The Bipolar Junction Transistor [BJT]

THE BIPOLAR JUNCTION TRANSISTOR [BJT]

Introduction

In 1951, William Schockley invented the modern version of transistor. It is a semiconductor device that led to a technological revolution in the twentieth century. The heat loss in transistor is very less. This has laid the foundation of integrated chips which contain thousands of miniaturized transistors. The emergence of the integrated chips led to increasing applications in the fast developing electronics industry.

Bipolar Junction Transistor

The BJT consists of a semiconductor (Silicon or Germanium) crystal in which an n-type material is sandwiched between two p-type materials (PNP transistor) or a p-type material sandwiched between two n-type materials (NPN transistor). To protect it against moisture, it is sealed inside a metal or a plastic case. The two types of transistors with their circuit symbols are shown in Figure 9.25.

The three regions formed are called as emitter, base and collector which are provided with terminals or ohmic contacts labeled as E, B, and C. As a BJT has two p-n junctions, two depletion layers are formed across the emitter-base junction (J_EB) and collector-base junction (J_CB) respectively. The circuit symbol carries an arrowhead at the emitter lead pointing from p to n indicating the direction of conventional current.

Emitter:

The main function of the emitter is to supply majority charge carriers to the collector region through the base region. Hence, emitter is more heavily doped than the other two regions.

Base:

Base is very thin (10^-6 m) and very lightly doped compared to the other two regions.

Collector:

The main function of collector is to collect the majority charge carriers supplied by the emitter through the base. Hence, collector is made physically larger than the other two as it has to dissipate more power. Its is modarately dopped.

Because of the differing size and the amount of dopping, the emitter and collector cannot be interchanged.

Transistor Biasing

The application of suitable dc voltages across the transistor terminals is called biasing.

Different modes of transistor biasing

Forward Active:

In this bias the emitter-base junction is forward biased and the collector-base junction is reverse biased. The transistor is in the active mode of operation. In this mode, the transistor functions as an amplifier.

Saturation:

Here, the emitter-base junction and collector-base junction are forward biased. The transistor has a very large flow of currents across the junctions. In this mode, transistor is used as a closed switch.

Cut-off:

In this bias, the emitter-base junction and collector-base junction are reverse biased. Transistor in this mode is an open switch.

In a PNP transistor, base and collector will be negative with respect to emitter indicated by the middle letter N whereas base and collector will be positive in an NPN transistor [indicated by the middle letter P]

Transistor circuit configurations

There are three types of circuit connections for operating a transistor based on the terminal that is used in common to both input and output circuits.

Common-Base(CB) configuration

The base is common to both the input and output circuits. The schematic and circuit symbol are shown in Figure 9.26(a) and 9.26(b). The input current is the emitter current I_E and the output current is the collector current I_C. The input signal is applied between emitter and base, the output is measured between collector and base.

Common-Emitter(CE) configuration

In this configuration, the emitter is common to both the input and output loops as shown in Figure 9.27. Base current, I_B is the input current and the collector current, I_C is the output current. The input signal is applied between the emitter and base and the output is measured between the collector and the emitter.

Common-Collector(CC) configuration

Here, the collector is common to both the input and output circuits as shown in Figure 9.28. The base current I_B is the input current, the emitter current I_E is the output current. The input signal is applied between the base and the collector, the output is measured between the emitter and collector.

As the output is taken from the emitter in common collector configuration, it is called an emitter follower.