What is a Transistor? Types of transistors…

#Technical studies-study 10


Transistor is a three terminal semiconductor device. It allows controlling the flow of current from one terminal to another by controlling flow of current in the third terminal.

 It replaced the vacuum tubes. After diode, transistor is the second fundamental semiconductor device in electronics. It is used in every electronics devices. Without it computers, mobile phones, laptops wouldn’t be possible.

Advantages of transistor over vacuum tubes:

  • Long life
  • Smaller than vacuum tubes in size
  • PC made using transistor is much smaller than the PC made using vacuum tubes
  • It operates in low voltage


Transistors are of two types:-

  1. Bipolar junction transistor(BJT)
  2. Field effect transistor(FET)

Field effect transistors are of two types:-

  1. Junction field effect transistor(JFET)
  2. Metal oxide semiconductor field effect transistor(MOSFET)

Bipolar junction transistor:

Structure of a transistor

The word bipolar means two polarities. It uses both types of charges free electrons and holes in operation.

It has three terminals, those are: – Base, Emitter, Collector.

The flow of current from collector to emitter is controlled by controlling the base current, so it is called as current controlled device.

It is a combination of two pn junctions, one between collector and base and another between emitter and base. Transistor is similar to two back to back diodes.

Bipolar junction transistors are available in two configurations, those are: – NPN and PNP.

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In NPN configuration a p-type material is placed between two n-type materials.

NPN transistor symbol
NPN transistor as two diodes

In PNP configuration an n-type material is placed between two p-type materials.

PNP transistor symbol
PNP transistor as two diodes

BJT Biasing:
Transistor biasing circuit

The above figure shows a biased transistor.

In figure the emitter base diode is forward biased and the collector base diode is reverse biased. The transistor connected in this connection produces useful output. So this connection is widely used. 

Field effect transistor:

It is a unipolar device, because it uses only one type of charge for operation, either holes or free electrons.

1.Junction field effect transistor:

Junction field effect transistor is a three terminal, voltage controlled semiconductor device.

The terminals are: – Gate, Drain, Source.

The flow of current from drain to source is proportional to the voltage applied to the gate terminal, so it called as voltage controlled device.

JFETs are available in two configurations, N-channel and P-channel.

N-channel JFET is made by doping donor impurities.

N-Channel JFET structure
N-Channel JFET symbol

P-channel JFETs are formed by doping acceptor impurities.

P-Channel JFET Structure
P-Channel JFET Symbol

  N-channel JFET has more power conduction power compared to P-channel so N- channel is widely used in applications.

JFET biasing:
JFET biasing

The above figure shows a biased JFET.

When voltage to the gate terminal is zero there is maximum current in drain of JFET. So JFET is called as a normally on device.

In figure the gate source diode is reverse biased. Because in this connection there is no current at the gate, so the JFET has infinite input resistance. This makes JFET suitable for the applications where high resistance is required.

2.Metal oxide semiconductor field effect transistor:

Metal oxide semiconductor field effect transistor is a three terminal, voltage controlled semiconductor device. The terminals are: – Gate, Drain, Source.

It is different from JFET, because in MOSFET the gate is insulated from the body. So MOSFET is called as insulated gate field effect transistor.

MOSFETs are of two types: – Depletion mode and Enhancement mode.

 Depletion mode MOSFET: –
N-Channel D-MOSFET Symbol
N-Channel D-MOSFET Structure

The above figure shows an n-channel depletion mode MOSFET.   

It is similar to JFET, because the gate voltage controls the flow of electrons from source to drain in reverse bias condition of gate. In this the gate is insulated from the body by placing a thin layer of silicon dioxide (SiO2) between the gate and the body. There is a p-type substrate which is internally connected to source terminal.

P-Channel D-MOSFET Structure

It is a normally on device, means when the gate voltage is zero there is current flow between drain to source. As the gate is insulated it conducts in positive gate voltage also. When the gate voltage is negative n-channel MOSFET turns off.

P-Channel D-MOSFET Symbol

Enhancement mode MOSFET: –
N-Channel E-MOSFET Structure
N-Channel E-MOSFET Symbol

The above figure shows an n-channel enhancement mode MOSFET.

P-Channel E-MOSFET Structure
P-Channel E-MOSFET Symbol

It has three terminals Gate, Drain and Source and the substrate is extended up to silicon dioxide.

It is a normally off device, means when the gate voltage is positive it is on and when the gate voltage is zero or negative it is off.

MOSFET Biasing circuit

When a positive voltage is applied to gate, it attracts the free electrons into the p-substrate. The free electrons combine with the holes near to the silicon dioxide and create a thin layer of n-type material called n-type inversion layer. The minimum voltage required to create an inversion layer is called threshold voltage.

Transistors are mainly used for switching and amplifying applications. we will discuss these topics in other studies.

Please write in the comment box below if you have any questions.

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