MOSFET as a switch

MOSFET

1. MOSFET as a switch, CMOS Inverter and
Comparison between BJT, FET and MOSFET

2. Figure shows a simple circuit which uses n-
channel E MOSFET as a switch.
Here the drain terminal (D) of the MOSFETis
connected to the supply voltage V
Svia the
drain resistor R
Dwhile its source terminal (S) is
grounded.
Further, it has an input voltage V
iapplied at its gate terminal (G)
while the output V
ois drawn from its drain.
MOSFET as a switch

3. Consider the case where V
i= 0V, which means the gate terminal of
the MOSFET is left unbiased. As a result, the MOSFET will be OFF
The output voltage V
owill become almost equal to V
S
Next, consider the case where the input voltage V
iapplied, the
MOSFET will start to conduct
This further means that the device will offer low resistance path
for the flow of constant I
DS, almost acting like a short circuit.
As a result, the output voltage will be ideally zero.
V
iDevice stateV
o
0Off V
s
1On 0

4. CMOS Inverter construction :
It is a combination of PMOS & NMOS
Gates are connected together & i/p is given
Both drains are connected together & o/p is taken
In PMOS, source is connected to supply voltage V
ddand source in
NMOS is connected to ground

5. CMOS Inverter
•It is a combination of PMOS & NMOS
Vin Q1 Q2 V
OUT
0 On Off 1 (Vdd)
1 Off On 0 (ground)
Vin Device
0 PMOS ON
1 NMOS ON

6. Comparison between BJT, FET and MOSFET
TERMS BJT FET MOSFET
Device type Current controlledVoltage controlledVoltage Controlled
Current flow Bipolar Unipolar Unipolar
Terminals Not interchangeableInterchangeableInterchangeable
Operational modes No modes D mode onlyBoth E and D modes
Input impedance Low High Very high
Output resistance Moderate Moderate Low
Operational speed Low Moderate High
Noise High Low Low
Thermal stability Low Better High

7. Parameters n-channel -JFET n-channel -D-MOSFET n-channel -E-MOSFET
Construction n-substrate is taken
p-material injected
p-substrate is taken
n-material injected
p-substrate is taken
n-material injected
Channel present at the time of
fabrication
present at the time of
fabrication
No channel at the time of
fabrication
Channel is induced
Working V
DS
(Creates I
D)
Positive (D is +ve & S is -
ve)
Positive (D is +ve & S is -
ve)
Positive (D is +ve & S is -
ve)
V
GS
(Controls I
D)
V
GS zero
(short circuit G&S)
–ve potential
(G is -ve & S is +ve)
V
GS zero
(short circuit G&S)
–ve potential
(G is -ve & S is +ve)
+ve potential
(G is +ve & S is -ve)
V
GS+ve potential
(G is +ve & S is -ve)
Drain curve
Comparison between FET devices

8. Parameters n-channel -JFET n-channel -D-
MOSFET
n-channel -E-
MOSFET
Transfer curve
Symbol
Name Voltage Controlled Device

9. Applications
JFET MOSFET
Phase Shift Oscillators
Current Limiter
Chopper
Analog Switch
Buffer Amplifier
Low Noise Amplifier
Switch
Amplifiers
Chopper
Linear Voltage Regulators