METAL OXIDE SEMICONDUCTOR FIELD

EFFECT TRANSISTOR

Depletion MOSFET N- channel & P- channel Enhancement MOSFET N- channel & P- channel

Types

N-Channel Depletion MOSFET Structure

Working ( VGS = 0V & VDS > 0V)

Cont.

Drain & Transfer Characteristics of N-channel D-MOSFET

Working – Enhancement mode

JFET & D-MOSFET have the same transconductance equation

Cont.

D-MOSFET Schematic Symbols

P-channel D-MOSFET Structure

P-channel D-MOSFET Characteristics

N- channel Enhancement Type MOSFET

VGS = 0V; VDS = +V

No Current ID flows

Working (VGS > 0V & VDS > 0V)

Cont.

It is the minimum gate to source voltage (VGS) that turns the E-MOSFET ON or that produces the induced channel which result in the flow of drain current (ID) in E-MOSFET

Threshold Voltage (VT or VGS(Th))

Working of E-MOSFET

Condition

Drain Current Induced Channel

VGS < VT VDS > 0V No current flows No channel

VGS = VT

VDS > 0V

No Current flows since the channel is smaller at the drain end than at the source end

Induced Channel is only just created

VGS > VT

VDS > 0VSignificant amount of

current flows

The density of free carriers in the

Induced Channel will increase

Drain Characteristics (n-channel E-MOSFET)

Drain Characteristics (n-channel E-MOSFET)

The saturation level for VDS is related to the level of applied VGS by

VDS (sat) = VGS – VT

Therefore for a fixed value of VT , the higher the level of VGS , the greater is the saturation level for VDS

Cont.

Regions Condition

Triode (Ohmic)VDS< VGS – VT

&VGS > VT

SaturationVDS > VGS – VT

&VGS > VT

Cut-off VGS < VT

Different Regions in Drain Characteristics (n-channel E-MOSFET)

Transfer & Drain Characteristics(n-channel E-MOSFET)

The equation for the E-MOSFET transconductance curve (for VGS > VT):

Cont.

P-channel E-MOSFET

P-channel E-MOSFET Drain & Transfer Characteristics

E-MOSFET Schematic Symbols

MOSFET as a Resistor

Used in Digital CMOS logic Used in almost all electronics appliances including

personal computers Used as Power Amplifers

Application of MOSFETs

JFET MOSFETTwo types – p channel & n channel JFET

Two types – Depletion & Enhancement type

Do not have the insulated gate Have the insulated gate structure

Input impedance is lower than MOSFET

Input Impedance is higher due to the insulated gate structure

Drain resistance is lower than MOSFET Drain resistance is higher than JFET

The transverse electric field across the reversed biased pn junction controls the channel conductivity

The transverse electric field induced across the insulating layer deposited on the semiconductor material controls the channel conductivity

JFET operated only in depletion mode D-MOSFET operated in both Depletion & Enhancement mode whereas E-MOSFET only in E-mode

Difficult to fabricate compared to MOSFETs

Easier to Fabricate

Not Susceptible to overload voltage Very Susceptible to overload voltage & needs special handling during installation. (easily damaged if not handled properly)

MOS Capacitor

Type Basic Relationships Symbol

Junction Field Effect Transistor (n-

channel)

Depletion type MOSFET (n-

channel)

Enhancement type MOSFET (n-

channel)

Summary of Field Effect Transistors

• Electronic Devices & Circuit Theory by Robert Boylestad and Louis Nashelsky

Reference