The document describes the structure and operation of a metal-oxide-semiconductor field-effect transistor (MOSFET). It details the three main components: the gate, source, and drain electrodes separated by a thin gate oxide layer. Depending on the gate voltage relative to the threshold voltage, the MOSFET can be in one of three operating modes - cutoff, linear, or saturation - determining whether current flows between the source and drain. Enhancement mode MOSFETs require a gate voltage to turn on, functioning like a normally open switch, while depletion mode MOSFETs require a gate voltage to turn off, functioning like a normally closed switch.

1. Metal Oxide Semiconductor
Field Effect Transistor

2. MOSFET STRUCTURE
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
3-D view of NMOS Transistor

3. CROSS-SECTION of NMOS Transistor
Gate Oxide
Gate
Source
Polysilicon
n+
Drain
n+
p-substrate
Bulk Contact
Field-Oxide
(SiO 2 )
p+ stopper

4. MODES OF OPERATION
For an enhancement-mode, n-channel MOSFET, the three operational modes are:
Cutoff, sub-threshold or weak-inversion region
When VGS < Vth :
 Basic threshold model ,no conduction between drain and source
 Boltzmann distribution ,energetic electrons at the source to enter the
channel and flow to the drain
 This results sub-threshold current
 Current between drain and source should ideally be zero when it is turned-off,
there is a weak-inversion region

5. Triode or linear region (also known as the ohmic region)
When VGS > Vth and VDS < ( VGS – Vth ) :
 The transistor is turned on, a channel has been created which allows current to
flow between the drain and the source
 The MOSFET operates like a resistor, controlled by the gate voltage relative to
both the source and drain voltages
 The current from drain to source is modeled as:

6. Cross section of a MOSFET operating in the linear (Ohmic) region
strong inversion region present even near drain

7. Saturation or active region
When VGS > Vth and VDS ≥ ( VGS – Vth )
 Since the drain voltage is higher than the source voltage, the electrons spread
out and conduction occurs
 This region is also known as pinch-off ,indicates the lack of channel region
near the drain.
 The drain current is now weakly dependent upon drain voltage and primarily
controlled by the gate–source voltage, and modeled approximately as:

8. Cross section of a MOSFET operating in the saturation (active) region
channel exhibits pinch-off near drain

9. For a Depletion-mode MOSFETs are doped so that a channel exists even with
zero voltage from gate to source
 To control the channel, a negative voltage is applied to the
gate(depleting the channel)
 Which reduces the current flow through the device

10. MOSFET Summary
1. Enhancement Type - the transistor requires a Gate-Source voltage, ( VGS ) to
switch the device "ON". The enhancement mode MOSFET is equivalent to a
"Normally Open" switch
2. Depletion Type - the transistor requires the Gate-Source voltage, ( VGS ) to
switch the device "OFF". The depletion mode MOSFET is equivalent to a
"Normally Closed" switch.
.
MOSFET type
VGS = +ve
VGS = 0
VGS = -ve
N-Channel Depletion
ON
ON
OFF
N-Channel Enhancement
ON
OFF
OFF
P-Channel Depletion
OFF
ON
ON
P-Channel Enhancement
OFF
OFF
ON

11. Symbols and basic construction for both configurations of MOSFET are shown below