TRANSISTORS

A transistor is a sandwich of one type of semiconductor (P-type or n-type) between two layers of other types.

Transistors are classified into two types;

1.      pnp transistor

pnp transistor is obtained when a n-type layer of silicon is sandwiched between two p-type silicon material.

2.      npn transisitor

npn transistor is obtained when a p-type layer of silicon is sandwiched between two n-type silicon materials.

Figure3.1  below shows the schematic representations of a transistor which is equivalent of two diodes connected back to back.

Ø  The three portions of transistors are named as emitter, base and collector. The junction between emitter and base is called emitter-base junction while the junction between the collector and base is called collector-base junction.

Ø  The base is thin and tightly doped, the emitter is heavily doped and it is wider when compared to base, the width of the collector is more when compared to both base and emitter.

Ø  In order to distinguish the emitter and collector an arrow is included in the emitter. The direction of the arrow depends on the conventional flow of current when emitter base junction is forward biased.

Ø  In a pnp transistor when the emitter junction is forward biased the flow of current is from emitter to base hence, the arrow in the emitter of pnp points towards the base.

3.1 Operating regions of a transistor

A transistor can be operated in three different regions as

a)      active region

b)      saturation region

c)      cut-off region

Active region

The transistor is said to be operated in active region when the emitter-base junction is forward biased and collector –base junction is reverse biased. The collector current is said to have two current components one is due to the forward biasing of EB junction and the other is due to reverse biasing of CB junction. The collector current component due to the reverse biasing of the collector junction is called reverse saturation current (I_CO or I_CBO) and it is very small in magnitude.

Saturation region

Transistor is said to be operated in saturation region when both EB junction and CB junction are forward biased as shown. When transistor is operated in saturation region I_C increases rapidly for a very small change in V_C.

Cut-off region

When both EB junction and CB junction are reverse biased, the transistor is said to be operated in cut-off region. In this region, the current in the transistor is very small and thus when a transistor in this region it is assumed to be in off state.

3.2 Working of a transistor (pnp)

Fig 3.6 Transistor in active region

·         Consider a pnp transistor operated in active region as shown in Figure 3.6

·         Since the EB junction is forward biased large number of holes present in the emitter as majority carriers are repelled by the +ve potential of the supply voltage V_EB and they move towards the base region causing emitter current I_E.

·         Since the base is thin and lightly doped very few of the holes coming from the emitter recombine with the electrons causing base current I_B and all the remaining holes move towards the collector. Since the CB junction is reverse biased all the holes are immediately attracted by the –ve potential of the supply V_CB. Thereby giving rise to collector current I_C.

·         Thus we see that I_E = I_B + I_C -----------------(1) (By KVL)

·         Since the CB junction is reverse biased a small minority carrier current I_CO flows from base to collector.

3.3 Current components of a transistor

Fig 3.7 above shows a transistor operated in active region. It can be noted from the diagram the battery V_EB forward biases the EB junction while the battery V_CB reverse biases the CB junction.

As the EB junction is forward biased the holes from emitter region flow towards the base causing a hole current I_PE. At the same time, the electrons from base region flow towards the emitter causing an electron current I_NE. Sum of these two currents constitute an emitter current I_E = I_PE +I_NE.

The ratio of hole current I_PE to electron current I_NE is directly proportional to the ratio of the conductivity of the p-type material to that of n-type material. Since, emitter is highly doped when compared to base; the emitter current consists almost entirely of holes.

Not all the holes, crossing EB junction reach the CB junction because some of the them combine with the electrons in the n-type base. If I_PC is the hole current at (Jc) CB junction. There will be a recombination current I_PE  - I_PC leaving the base as shown in figure 3.7.

If emitter is open circuited, no charge carriers are injected from emitter into the base and hence emitter current I_E =o. Under this condition CB junction acts a a reverse biased diode and therefore the collector current ( I_C = I_CO) will be equal to te reverse saturation current. Therefore when EB junction is forward biased and collector base junction is reverse biased the total collector current I_C = I_PC +I_CO.