Field Effect Transistor
FET stands for Field Effect Transistor
It is a voltage-controlled device
Controls current using an electric field
Has high input impedance
Types of FET:
JFET
MOSFET
MOSFET
MOSFET = Metal Oxide Semiconductor Field Effect Transistor
Most commonly used transistor
Used in digital and analog circuits
Operates by controlling current with electric field
MOSFET Structure
Four terminals:
 Gate (G)
 Drain (D)
 Source (S)
 Substrate (Body)
Gate is separated by oxide layer (SiO₂)
Provides insulation between gate and
channel
 Substrate: Base material (P-type or N-type)
 Source & Drain: Heavily doped regions
 Gate: Metal or polysilicon
 Oxide layer: Insulating layer
Types of MOSFET
Two main types:
1. Enhancement Mode MOSFET
2. Depletion Mode MOSFET
Each type includes:
 N-channel MOSFET
 P-channel MOSFET
Enhancement Mode MOSFET
 Normally OFF at Vgs = 0
 Requires gate voltage to
turn ON
 Most widely used type
 Forms channel only
when voltage is applied
Depletion Mode
MOSFET
 Normally ON at Vgs = 0
 Can be turned OFF by
applying voltage
 Has pre-existing channel
N-Channel vs P-Channel MOSFET
N-Channel:
Uses electrons
Faster switching
P-Channel:
Uses holes
Slower than N-channel
Physical Operation of MOSFET
Gate voltage controls channel formation
Electric field attracts charge carriers
Conductive path forms between source and drain
Working of N-Channel MOSFET
 At Vgs = 0 → No conduction (Enhancement type)
Apply positive Vgs:
 Electrons accumulate
 Channel forms
Current flows from drain to source
Working of P-Channel MOSFET
 Requires negative gate voltage
 Holes form conductive channel
 Current flows in opposite direction