Semiconductor Electronics: Materials, Devices and Simple Circuits

Bipolar Junction Transistor

The word transistor is derived from words transfer and resistor. The operation of a bipolar junction transistor is the transmission of the input signal from low to high resistance circuit. Thus, in this article, we are going to discuss in detail about the bipolar junction transistor.

bipolar junction transistor

Introduction to Bipolar Junction Transistor

The bipolar junction transistor is a current-controlled and 3-terminal semiconductor device. Moreover, it consists of two p-n junctions. These junctions magnify the signal. Further, the three terminals are collector, base, and emitter. The signal of less amplitude applies to the base becomes available in the amplified way at the transistor collector. It doesn’t need an external source like DC for carrying the amplification.

Construction of Bipolar Junction Transistor

It composes of three doped semiconductors like a collector, base, and an emitter that separate by 2 p-n junctions. The transistors prepare in two ways- NPN and PNP. They are available in large quantities and as distinct components. The key purpose is to magnify the current. We can use them as amplifiers or switches. Moreover, we widely use them in other devices such as mobile, radio transmitter, television, and industrial control.

Operation of Bipolar junction transistor

There are three regions of operation in bipolar junction transistor

Active region- It is the region where the transistor remains functional as an amplifier.

Saturation region- It is the region where the transistor is on and functions as a switch. The collector current is equivalent to the saturation current.

Cut-off region- It is the region in which the transistor is off and the collector current is zero.

Types of Bipolar Junction Transistor

Mainly there are two kinds of bipolar junction transistors- PNP and NPN.

PNP- Here, the n-type semiconductor is between two p- kinds of the semiconductor. The two p- semiconductors are emitter and collector while n-type is the base. Here, the current enter transistor via emitter and the emitter-base junction is forward and the collector base is reverse biased.

NPN- Here, a p-type semiconductor is between two n-kind semiconductors. The two n-semiconductors are emitter and collector while p-semiconductor is base.  The current entering base, emitter, and collector have positive signs while the current leaving transistor has a negative sign.

The combination of PNP and NPN creates three different types of Bipolar Transistors

  1. General Bipolar Transistor II) Bias Resistor Built in Transistors III) Radio Frequency Bipolar Transistors

Another important aspect to consider is that the transistor is biased in an accurate manner. Failure to do so can result in issues like

  1. The shift of operating parameters
  2. Distorted output signal
  3. Reduced work efficiency of transistors

Moreover, one must not confuse BJT with MOSFET as both are separate entities. While BJT is a bipolar Junction Transistor with three terminals called as Base, emitter and collector. Further, MOSFET is a metal oxide based semiconductor Transistor with terminal called source, drain and gate.

Function of the Bipolar junction Transistor

They are of two kinds of transistors based on doping kinds of three main terminals.  NPN transistor has two semiconductor junctions with the thin p-doped anode. PNP transistor has two semiconductor junctions with a thin n-doped cathode.

The charge flow is due to the diffusion of charge carriers. The regions are collector, base, and emitter. The emitter is highly doped. The base and collector have the same concentration of charge carrier. The base-emitter junction is forward while the base-collector is reverse biased. The forward biased is p-doped and has more potential as compared to the n-doped one.

Further, always remember that there is a fixed maximum limit of a transistor of any kind, the collector supply voltage.  Any over the limit charge can damage the transistor.

Application of Bipolar Junction Transistor

We generally use BJT as a switch, filter, amplifier, and oscillator.  Additionally, we also apply it as an amplifier in electronic signals. Thus, we can also use it as a component of Modulator.

FAQ on Bipolar Junction Transistor

Question 1: Who invented Bipolar Junction Transistor?

Answer 1: BJT was a joint discovery by Brattin, Bardeen and Shockley

Question 2: Which are common operating region of a Bipolar Junction Transistor?

Answer 2: The key region of operation of BJT are

  • Cut off region and Saturation region
  • Active region and Inverted region also referred to as Forward Active and Reverse Active regions.

Question 3: Is Bipolar Junction Transistor and Field-Effect Transistors the same thing?

Answer 3: No, both are different in their mechanism of operations. While BJT has both majority and minority charge flow, in the case of FET only minority charge flows.

However, FET is much easier to fabricate and is much more temperature stable that BJT.

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