Unit 1.2 Introduction1

7/27/2019 Unit 1.2 Introduction1

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MOS TRANSISTORTHEORY

UNIT -1

Chapter -2

(Neil weste p:- 41- 91)


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Introduction

A MOS transistor is a majority-carrier device,in which the current in a conducting channel

between the source and the drain is

modulated by a voltage applied to the gate.


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NMOS (n-type MOS transistor)

1) Majority carrier = electrons

2) A positive voltage applied on the gate with respect tothe substrate enhances the number of electrons in the

channel and hence increases the conductivity of the

channel.

3) If gate voltage is less than a threshold voltage Vt , thechannel is cut-off (very low current between source &

drain).

PMOS (p-type MOS transistor)

1) Majority carrier = holes2) Applied voltage is negative

with respect to substrate.


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Relationship between Vgs andIds, for a fixed Vds:

n-channel enhancement

n-channel depletionp-channel enhancement

n-channel depletion

Ids

Ids

Ids Ids

VgsVgs

VgsVgs

+ Vt

+ Vt

- Vt

- Vt


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Devices that are normally cut-off with zero

gate bias are classified as

"enhancementmode"devices.

Devices that conduct with zero gate bias are

called "depletion-mode"devices.

Enhancement-mode devices are more

popular in practical use.


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nMOS EnhancementTransistor

At Vds=+V, Vgs=0V, no current flows

from source to drain because they are

insulated by two reverse biased pn

junction

1. Accumulation mode (Vgs Vt)


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The factors that influence the level of drain current

Ids (b/w S and D)

Distance b/w S and D

Channel width Threshold voltage Vt

Thickness of SiO2

Dielectric constant of insulator

Carrier mobility


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Threshold voltage (Vt):

The voltage at which an MOS device begins to

conduct ("turn on"). The threshold voltage is a

function of

(1) Gate conductor material

(2) Gate insulator material

(3) Gate insulator thickness

(4) Impurity at the silicon-insulator interface

(5) Voltage between the source and the substrate Vsb

(6) Temperature


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Threshold voltage (Vt) Equations


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Threshold voltage (Vt)equation

Vt ,can be expressed as

Where Vt-mos is the ideal thresholdvoltage of an ideal transistor and Vfb istermed as flat-band voltage.


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Body Effect

Effect due to series connection oftransistors

Increases if source voltage increasesbecause source is connected to thechannel

Increase in Vt with Vs is called the bodyeffect


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( )2

cutoff

linear

saturatio

0

2

2n

gs t

dsds gs t ds ds dsat

gs t ds dsat

V V

VI V V V V V

V V V V

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Unit 1.2 Introduction1