Construction of pnp and npn Transistors

Construction of pnp and npn Transistors

• When a transistor is formed by sandwiching a single p‒region between two n‒regions, as shown in Fig. 4.2.1 (a), it is an n‒p‒n type transistor. The p‒n‒p type transistor has a single n‒region between two p‒regions, as shown in Fig. 4.2.1 (b).

Fig. 4.2.1 Bipolar transistor construction

• The middle region of each transistor type is called the base of the transistor. This region is very thin and lightly doped.

• The process by which impurities are added to a pure semiconductor is called doping.

• The remaining two regions are called emitter and collector.

• The emitter and collector are heavily doped. But the doping level in emitter is slightly greater than that of collector.

• The collector region‒area is slightly more than that of emitter. It helps in better power dissipation.

Transistor Symbols

• Fig. 4.2.2 (a) and (b) shows the symbols of and pnp transistors. Arrowhead on a transistor symbol indicates the conventional current which is opposite to the direction of electron current in emitter.

Diode Equivalent Structure of Transistor

• A transistor can be considered as two p‒n junction diodes connected back to back as shown in Fig. 4.2.3 (a) and (b).

Junctions of Transistor

• A transistor has two p‒n junctions.

• One junction is between the emitter and the base and is called the emitter base junction or simply the emitter junction J_E.

• The other junction is between the base and the collector and is called collector‒base junction or simply collector junction J_C.

Cannot Replace Transistor by Back to Back Diodes

• However, we cannot replace transistor by back to back connected diodes because of the following reasons:

1. Relative doping levels in the base, emitter and collector junctions must be satisfied to work that device as a transistor. Two normal p‒n junction diodes cannot satisfy this requirement.

2. In a transistor, emitter to base junction is forward biased while base to collector junction is reversed biased. But due to diffusion process almost entire emitter current reaches to collector and base current is negligibly small. Thus due to diffusion, device works as a transistor. While in back to back connected diodes there are two separate diodes, one forward biased and one reverse biased and diffusion cannot take place. Thus maximum series current which can flow is reverse saturation current of a reverse biased diode. Hence the combination of back to back connected diodes cannot be used as transistor.

• Another important point is that, the emitter area in the transistor is considerably smaller than the collector area. This is because the collector region has to handle more power than the emitter and more surface area is required for heat dissipation.

Review Questions

1. Why collector region of a transistor is larger in area than emitter region?

2. Explain the construction of n‒p‒n and p‒n‒p transistors.

3. Why transistor can not be replaced by back to back diode connection?

4. Explain the terms emitter, collector and base.

5. Sketch the symbols of transistors.