Bipolar Junction Transistor
PNP transistor
When a single n-type semiconductor layer is sandwiched between two p-type semiconductor layers, a pnp transistor is formed.
PNP transistor symbol
The circuit symbol and diode analogy of pnp transistor is shown in the below figure.
PNP transistor construction
The pnp transistor is made up of three semiconductor layers: one n-type semiconductor layer and two p-type semiconductor layers.
The n-type semiconductor layer is sandwiched between two p-type semiconductor layers.
The pnp transistor has three terminals: emitter, base and collector. The emitter terminal is connected to the left side p-type layer. The collector terminal is connected to the right side p-type layer. The base terminal is connected to the n-type layer.
The pnp transistor has two p-n junctions. One junction is formed between the emitter and the base. This junction is called emitter-base junction or emitter junction. The other junction is formed between the base and the collector. This junction is called collector-base junction or collector junction.
Working of a pnp transistor
Unbiased pnp transistor
When no voltage is applied to a pnp transistor, it is said to be an unbiased pnp transistor. At the left side p-region (emitter) and right side p-region (collector), holes are the majority carriers and free electrons are the minority carriers whereas in n-region (base), free electrons are the majority carriers and holes are the minority carriers.
We know that the charge carriers (free electrons and holes) always try to move from higher concentration region to lower concentration region.
For holes, p-region is the higher concentration region and n-region is the lower concentration region. Similarly, for free electrons, n-region is the higher concentration region and p-region is the lower concentration region.
Therefore, the holes at the left side p-region (emitter) and right side p-region (collector) experience a repulsive force from each other. As a result, the holes at the left side and right side p-regions (emitter and collector) will move into the n-region (base).
During this process, the holes meet the free electrons in the n-region (base) and recombines with them. As a result, depletion region (positive and negative ions) is formed at the emitter to base junction and base to collector junction.
At emitter to base junction, the depletion region is penetrated more towards the base side, similarly; at base to collector junction, the depletion region is penetrated more towards the base side.
This is because at emitter to base junction, the emitter is heavily doped and base is lightly doped so the depletion region is penetrated more towards the base side and less towards the emitter side. Similarly, at base to collector junction, the collector is heavily doped and base is lightly doped so the depletion region is penetrated more towards the base side and less towards the collector side.
The collector region is lightly doped than the emitter region, so the depletion layer width at the collector side is more than the depletion layer width at emitter side.
Biased pnp transistor
When external voltage is applied to a pnp transistor, it is said to be a biased pnp transistor. Depending on the polarity of the applied voltage, the pnp transistor can be operated in three modes: active mode, cutoff mode and saturation mode.
The pnp transistor is often operated in active mode because in active mode the pnp transistor amplifies the electric current.
So let’s see how a pnp transistor works in active mode.
Let us consider a pnp transistor as shown in the below figure. In the below figure, the emitter-base junction is forward biased by the DC voltage VEE and base-collector junction is reverse biased by the DC voltage VCC.
Emitter-base junction:
Due to the forward bias, a large number of holes in the left side p-region (emitter) experience a repulsive force from the positive terminal of the DC battery and also they experience an attractive force from the negative terminal of the battery. As a result, the holes start flowing from emitter to base. In the similar way, free electrons in base experience a repulsive force from the negative terminal of the battery and also experience an attractive force from the positive terminal of the battery. As a result, the free electrons start flowing from base to emitter.
The majority carriers holes carry most of the current from emitter to base. Thus, the electric current flows from emitter to base.
This electric current flow reduces the width of the depletion region at emitter-base junction.
Base-collector junction:
Due to the reverse bias, a large number of holes in the right side n-region (collector) experience an attractive force from the negative terminal of the battery. Hence, the holes move away from the junction and flow towards the negative terminal of the battery. As a result, a large number of neutral collector atoms gains electrons and becomes negative ions. On the other hand, free electrons in the n-region (base) experience an attractive force from the positive terminal of the battery. Hence, the free electrons move away from the junction and flow towards the positive terminal of the battery. As a result, a large number of neutral base atoms loses electrons and becomes positive ions.
Thus, the width of depletion region increases at base-collector junction. In other words, the number of positive and negative ions increases at the base-collector junction.
Emitter-base-collector current:
The holes that are flowing from emitter to base due to forward bias will combines with the free electrons in the base. However, the base is very thin and lightly doped. So only, a small percentage of emitter holes will combine with the free electrons in the base region. The remaining large number of holes will cross the base region and reaches to the collector region. This is due to the negative supply voltage applied at collector. Hence, the holes flow from emitter to collector. At collector, both the emitter holes and collector holes produces current by flowing towards the negative terminal of the battery. Therefore, an amplified current is produced at the output.
In pnp transistor, the electric current is majorly conducted by holes.
