Government Engineering College, Bharuch

VLSI Presentation

Basic MOSFET Structure

Prepared by:

Xyz

6th Sem E.C.(A)

Electronics and Comm. Dept.

MOSFET Structure

p-Si

n+L

Source Gate Drain

Field Oxide

Gate Oxide

Bulk (Substrate)

• Importance for LSI/VLSI

–Low fabrication cost

–Small size

–Low power consumption

• Applications

–Microprocessors

–Memories

–Power Devices

i. Basic Properties

i. Unipolar device

ii. Very high input impedance

iii. Capable of power gain

iv. Two possible device types:

enhancement mode; depletion mode

v. Two possible channel types: n-channel;

p-channel

Symbols

G

D

S

BG

D

S

B

p Channel MOSFET n Channel MOSFET

Current-Voltage Characteristic

A

B CDIDS

VDS

Channel Formation

p-Si

B

VG +VDS

n-Channel

S D

Increased VDS

p-Si

B

VG +VDS

n-Channel

S D

Triode Region A voltage-controlled resistor @small VDS

G

pn+n+

metal

S DB

oxide

+-

+++

+++

- - - -

VGS1>Vt

pn+n+

metal

S DB

oxide

+-

+++

++++++

- - - - - -

VGS2>VGS1

pn+n+

metal

S DB

oxide

+-

+++

++++++

- - - - - - - - -

VGS3>VGS2+++

ID

VDS

0.1 v

increasingVGS

Increasing VGS puts more charge in the channel, allowing more drain current to flow

cut-off

Saturation Regionoccurs at large VDS

p

n+n+

metal

source

S

gate

Gdrain

D

body

B

oxide

+-

+++

++++++

VDS large

As the drain voltage increases, the difference in voltage between the drain and the gate becomes smaller. At some point, the difference is too small to maintain the channel near the drain pinch-off

Simplified MOSFET I-V Equations

Cut-off: VGS< VT

ID = IS = 0Triode: VGS>VT and VDS < VGS-VT

ID = kn’(W/L)[(VGS-VT)VDS -

1/2VDS2]

Saturation: VGS>VT and VDS > VGS-VT

ID = 1/2kn’(W/L)(VGS-VT)2

where kn’= (electron mobility)x(gate capacitance)

= mn(eox/tox) …electron velocity = mnE

and VT depends on the doping concentration and gate material used (…more details later)

Thank You