This document compares BJT, FET, and MOSFET transistors and describes how MOSFETs can be used as switches. It also explains the construction and operation of a CMOS inverter. A MOSFET can act as a switch by turning fully on or off depending on the input voltage. A CMOS inverter consists of a PMOS and NMOS device with common gates and drains. It outputs a high voltage when the input is low and vice versa. MOSFETs have advantages over BJTs and FETs like higher input impedance, lower output resistance, and faster switching speed.

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 MOSFET is
connected to the supply voltage VS via the
drain resistor RD while its source terminal (S) is
grounded.
Further, it has an input voltage Vi applied at its gate terminal (G)
while the output Vo is drawn from its drain.
MOSFET as a switch

3. Consider the case where Vi = 0V, which means the gate terminal of
the MOSFET is left unbiased. As a result, the MOSFET will be OFF
The output voltage Vo will become almost equal to VS
Next, consider the case where the input voltage Vi applied, the
MOSFET will start to conduct
This further means that the device will offer low resistance path
for the flow of constant IDS, almost acting like a short circuit.
As a result, the output voltage will be ideally zero.
Vi Device state Vo
0 Off Vs
1 On 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 Vdd and source in
NMOS is connected to ground

5. CMOS Inverter
• It is a combination of PMOS & NMOS
Vin Q1 Q2 VOUT
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 controlled Voltage controlled Voltage Controlled
Current flow Bipolar Unipolar Unipolar
Terminals Not interchangeable Interchangeable Interchangeable
Operational modes No modes D mode only Both 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 VDS
(Creates ID)
Positive (D is +ve & S is -
ve)
Positive (D is +ve & S is -
ve)
Positive (D is +ve & S is -
ve)
VGS
(Controls ID)
VGS zero
(short circuit G&S)
–ve potential
(G is -ve & S is +ve)
VGS zero
(short circuit G&S)
–ve potential
(G is -ve & S is +ve)
+ve potential
(G is +ve & S is -ve)
VGS +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