2. JFETs_MOSFETS BJT ANALOG ELECTRONICS Engineering

Analog electronics notes
bjt mosfet

1. FET is a three terminal device used for applications similar to BJTs.
However , there are important differences between the two
FETs
BJT transistor is a current-controlled device as
depicted , while the JFET transistor is a voltage-
controlled device as shown

2. Similarities in Applications:
-Amplifiers
-Oscillators
-Switching devices
-Impedance matching circuits
Differences:
-FETs are unipolar devices where as BJTs are bipolar
-FETs are voltage controlled devices. BJTs are current controlled
devices.
-FETs have a higher input impedance (useful in buffer). BJTs have higher gains.
-FETs are less sensitive to temperature variations and are more easily
integrated on ICs. (Compact size)
-FETs are generally more static sensitive than BJTs.
-FETs are less noisy than bipolar devices as they have only one type of charge
carriers
FETs vs. BJTs

3. •JFET: Junction FET
•MOSFET: Metal–Oxide–Semiconductor FET
▪D-MOSFET:Depletion MOSFET
▪E-MOSFET:Enhancement MOSFET
FET Types

4. JFET Construction
There are two types of JFETs
•n-channel
•p-channel
The n-channel is more widely used.
There are three terminals:
•Drain (D) and Source(S) are connected to the n-channel
•Gate(G) is connected to the p-type material

5. JFET Operating Characteristics
There are three basic operating conditions for a JFET:
•V
GS= 0, V
DSincreasing to some positive value
•V
GS< 0, V
DSat some positive value
•Voltage-controlled resistor

6. JFET Operating Characteristics: V
GS= 0 V
•The depletion region between p-gate
and n-channel increases as electrons
from n-channel combine with holes
from p-gate.
•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 (I
D) from
source to drain through the n-channel
is increasing. This is because V
DSis
increasing.
Three things happen when V
GS= 0 and V
DSis increased from 0 to a more positive
voltage

7. If V
GS= 0 and V
DSis further increased to a
more positive voltage, then the depletion
zone gets so large that it pinches offthe n-
channel.
This suggests that the current in the n-
channel (I
D) would drop to 0A, but it does
just the opposite–as V
DSincreases, so does
I
D.
JFET Operating Characteristics: Pinch Off

8. At the pinch-off point:
•Any further increase in V
DSdoes not
produce any increase in I
D. V
DSat
pinch-off is denoted asV
p.
•I
Dis at saturation or maximum. It is
referred to as I
DSS.
•The ohmic value of the channel is
maximum.
JFET Operating Characteristics: Saturation

9. JFET Operating Characteristics
As V
GSbecomes more negative, the
depletion region increases.

10. As V
GSbecomes more negative:
•The JFET experiences pinch-
off at a lower voltage (V
P).
•I
Ddecreases (I
D< I
DSS) even
though V
DSis increased.
•Eventually I
Dreaches 0 A. V
GS
at this point is called V
por
V
GS(off)..
JFET Operating Characteristics
Also note that at high levels of V
DSthe JFET reaches a breakdown situation. I
D
increases uncontrollably if V
DS> V
DSmax.

11. 2
P
GS
o
d
V
V
1
r
r








−
= The region to the left of the
pinch-off point is called the
ohmic region.
The JFET can be used as a
variable resistor, where V
GS
controls the drain-source
resistance (r
d). As V
GSbecomes
more negative, the resistance
(r
d)increases.
JFET Operating Characteristics:
Voltage-Controlled Resistor

12. p-Channel JFETS
The p-channel JFET behaves the
same as the n-channel JFET, except
the voltage polarities and current
directions are reversed.

13. p-Channel JFET Characteristics
Also note that at high levels of V
DSthe JFET reaches a breakdown situation: I
Dincreases
uncontrollably if V
DS> V
DSmax.
As V
GSincreases more positively
•The depletion zone
increases
•I
Ddecreases (I
D< I
DSS)
•Eventually I
D= 0 A

14. N-Channel JFET Symbol

15. 2
V
V
1
DSSD
P
GS
II










−= The transfer characteristic of input-to-output is not as straightforward in a
JFET as it is in a BJT.
In a BJT, indicates the relationship between I
B(input) and I
C(output).
In a JFET, the relationship of V
GS(input) and I
D(output) is a little more
complicated:
JFETTransfer Characteristics

16. JFET Transfer Curve
This graph shows the
value of I
Dfor a given
value of V
GS.
16

17. Using I
DSSand Vp (V
GS(off)) values found in a specification sheet, the transfer
curve can be plotted according to these three steps:
Solving for V
GS= 0V I
D = I
DSS2
P
GS
DSSD
V
V
1II








−=
Step 1
Solving for V
GS= V
p(V
GS(off)) I
D= 0A2
P
GS
DSSD
V
V
1II








−=
Step 2
Solving for V
GS= 0V to V
p 2
P
GS
DSSD
V
V
1II








−=
Step 3
Plotting the JFET Transfer Curve
17

18. MOSFETs
There are two types of MOSFETs:
•Depletion-Type
•Enhancement-Type
MOSFETs have characteristics similar to JFETs and additional
characteristics that make then very useful.
18

19. 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. Thisn-channel is connected to
the Gate (G) via a thin insulating layer
of SiO
2.
The n-doped material lies on a p-doped
substrate that may have an additional
terminal connection called Substrate
(SS).
19

21. Reduction in free carriers in a channel due to a
negative potential at the gate terminal

22. Basic MOSFET Operation
A depletion-type MOSFET can operate in two modes:
•Depletion mode
•Enhancement mode
22

23. D-Type MOSFET in Depletion Mode
•When V
GS= 0 V, I
D= I
DSS
•When V
GS < 0 V, I
D< I
DSS
•The formula used to plot the transfer
curve still applies:
Depletion Mode
The characteristics are similar
to a JFET.2
P
GS
DSSD
V
V
1II








−=
23

24. D-Type MOSFET in Enhancement Mode
•V
GS> 0 V
•I
Dincreases above I
DSS
•The formula used to plot
the transfer curve still
applies:2
P
GS
DSSD
V
V
1II








−=
Enhancement Mode
Note that V
GSis now a positive polarity
24

25. p-Channel D-Type MOSFET
25

26. D-Type MOSFET Symbols
26

27. E-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
•The Gate(G) connects to the p-doped
substrate via a thin insulating layer of
SiO
2
•There is no channel
•The n-doped material lies on a p-doped
substrate that may have an additional
terminal connection called the
Substrate(SS)
27

28. Channel formation in the n-channel
enhancement-type MOSFET

29. Change in channel and depletion region with
increasing level of V DS for a fixed value of V
GS .

30. Basic Operation of the E-Type MOSFET
•V
GSis always positive
•As V
GSincreases, I
D
increases
•As V
GSis kept constant
and V
DSis increased,
then I
Dsaturates (I
DSS)
and the saturation level,
V
DSsatis reached
The enhancement-type MOSFET operates only in the enhancement mode.
30

31. E-Type MOSFET Transfer Curve
To determine I
Dgiven V
GS:
Where:
V
T= threshold voltage
or voltage at which the
MOSFET turns on2
TGSD
)VV(kI −=
k, a constant, can be determined by using
values at a specific point and the formula:2
T
GS(ON)
D(ON)
)V(V
I
k
−
=
V
DSsatcan be calculated by:TGSDsat
VVV −=
31

32. p-Channel E-Type MOSFETs
The p-channel enhancement-type MOSFET is similar to the n-
channel, except that the voltage polarities and current directions
are reversed.
32

33. MOSFET Symbols
33

34. Advantages
•Useful in logic circuit designs
•Higher input impedance
•Faster switching speeds
•Lower operating power levels
CMOS Devices
CMOS (complementary
MOSFET) uses a p-channel
and n-channel MOSFET;
often on the same substrate as
shown here.
34

35. Summary Table
35