Biasing Of A Transistor

A transistor can be defined as a semiconducting electrical component that is majorly used for amplifying electrical signals over large distances, in other words, a conduit for communication. It is nearly impossible to imagine a modern-day electrical circuit without a transistor. It acts as a gate (or 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: a bipolar junction transistor (abbreviated as BJT) and a 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
• a base

The purpose of the heavily doped emitter region is to carry conventional 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, where most of the conventional current carriers accumulate. The collector is moderately doped.

Modes of Operation of Bipolar Junction Transistors

If the bipolar junction transistor is emitter-based, then it would be forward-biased. If, however, the BJT is collector-based, it is reverse biased. 

When the electric current goes out from a transistor, it is considered negative. However, if the electric current goes into a transistor, it is positive. 

• The emitter current is represented by the symbol IE.
• The collector current is represented by the symbol IC.
• The base current is represented by the symbol IB.

There can be two types of junction transistors, which are the NPN transistor and the PNP transistor.

NPN Transistor 

In this, a P-type semiconductor is sandwiched between two N-type semiconductors. It is more widely used than PNP 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 vast.

In NPN transistors, the recombination between electrons and holes is limited. Most of the electrons (majority charge carriers) pass through the base and reach the collector region. 

In these transistors, the:

• emitter current (IE) has a negative sign
• base current (IB) has a positive sign
• collector current (IC) has a positive sign 

For the emitter-base junction to be forward biased, 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).

PNP 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 entirely reversed. In these transistors, the:

• emitter current (IE) has a positive sign
• base current (IB) has a negative sign
• collector current (IC) has a negative sign

   

Working Principle of Junction Transistors

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

The base-emitter junction is forward biased, but the collector-base junction is reverse biased. 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 the collector current. It is known as reverse saturation current and is very small in magnitude. 

Applications of Bipolar Junction Transistors

Bipolar junction transistors are widely used in electrical appliances because they exhibit high transconductance. There is also a higher output resistance, which is advantageous in that they can be conveniently used in areas where there are high frequencies involved. 

BJTs are also 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. Depending upon the arrangement, they can be of two types: PNP type or NPN type. A typical transistor consists of three components — the emitter, collector, and base. The emitter is the most heavily doped region, and the base is the least heavily doped. The widest region is the collector. Most charge carriers travel from the emitter to the collector directly, with very few recombining with minority charge carriers in the base component of the transistor, and this is also a reason why the base current in transistors is very low in magnitude. Transistors have several applications. They are used in wireless system radio frequencies and metal proximity photocells, thus they are also categorised as a conduit for communication.