All About Junction transistor

A transistor can be defined as a semiconducting electrical component that is majorly used for amplifying electrical signals over large distances. It is nearly impossible to imagine a modern-day electrical circuit without a transistor. It acts as a gate (or as a switch) in electrical circuits. The output power, current, and voltage of a transistor are strictly regulated by the input current. 

Transistors are broadly of two types – bipolar junction transistor (abbreviated as BJT) and field-effect transistor. A junction transistor involves two p-n junctions. An n-type (or a p-type) semiconducting material is sandwiched between two p-type (or two n-type) semiconducting materials. Holes and electrons are current carriers.

Bipolar Junction Transistor Construction

A bipolar junction transistor is the most widely used electrical component for the amplification of electrical signals. A typical BJT consists of three terminals – an emitter, a collector, and a base. The purpose of the heavily doped emitter region is to carry current carriers (either electrons or holes, depending upon the type of transistor) into the base. 

The base is the sandwiched region, which is very thin, and the doping is light. The collector region is the widest of the three regions, and it is where most of the charge carriers accumulate. The collector is moderately doped.

If the BJT is emitter-based, then it would be forward-biassed. If, however, the BJT is collector-based, it would be reverse biassed. 

When the electric current goes out from the transistor, then it is considered negative. However, if the electric current goes into the transistor, then its sign is taken as positive.

• The emitter current is represented by the symbol IE.
• Collector current is represented by the symbol IC.
• Base current is represented by the symbol IB.

Types of junction transistors

There can be two types of junction transistors which are described as follows:

1. n-p-n transistor: 

In this, a p-type semiconductor is sandwiched between two n-type semiconductors. It is more widely used than p-n-p junction transistors because of the ease with which the electrons move. Electrons are the majority charge carriers, and holes are the minority charge carriers. The flow of current from emitter to collector is huge.

In n-p-n transistors, the recombination between electrons and holes is limited. Most of the electrons (majority charge carriers) actually pass through the base and reach the collector region. In these transistors, emitter current (IE) has a negative sign, base current (IB) has a positive sign, and collector current (IC) has a positive sign.

For the emitter-base junction to be forward biassed, the potential to be supplied has to be larger than the barrier potential (which is 0.7V for silicon and 0.3V for germanium semiconducting transistors).

1. p-n-p transistor: 

In this, an n-type semiconductor is sandwiched between two p-type semiconductors. Electrons are the minority charge carriers, and holes are the majority charge carriers.
The signs here are completely reversed. Emitter current (IE) has a positive sign, base current (IB) has a negative sign, and collector current (IC) has a negative sign.

Working Principle of Junction Transistors

To understand the working principle of junction transistors, it is crucial to have a basic understanding of biassing of transistors first. Transistor biassing can be defined as the process of fixing the DC operator voltage in such a manner that the AC current can be amplified to the highest possible extent.

The collector-base junction is reverse-biassed, whereas the base-emitter junction is forward-biassed. Also, the width of the depletion layer across the base-emitter junction is shorter than the width of the depletion layer across the collector-base junction.

Now, since the base current is small, IE ≃ IC.

Thermally generated carriers give rise to one more component of collector current. It is known as reverse saturation current and is actually very small in magnitude. 

Applications of Bipolar Junction Transistors

BJTs are very widely used in electrical appliances because they exhibit high transconductance. There is also a higher output resistance which is advantageous for us. They can be conveniently used in areas where there are high frequencies involved. 

BJTs are most widely used as amplifiers, i.e., they can amplify electrical signals over large distances.

Conclusion

Junction transistors are a combination of two p-n diodes. This is called the two diode analogy of transistors. They can be of two types – p-n-p type or n-p-n type depending upon the arrangement. A typical transistor consists of three components – emitter, collector, and base. The emitter is the most heavily doped region, and the base is the least heavily doped region. The widest region is the collector. Most of the majority charge carriers travel from the emitter to the collector directly, with very few of them recombining with minority charge carriers in the base component of the transistor. This is why the base current in transistors is very low in magnitude. Transistors have several applications. They are used in wireless system radio frequencies and also in metal proximity photocells.