Basic Concept of Transistor

Transistor is a semiconductor devices that is made for amplification or oscillations and generally used as a electronic switch in the electronic devices. Transistors have become the key ingredient of all digital circuits, including computers. Every chip or microprocessors are made up of the logic integration of transistors.

Invention:-

It was invented in 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley. The junction version known as the bipolar junction transistor, invented by Shockley in 1948.

Structure and Working:-

Transistors are manufactured in different shapes but they have three leads (legs).

The BASE - which is the lead responsible for activating the transistor.

The COLLECTOR - which is the positive lead.

The EMITTER - which is the negative lead.

The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts – OFF.

Transistor is defined into three types of structures:-

1. NPN - It consists of a layer of P-doped semiconductor between two N-doped semiconductors. When a small current enters through the base (B), it gets amplified and produces the large collector (C) and emitter (E) current.

- Active state:- Transistors becomes active when there is a positive potential difference measured from the emitter of an NPN transistor to its base as well as positive potential difference measured from the base to the collector. In this active state, current flows between the collector and emitter of the transistor.

2. PNP - It is another type of BJT, consisting of N-doped semiconductor between two layers of P-doped material. A small current leaving the base is amplified in the collector output.

3. Heterojunction BJT - It is improvement in the structure of BJT that can handle signals of very high frequencies up to GHZ that's why it is used in ultra fast operations or RF systems. As emitter is composed of large bandgap as compared to base, it increases the efficiency of emitter injection into the base and switching speed as well as reduces the base current.

Modes of operation:-

The modes of operation can be described in terms of the applied voltages:-

1. Forward active: base higher than emitter, collector higher than base (in this mode the collector current is proportional to base current by ).

2. Saturation: base higher than emitter, but collector is not higher than base.

3. Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor is not letting conventional current to go through collector to emitter.

4. Reverse-active: base lower than emitter, collector lower than base: reverse conventional current goes through transistor.