Introduction:

It was in the year 1951 that a scientist named William Shockley invented the transistor. The transistor which represented the junction transistor had the presence of p-n junctions in them. However, it was not the first time that a transistor came into existence. 

In 1947, J. Bardeen and W.H. Brattain were the first to invent something that the world called a transistor. In this type of transistor, it was purely point-contact. 

Body: 

Junction Transistor

The Junction Transistor that is also known as the Bipolar Junction Transistor (BJT) is used in the amplification of a signal. It is a semiconductor device that consists of two p-n junctions. It consists of three terminals: 

• Emitter: It is heavily doped. That means, it has many impurities. It is not too big or not too small in size. It aids in the current flow by supplying the major carriers.  
• Base: It is lightly doped. It forms the central segment. 
• Collector: It is bigger than the emitter. It is moderately doped. It helps in the supply of the majority charge from the emitter and regulates the flow of current. 

Types of Junction Transistors

• NPN Transistor– A n-doped emitter and an n-doped collector are connected to a p-type semiconductor base in an NPN transistor. Because electron mobility is easier than electron-hole mobility, NPN transistors are the most often utilised bipolar transistors. Electrons make up the majority of the charge carriers in an n-p-n transistor, while holes make up the minority. A big quantity of current flows from the emitter to the collector whereas only a tiny amount of current flows via the base terminal. The bulk of charge carriers in the emitter are repulsed towards the base due to the transistor’s forward biassing. In the base region, electron-hole recombination is extremely rare, with the majority of electrons travelling to the collector region instead.
• PNP Transistor– Emitter and collector of a PNP transistor are made up of a p-doped semiconductor, whereas the base is n-doped. These transistors use holes as majority carriers, whereas electrons are used as minority carriers in these devices. The emitter of a PNP transistor is biassed forward, while the collector is biassed backward.

Theory and Modelling

To understand BJTs, think of them as two diodes (P–N junctions) that are connected to each other. Two diodes sharing an N- and P-type cathode region in a PNP BJT and two diodes sharing a P-type anode region in an NPN BJT are analogous. Using wires to connect two diodes does not create a BJT because the minority carriers cannot travel through the wire. BJTs work by allowing the base current to regulate an amplified output from the collector, which both types of BJTs do. An excellent switch can be made by using the base input of the BJT. The BJT is also a good amplifier since it can increase the strength of a weak signal by around 100 times. BJT networks can be utilized to create powerful amplifiers for a wide range of purposes.

History of the Transistor

It is a semiconductor device with three terminals that can be connected to an electrical circuit. Commonly, the third terminal acts as a switch to regulate how much electricity flows between the two other endpoints. As in a radio receiver, this is for amplification, as in digital circuits, or for fast switching. Triodes, also known as (thermionic) valves, were much larger in size and required a lot more power to operate than transistors. In 1947, Bell Labs in Murray Hill, New Jersey successfully demonstrated the first transistor. Founded by American Telephone and Telegraph, Bell Labs is the company’s research department (AT&T). Shockley, Bardeen, and Brattain are the three inventors that have been attributed with the transistor’s creation. The transistor’s introduction is frequently regarded as one of the most significant technological developments of all time.

V-I characteristics of p-n junction diode

A curve between the voltage and current through the circuit defines the V-I properties of P-N junction diodes. The x-axis represents voltage, while the y-axis represents current. Suppose the V-I characteristics of the p-n junction diode are plotted on a graph; we can see that the diode works in three different zones, which are:

1. Zero Bias: No external voltage is delivered to the p-n junction in a zero bias condition,

As a result, the potential barrier at the junction prevents current flow.

As a result, when V = 0 , the circuit current is zero.

2. Forward Bias:

The p-type of the p-n junction forward bias is connected to the positive terminal of the external voltage, while the n-type is connected to the negative terminal.

As a result, the potential barrier is minimised.

The forward V-I characteristic of the P-N junction diode shows that the current grows exceptionally slowly at the beginning. The curve is nonlinear because the external voltage delivered to the p-n junction is used to overcome the potential barrier in this region.

The potential barrier is removed once the external voltage surpasses the possible barrier voltage, and the p-n junction functions like an ordinary conductor. As a result, the curve AB climbs significantly as the external voltage rises, which is nearly linear.

3. Reverse Bias: The p-type of the p-n junction is connected to the negative terminal of the external voltage, while the n-type is connected to the positive terminal.

As a result, the potential barrier at the intersection is enhanced.

The junction resistance rises to an extremely high level, and virtually no current flows through the circuit.

In practice, however, a very modest current of the order of micro-amperes travels across the circuit. Because of the minority carriers in the junction, this is known as reverse saturation current.

Conclusion:

A p-n junction is the fundamental component of many semiconductor devices such as diodes and transistors. Understanding the development and operation of a p-n Junction is critical to understanding how semiconductor devices work.