Introduction to FET’s

Current Controlled vs Voltage Controlled Devices

Types of Field Effect Transistors (The Classification)

» JFET

MOSFET (IGFET)

n-Channel JFET

p-Channel JFET

n-Channel EMOSFET

p-Channel EMOSFET

Enhancement MOSFET

Depletion MOSFET

n-Channel DMOSFET

p-Channel DMOSFET

FET

Figure: n-Channel JFET.

The Junction Field Effect Transistor (JFET)

ELEC 121

JFET ConstructionThere are two types of JFET’s: n-channel and p-channel.

The n-channel is more widely used.

There are three terminals: Drain (D) and Source (S) are connected to n-channelGate (G) is connected to the p-type material

Gate

Drain

Source

SYMBOLS

n-channel JFET

Gate

Drain

Source

n-channel JFETOffset-gate symbol

Gate

Drain

Source

p-channel JFET

N-Channel JFET Symbol

Figure: n-Channel JFET and Biasing Circuit.

Biasing the JFET

Figure: The nonconductive depletion region becomes broader with increased reverse bias. (Note: The two gate regions of each FET are connected to each other.)

Operation of JFET at Various Gate Bias Potentials

Transfer CharacteristicsThe input-output transfer characteristic of the JFET is not as straight forward as it is for the BJT

In a BJT, (hFE) defined the relationship between IB (input current) and IC (output current).

In a JFET, the relationship (Shockley’s Equation) between VGS (input voltage) and ID (output current) is used to define the transfer characteristics, and a little more complicated (and not linear):

As a result, FET’s are often referred to a square law devices

2GS

D DSSP

V I = I 1 -

V

JFET Operating Characteristics

There are three basic operating conditions for a JFET:JFET’s operate in the depletion mode onlyA. VGS = 0, VDS is a minimum value depending on IDSS and the drain and source

resistanceB. VGS < 0, VDS at some positive value andC. Device is operating as a Voltage-Controlled Resistor

For an n channel JFET, VGS may never be positive*For an p channel JFET, VGS may never be negative*

Saturation

At the pinch-off point: • any further increase in VGS does not produce any increase in ID. VGS at pinch-off is denoted as Vp. • ID is at saturation or maximum. It is referred to as IDSS. • The ohmic value of the channel is at maximum.

Specification Sheet (JFETs)