an excessive value.
17. 4. Cut Off Region
๏As Vgs is made more and more negative, the gate
reverse bias increases which increases the thickness
of the depletion region.
๏As negative value of Vgs is increased, a stage comes
when the 2 depletion regions touch each other.
Vgs (off) = -Vp
/Vp/ = /Vgsoff/
18.
19. JFET OPERATING CHARACTERISTICS: PINCH OFF
๏ If VGS = 0 and VDS is further
increased to a more positive
voltage, then the depletion zone
gets so large that it pinches off the
n-channel.
๏ As VDS is increased beyond |VP |,
the level of ID remains the same
(ID= IDSS)
20. ๐ฝ ๐ฎ๐บ โฅ ๐
Voltage from gate to source is controlling
voltage of the JFET.
๏ As ๐ ๐๐ becomes more negative, the
depletion region increases.
๏ The more negative ๐ ๐๐, the
resulting level for ๐ ๐ is reduced.
๏ Eventually, when ๐ ๐๐ = ๐๐ฉ [Vp = VGS
(off)], ID is 0 mA. (the device is
โturned offโ.
22. JFET OPERATING CHARACTERISTICS: VOLTAGE-
CONTROLLED RESISTOR
The region to the left of the
pinch-off point is called the
ohmic region/Voltage controlled
resistance region.
The JFET can be used as a variable resistor, where VGS controls the drain-
source resistance (rd). As VGS becomes more negative, the resistance (rd)
increases
23. where ro is the resistance with VGS=0 and rd is the
resistance at a particular level of VGS.
24. 1. For an n-channel JFET with ๐๐ = 10๐ฮฉ ๐๐บ๐ =
FOR EXAMPLE:
๐๐ =
10๐ฮฉ
(1 โ
โ3
โ6
)2
๐๐ = 40๐ฮฉ
25. P-CHANNEL JFETS
The p-channel JFET
behaves the same as the
n-channel JFET, except the
voltage polarities and
current directions are
reversed.
26. P-CHANNEL JFET CHARACTERISTICS
Also note that at high levels of VDS
the JFET reaches a breakdown
situation: ID increases uncontrollably
if VDS > VDSmax
28. SUMMARY:
Important parameters to remember:
VGS = 0V, ID = IDSS
Cutoff (๐ผ ๐ท = 0๐ด)
๐๐บ๐ less than the pinch
off level
29. ๐ผ ๐ท is between 0 A and ๐ผ ๐ท๐๐ ๐๐๐ ๐๐บ๐ โค
0๐ ๐๐๐ ๐๐๐๐๐ก๐๐ ๐กโ๐๐ ๐กโ๐ ๐๐๐๐โ ๐๐๐ ๐๐๐ฃ๐๐.
30. JFET TRANSFER CHARACTERISTICS
In a BJT, ๐ฝ indicates the relationship between ๐ผ ๐ต (input) and ๐ผ ๐ถ (output).
๐ผ ๐ถ = ๐ ๐ผ ๐ต = ๐ฝ๐ผ ๐
Constant
Control variable
In a JFET, the relationship of ๐๐บ๐ (input) and ๐ผ ๐ท (output) is defined by
Shockleyโs Equation
๐ผ ๐ท = ๐ผ ๐ท๐๐ 1 โ
๐๐บ๐
๐๐
2
Constant
Control variable
32. This graph shows the value of ๐ฐ ๐ซ for a given value of ๐ฝ ๐ฎ๐บ.
33. PLOTTING THE JFET TRANSFER CURVE
Using ๐ผ ๐ท๐๐ and ๐๐ (๐๐บ๐(๐๐๐)) values found in a specification sheet, the transfer
curve can be plotted according to these three steps:
Step 1:
Solving for ๐๐บ๐ = 0, ๐ผ ๐ท = ๐ผ ๐ท๐๐ 1 โ
๐๐บ๐
๐๐
2
, ID = IDSS
Step 2:
Solving for ๐๐บ๐ = ๐๐(๐๐บ๐ ๐๐๐ ), ๐ผ ๐ท = ๐ผ ๐ท๐๐ 1 โ
๐๐บ๐
๐๐
2
, ID = 0 A
Step 3:
Solving for ๐ผ ๐ท if we substitute ๐๐บ๐ = โ1 ๐ , ๐ผ ๐ท๐๐ = 8mA and ๐๐ =-4
๐ผ ๐ท = ๐ผ ๐ท๐๐ 1 โ
๐๐บ๐
๐๐
2
, ๐ผ ๐ท = 8๐๐ด 1 โ
(โ1)
(โ4)
2
,
ID = 4.5mA
38. THERE ARE TWO TYPES OF MOSFETS:
๏ Depletion-Type
๏ Enhancement-Type
39. DEPLETION-TYPE MOSFET CONSTRUCTION
๏ The Drain (D) and Source (S)
connect to the to n-doped regions.
๏ These n-doped regions are
connected via an n-channel.
๏ This n-channel is connected to the
Gate (G) via a thin insulating layer
of SiO2.
๏ The n-doped material lies on a p-
doped substrate that may have an
additional terminal connection
called Substrate (SS).
n-Channel depletion-type
MOSFET
Dielectric
insulator
40. SILICON DIOXIDE:
๏ Insulator refer to as DIELECTRIC.
๏ It sets up opposing electric field within the dielectric
when exposed to an externally applied field.
๏ The fact that SiO2 layer is an insulating layer
means that:
There is no direct electrical connection between the
gate terminal and the channel of a MOSFET.
It is the insulating layer of SiO2 in the MOSFET
construction that accounts for the very desirable high
input impedance of the device
41. WHY MOSFET?
๏ Metal:
๏ถ For the drain, source, and gate connection for the proper
surface โ in particular, the gate terminal and the control to be
offered by the surface area of the contact.
๏ Oxide:
๏ถ For the Silicon dioxide insulating layer.
๏ Semiconductor:
๏ถ For the basic structure on which the n- and p-type region are
DIFFUSED.
MOSFET is also called
INSULATED GATE-FET or IGFET
42. DEPLETION-TYPE MOSFET :BASIC OPERATION
AND CHARACTERISTICS
๏ผ VGS = 0 and VDS is applied
across the drain to source
terminals.
๏ผ This results to attraction of
free electrons of the n-
channel to the drain, and
hence current flows.
43. Continuationโฆ.
๏ ๐๐บ๐ is set at a negative
voltage such as -1 V
๏ The negative potential at the
gate pressure electrons
toward the p-type substrate
and attract the holes for the
p-type substrate.
๏ This will reduce the number
of free electrons in the n-
channel available for
conduction.
๏ The more negative the ๐๐บ๐ ,
the resulting level of drain
current ๐ผ ๐ท is reduced.
๏ When ๐๐บ๐ is reduced to ๐๐
(pinch off voltage), then ๐ผ ๐ท =
0๐๐ด.
44. When ๐๐บ๐ is reduced to ๐๐ (pinch off) {i.e ๐๐ = โ6๐} then ๐ผ ๐ท = 0๐๐ด.
For positive values of ๐๐บ๐, the positive gate will draw additional
electrons (free carriers from the p-type substarte and hence ๐ผ ๐ท
increases.)
50. ENHANCEMENT-TYPE MOSFET CONSTRUCTION
๏ The Drain (D) and Source (S)
connect to the to n-doped
regions.
๏ The Gate (G) connects to the
p-doped substrate via a thin
insulating layer of SiO2
๏ There is no channel
๏ The n-doped material lies on
a p-doped substrate that may
have an additional terminal
connection
51. ๏ For ๐๐บ๐ = 0, ๐ผ ๐ท = 0(no channel)
๏ For ๐๐ท๐ some positive voltage
and ๐๐บ๐ = 0, two reversed biased
n-junctions and no significant
flow between drain and source.
๏ For ๐๐บ๐ > 0 and ๐๐บ๐ > 0, the
positive voltage at gate pressure
holes to enter deeper regions of
the p-substrate, and the electrons
in p-substrate and the electrons
in p-substrate will be attracted to
the positive gate.
๏ The level of ๐๐บ๐ that results in the
significant increase in drain
current in called:
THRESHOLD VOLTAGE (Vt)
๏ For ๐๐บ๐ < ๐๐, ๐ผ ๐ท = 0๐๐
52. BASIC OPERATION OF THE E-TYPE MOSFETNote:
The enhancement-type
MOSFET operates only in the
enhancement mode
๏ ๐๐บ๐ is always positive
๏ As ๐๐บ๐ increases, ๐ผ ๐ท increases
๏ As ๐๐บ๐ is kept constant and ๐๐ท๐ is
increased, then ๐ผ ๐ท saturates (๐ผ ๐ท๐๐)
and the saturation level, ๐๐ท๐๐ ๐๐ก is
reached.
๏ ๐๐ท๐๐ ๐๐ก can be calculated by
๐๐ท๐ ๐๐ก = ๐๐บ๐ โ ๐๐
53. E-TYPE MOSFET TRANSFER CURVE
To determine ๐ผ ๐ท given ๐๐บ๐:
Where,
๐๐ is the threshold voltage or voltage at which the MOSFET turns on.
๐, a constant, can be determined by using values at a specific point and the
formula:
54. FOR EXAMPLE:
Substituting ๐ผ ๐ท(๐๐) = 10๐๐ด ๐คโ๐๐ ๐๐บ๐(๐๐) = 8๐
The level of ๐๐ is 2V, as revealed by
the fact that the drain current has
dropped to 0 mA.
๐ =
10๐๐ด
(8๐ โ 2๐)2
= ๐. ๐๐๐๐๐๐โ๐ ๐จ
๐ฝ ๐
55. Note:
For values of ๐๐บ๐ less than the threshold level, the drain current of an
enhancement type MOSFET is 0 mA.
Substituting the ๐๐บ๐ from the general equation:
๐ผ ๐ท = ๐(๐๐บ๐ โ ๐๐)2
๐ = 0.278๐ฅ10โ3
๐ผ ๐ท = 0.278๐๐ด/๐2
(๐๐บ๐ โ 2๐)2
i.e substituting ๐๐บ๐ = 4๐, we find that
๐ผ ๐ท = 0.278๐๐ด/๐2(4๐ โ 2๐)2
๐ฐ ๐ซ = ๐. ๐๐๐๐จ
58. VMOS CHARACTERISTICS:
๏ Compared with commercially available planar
MOSFETs, VMOS FETs have reduced channel
resistance levels and higher current and power
ratings
๏ VMOS FETs have a positive temperature
coefficient that will combat the possibility of
thermal runaway
๏ The reduced charge storage levels result in faster
switching times for VMOS construction compared
to those for conventional planar construction
62. ASSIGNMENT:
1. In what ways is the construction of a depletion type
of MOSFET similar to that of a JFET? In what ways is
it different?
2. What is the significant difference between the
construction of an enhancement type MOSFET and a
depletion type MOSFET?
3. Sketch the transfer characteristics of a p-channel
enhancement type MOSFET if ๐๐ = โ5๐ and ๐ =
0.45๐ฅ10โ3 ๐ด ๐2.
63. SOURCE:
Electronic Devices and Circuit Theory, 10th Ed., R.
Boylestad & L. Nashelsky, Copyright ยฉ2009 by
PEARSON Education, Inc.