The document discusses the fundamentals of MOSFETs (metal-oxide-semiconductor field-effect transistors). It describes the structure of an n-channel MOSFET, including the gate, body, source, and drain. It explains the different operating modes - depletion, enhancement - for n-channel and p-channel MOSFETs depending on the applied gate-source voltage. Finally, it notes that MOSFETs are generally considered three-terminal devices due to the body effect, which influences the threshold voltage.

1. MOSFET
Fundamentals
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
5/31/2021 1
Arpan Deyasi, RCCIIT, India

2. 5/31/2021 Arpan Deyasi, RCCIIT, India 2
Al-SiO2-Si(p) under different biasing conditions
Ec
EV
EFP
Ei
Al SiO2 Si
VM>0 & small
Depletion
VM>0 & large
Al SiO2 Si
1
2
point of
inversion
3
Inversion
ΨF
ΨS

3. 5/31/2021 3
Arpan Deyasi, RCCIIT, India
ΨF
ΨS
p
n
Conductor
Insulator
VM>0 & large

4. 5/31/2021 4
Arpan Deyasi, RCCIIT, India
Semiconductor (p-substrate)
Insulator
Conductor
n+ n+
n region (formed due
to inversion)

5. 5/31/2021 5
Arpan Deyasi, RCCIIT, India
Structural review of MOSFET
MOSFET is a 4-terminal device
Gate (conducting layer at top)
Body (semiconducting substrate at bottom)
Source (from where carriers start to flow)
Drain (where carriers are collected)
Body
Channel
Insulator
Gate
S D
n-channel MOSFET / n-MOSFET

6. 5/31/2021 6
Arpan Deyasi, RCCIIT, India
p-MOSFET n-MOSFET
Body
Channel
Insulator
Gate
S D
Body
Channel
Insulator
Gate
D
S

7. 5/31/2021 7
Arpan Deyasi, RCCIIT, India
Different Modes of Operation
MOSFET
type
Operating
mode
VGS
positive
VGS = 0 volt VGS
negative
n-channel
Depletion ON ON OFF
Enhancement ON OFF OFF
p-channel
Depletion OFF ON ON
Enhancement OFF OFF ON

8. 5/31/2021 8
Arpan Deyasi, RCCIIT, India
Symbols of MOSFET under Different Modes
p-MOSFET
n-MOSFET

9. 5/31/2021 9
Arpan Deyasi, RCCIIT, India
Why MOSFET is generally considered
a three-terminal device?
Ans: Body Effect

10. 5/31/2021 Arpan Deyasi, RCCIIT, India 10
Body
Effect
in
n-channel
MOSFET
Body
Channel
Insulator
Gate
S
(n+)
D
(n+)
(p)
(p)

11. 5/31/2021 Arpan Deyasi, RCCIIT, India 11
Body
Effect
in
p-channel
MOSFET
Body
Channel
Insulator
Gate
D
(p+)
S
(p+)
(n)
(n)