Bipolar Junction Transistor (BJT)

The transistor is an electrical device categorised into semiconductor components. It was invented by Shockley, Brattain and Barden in 1947. It serves the function of amplifying the signals for large distances by acting as a switch or gate. The output-generated specifications depend on the input values of variables such as current, voltage and power.

Table of Contents

Bipolar Junction Transistor
Construction
Types of BJT
Advantages and Disadvantages of BJT Transistors
BJT Configuration
Practice Problems
Frequently Asked Questions

Bipolar Junction Transistor

It is a three-terminal device comprising p and n junctions attached in any one of the three types of combination. It amplifies or magnifies the signal. Abbreviated as BJT, it is made of three components. The input is received at the base, and the output is made available at the collector for further transfer. It works in combination with a DC power supply.

Construction

It comprises three regions, emitter, collector and base, which are available in two types:

n-p-n
p-n-p

In BJT, the emitter region is concerned with current transport via the carriers, such as holes or electrons, into the base. The base is present in the middle. The collector present on one end is responsible for the accumulation of charge carriers. The emitter is heavily doped, the collector is moderately doped, and the base is very lightly doped.

The BJT will be forward-biased if it is emitter-base while reversing-biased if it is collector based. It will be negative if the electric current goes out from the transistor, while the sign will be positive if the electric current goes into the transistor. The symbolic representation of emitter, collector and base current is IE, IC and IB, respectively.

Types of BJT

As stated, the bipolar junction transistors are either NPN or PNP.

NPN transistor

The name indicates the position of the semiconductor, where it will be at the terminal or in the middle. The NPN transistor has a p-type semiconductor in the middle of two n-type semiconductors. It provides higher mobility of electrons, electrons, and hence finds widespread application. In the NPN transistor, most charge carriers are electrons, holes are the minority charge carriers, and the current flow direction is emitter to collector.

Commonly electrons pass from the base to the collector. Thus, these regions have a positive sign for electric current. Consequently, the emitter has a negative sign for the electric current. The forward-biassed emitter-base junction must have received a higher potential than the barrier potential. Hence, the demand for silicon-based npn transistors is 0.7 Volts, and for germanium-based semiconductors is 0.3 Volts.

PNP transistor

The specifications here are opposite to the NPN transistor. Here n is sandwiched between p-type semiconductors. Thus, n will be the base, and p-type will be the emitter and collector. Here electrons are minority charge carriers, and holes are the majority ones. Emitter has a positive current, while the base and collector have a negative one. The input current location is at the emitter, making the emitter-base junction forward-biassed and the collector-base junction reverse-biased.

Advantages and Disadvantages of BJT Transistors

The different advantages are enlisted below:

They have low forward voltage.
The performance is efficient at higher frequency
They also have higher current density and efficient voltage gain
The gain bandwidth is also large
They are capable of operation in the variable power range

The BJT transistors are also disadvantageous in certain scenarios, such as:

Low thermal stability
Huge noise
Dangerous radiations
Low switching frequency and time
Complex control

BJT Configuration

There are three possible configurations with one terminal for both output and input.

Common emitter configuration that has both voltage and current gain
A common base configuration that has neither current nor voltage gain
Common collector configuration that has only current gain but no voltage gain

Practice Problems

Q1. What is true about cut-off region characteristics?

A. Current flow in the collector is zero
B. Base emission voltage is more than 0.7 Volts
C. BJT acts as an open switch
D. All of the above

Ans. D. All of the above

All the stated characteristics are of the cut-off region.

Q2. Which region has the highest current during saturation?

A. Base
B. Collector
C. Emitter
D. BJT

Ans. A. Base

The base has the highest current during the saturation region.

Q3. Which voltage determines the power lost by the transistor with collector current?

A. Base supply voltage
B. 0.7 Volts
C. Collector emitter voltage
D. Base emitter voltage

Ans. C. Collector emitter voltage

The power dissipated is the product of collector current and collector-emitter voltage.

Frequently Asked Questions

Q1. What is FET?
Ans. FET or Field Effect Transistor is a type of transistor where the electric field regulates the flow of current.

Q2. Why is BJT called polar?
Ans. Bipolar junction transistors comprise the p and n junction, with electrons and holes as charge carriers. Thus, it is called bipolar.

Q3. What are the different methods for using BJT?
Ans. The BJT can be used as a modulator and demodulator. It is also commonly used as an amplifier and oscillator.