JFET

Junction field effect transistor JFET (2m)

Is a three terminal semi conductor device in which current conduction is by one type of carrier i.e electrons or holes. The output characteristics are controlled by input voltage and not by input current.

There are 2 types of JFET: n-channel JFET & p-channel JFET

Three Terminal: Drain – D, Gate -G Source – S 

N channel JFET: (5m/8m)

Major structure is  n-type material (channel) between embedded p-type material to form 2 p-n junction.

In the normal operation of an n-channel device, the Drain (D) is positive with respect to the Source (S). Current flows into the Drain (D), through the channel, and out of the Source (S)

Because the resistance of the channel depends on the gate-to-source voltage (V_GS), the drain current (I_D) is controlled by that voltage

WORKING PRINICIPLE (8 m)

There are three basic operating conditions for a JFET:

JFET’s operate in the depletion mode only

A. VGS = 0, VDS is a minimum value depending on IDSS and the drain and source resistance

B. VGS < 0, VDS at some positive value and

C. Device is operating as a Voltage-Controlled Resistor

When a voltage Vds is applied between drain and source terminals and the voltage on the gate is zero. the two pn junction at the side of the bar establish depletion layers. The electrons will flow from source to drain through a channel between the  depletion layers. the size of these layers determined the width of the channel and hence the current conduction through the bar.

When a reverse voltage VGS is applied between the Gate and source, width of the depletion layer is increased this reduces width of conducting channel , their by increasing the resistance of n type bar .the current from source to drain is decreased. if the reverse voltage on the gate is decreased width of the depletion layer also decreases. this increases the width of the conducting channel and hence source to drain current

The gate source voltage controls the drain current:

o        as reverse gate voltage increased, the depletion region widens reducing the drain current.

o        as the gate voltage reduced the depletion region recedes increasing the drain current

o        depletion region acts like an internal resistance effecting the flow of drain current gate current is normally extremely low.