What is Transistor ? Difference Between FET and BJT.

 Objective :- In this article we will discuss following aspects about Transistor

• What is transistor?

• Types of transistor.

• How transistor works?

• Transistor uses.

Transistor Definition

Transistor is an electronic device which transfers current from low impedance level to high impedance level. It is an active device which provides gain. Transistor is a three terminals device which consists of two pn-junctions, each junction represents diode. Depending upon the biasing whether it is forward or reverse of each junction different modes of operations are obtained.

 

Types of Transistor

 

Transistor mainly is off two types

• BJT (Bipolar Junction Transistor)

• FET (Field Effect Transistor)

 

 

FET (Field Effect Transistor) further classified in two types

• JFET (Junction Field Effect Transistor)

• MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

Introduction to BJT (Bipolar junction Transistor)

As we studied above transistor is a three terminal device, similarly BJT is a three terminal device namely Emitter, Base and Collector which is consists of two pn-junctions one is emitter-base junction and other is collector-base junction each junction refers a diode. As the name implies Bipolar Junction Transistor it conducts on both the polarities i.e. both electrons and holes are participate in conduction process in transistor. So we can say BJT is current controlled Device because majority and minority carrier current control the conduction process. BJT is formed either by using a p-type semiconductor sandwiched between two n-type semiconductors (i.e. pnp transistor) or by using a n-type semiconductor sandwiched between two p-type semiconductors (i.e. npn transistor).

 

BJT Symbols

 

FET (Field Effect Transistor)

FET is a unipolar device. Means only majority charge carriers participate in conduction process there is no minority charge carriers. FET is a voltage controlled device means by varying the voltage of two terminals different modes of operations can be obtained.

 

JFET (Junction Field Effect Transistor)

JFET is three terminals device namely Source, Gate and Drain. By controlling gate to source voltage different modes of operations can be obtained. Constructionally a n-channel JFET consists of n-type material, which forms the channel between two p-type layer. One side of the n-type channel is connected through an ohmic contact to a terminal which is referred to as the Drain (D), whereas other end of this channel is connected through an ohmic contact to a terminal referred to as the Source (S). The two p-type materials are connected together and forms Gate (G) terminal. Similarly p-channel JFET is formed. In n-channel JFET, n-type channel and two p-type materials forms two pn-junctions.  

JFET Symbol

        

MOSFET

MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. It is type of FET in which a thin metal oxide layer is used for fabrication process. Basically metal oxide layer is used to insulate the Gate electrode from semiconductor. It has electrically insulated gate whose voltage determine the operation of MOSFET. So basically we can say MOSFET is voltage controlled device. Generally silicon dioxide is used for fabrication of MOSFET. MOSFET is most widely used in modern electronics.

MOSFET Symbol

MOSFET is a four terminals namely Source, Gate, Drain and Substrate semiconductor device.

 

             

 

Types of MOSFET

MOSFET is of two types

• Depletion MOSFET

• Enhancement MOSFET

 

Depletion MOSFET

Depletion MOSFET is type of MOSFET which is less commonly used. Constructionally we can say a channel already provided in between source and drain. So even when there is no Gate to Source voltage V_GS depletion type MOSFET conducts. To control the conduction process negative gate to source voltage is applied.

 

 

Enhancement MOSFET

The enhancement type MOSFET is most widely used FET. Generally we can say it is a normally open (Off) switch. Means there is no channel provided between source and drain. So we have to apply positive gate to source voltage to start the conduction process.

Some Basic Difference between BJT and FET

BJT	FET
1. BJT is a current controlled device	1. FET is a voltage controlled device
2. BJT is bipolar device	2. FET is unipolar device
3. BJT is both majority and minority carrier device.	FET is majority carrier device
4. BJT is faster	4. FET is slow than BJT
4. Sensitivity of BJT is higher.	5. FET is less sensitive as compared to BJT.

See also :-

SCR (Silicon Controlled Rectifier).