A bipolar junction transistor is a current controlled device which means that output characteristics of the device are controlled by Base current and not by voltage.However in a field effect transistor (FET),the output characteristics are controlled by input voltage and not by input current.There are two basic types of field effect transistors.
(i) Junction field effect transistor
(ii)Metal oxide semiconductor field effect transistor.
Junction field effect transistor:
A junction field effect transistor is a three terminal semiconductor device in which current conduction by on type of carrier.
The structure of n-channels JFET and p-channel JFET are shown in below. Ohmic contacts are made to the two ends of a semiconductor bar o n-type material.current is caused to the flow along the length of the bar because of the supply voltage connected between the bars.This current consists of majority carriers,which in this case are electrons.
schematic symbol of JFET :
Source :
The source S is terminal through which the majority carriers enter the bar.Conventional current entering the bar at S designated by Is.
Drain :
The Drain is the terminal through which majority carriers have leave the bar.Conventional current entering the bar at D is designated by Id.The Drain-to-source voltage is called Vds.
Gate :
On both sides of n-type bar heavily doped (p+) regions acceptor impurities have been formed by alloying,by diffusion or any other procedure available for creating pn junction.
constructional details :
Importance of JFET :
JFET act like a voltage controlled device which means that input voltage (Vgs) controls the output current.This is different from ordinary transistor where the input current controls the output current.Thus JFET is a semiconductor device acting like a vacuum tube.The JFET devices have not only taken over the functions of vacuum tubes.The JFET has higher input impedance than that of a conventional transistor,generates less noise and has greater resistance to nuclear radiations.
Salient features of JFET :
- A JFET is a three terminal voltage controlled semiconductor device which means input voltage controls the output characteristics.
- The JFET is always operated reverse bias.
- In a JFET the gate current is zero.
- since there is no gate current,Id=Is.
- The JFET is not subjected to the thermal runaway when temperature of the device increases.
Important terms of JFET:
The following important terms are often used in JFET
1.Shorted-gate drain current
2.Pinch off voltage
3.Gate-source cut off voltage
No comments:
Post a Comment