Transistor has two specific families to deal with. The first one is Bipolar Junction Transistors (BJT), and the second is Field Effect Transistors (FETS). The BJT consists of two junction semiconductor devices, three layers and three terminals. It has two PN junctions sandwiched with a middle layer. When it comes to learning about transistors, it is referred to as the Bipolar Junction Transistors.

Transistor

Basically, the transistor has three terminals, i.e., base, emitter, and collector. The function of the emitter is to emit the electrons to the base as the emitter has a highly doped terminal. 

The function of the base as a lightly doped terminal is to make way for the emitter-injected electrons to the collector. In contrast, the collector grabs the electrons from the base as the collector has moderately doped properties. Compared to other terminals, the collector is large and sometimes starts generating less heat for the operation’s working.

History of the Transistor

It is a semiconductor device with three terminals 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, digital circuits, or fast switching. Triodes, also known as (thermionic) valves, were much larger and required 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 attributed to the transistor’s creation. The transistor’s introduction is frequently regarded as one of the most significant technological developments of all time.

Junction Transistor

The Junction Transistor, also known as the Bipolar Junction Transistor (BJT), is used to amplify 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 supply 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 often utilised as bipolar transistors. Electrons make up most of the charge carriers in an n-p-n transistor, while holes make up the minority. A large 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 biasing. In the base region, electron-hole recombination is extremely rare, with most electrons travelling to the collector region instead.

• PNP Transistor- The 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 biased forward, while the collector is biased backward.

Theory and modelling of a transistor

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 utilised to create powerful amplifiers for a wide range of purposes.

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.