Bipolar Junction Transistor
NPN transistor
When a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers, an npn transistor is formed.
NPN transistor symbol
The circuit symbol and diode analogy of npn transistor is shown in the below figure.
In the above figure, it is shown that the electric current always flows from p-region to n-region.
NPN transistor construction
The npn transistor is made up of three semiconductor layers: one p-type semiconductor layer and two n-type semiconductor layers.
The p-type semiconductor layer is sandwiched between two n-type semiconductor layers.
The npn transistor has three terminals: emitter, base and collector. The emitter terminal is connected to the left side n-type layer. The collector terminal is connected to the right side n-type layer. The base terminal is connected to the p-type layer.
The npn 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 npn transistor
Unbiased npn transistor
When no voltage is applied to a transistor, it is said to be an unbiased transistor. At the left side n-region (emitter) and right side n-region (collector), free electrons are the majority carriers and holes are the minority carriers whereas in p-region (base), holes are the majority carriers and free electrons 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 free electrons, n-region is the higher concentration region and p-region is the lower concentration region. Similarly, for holes, p-region is the higher concentration region and n-region is the lower concentration region.
Therefore, the free electrons at the left side n-region (emitter) and right side n-region (collector) experience a repulsive force from each other. As a result, the free electrons at the left side and right side n-regions (emitter and collector) will move into the p-region (base).
During this process, the free electrons meet the holes in the p-region (base) near the junction and fill 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.
Why depletion region penetrates more towards lightly doped side than the heavily doped side?
We know that doping is the process of adding impurities to the intrinsic semiconductor to improve its electrical conductivity. The electrical conductivity of the semiconductor is depends on the doping level added to it.
If the semiconductor material is heavily doped, its electrical conductivity is very high. That means the heavily doped semiconductor material has a large number of charge carriers which conduct electric current.
If the semiconductor material is lightly doped, its electrical conductivity is very low. That means the lightly doped semiconductor material has a small number of charge carriers which conduct electric current.
We know that in n-type semiconductor, free electrons are the majority charge carriers and holes are the minority charge carriers.
In npn transistor, the left side n-region (emitter) is heavily doped. So the emitter has a large number of free electrons.
We know that in p-type semiconductor, holes are the majority charge carriers and free electrons are the minority charge carriers.
The p-region (base) is lightly doped. So the base has a small number of holes.
The right side n-region (collector) is moderately doped. Its doping level lies between that of emitter and base.
When the atom loses or donates an electron, it becomes a positive ion. On the other hand, when the atom gains or accepts an electron, it becomes a negative ion.
The atoms which donate electrons are known as donors and the atoms which accept electrons are known as acceptors.
Emitter-base junction:
Let us assume that, at left side n-region (emitter), each atom has three free electrons, and at p-region, each atom has one hole.
During the diffusion process, the free electrons move from emitter (n-region) to base (p-region). Similarly, the holes move from base (p-region) to emitter (n-region).
At emitter-base junction, when the n-region (emitter) atoms meet the p-region (base) atoms, each n-region atom donates three free electrons to three p-region atoms. As a result, the n-region (emitter) atom which donates three free electrons will become a positive ion and the three p-region (base) atoms which accepts (each accept one free electron) three free electrons will become negative ions. Thus, each n-region (emitter) positive ion produces three p-region (base) negative ions.
Therefore, the depletion region at the emitter-base junction contains more negative ions than the positive ions. The negative ions reside at the p-region (base) near the junction and the positive ions reside at the n-region (emitter) near the junction.
Therefore, the depletion region is penetrated more towards the p-region (base) than the n-region (emitter).
Base-collector junction:
Let us assume that, at right side n-region (collector), each atom has two free electrons, and at p-region, each atom has one hole.
During the diffusion process, the free electrons move from collector (n-region) to base (p-region). Similarly, the holes move from base (p-region) to collector (n-region).
At base-collector junction, when the n-region (collector) atoms meet the p-region (base) atoms, each n-region (collector) atom donates two free electrons to two p-region (base) atoms. As a result, the n-region (collector) atom which donates two free electrons will become a positive ion and the two p-region (base) atoms which accepts (each accept one free electron) two free electrons will become negative ions. Thus, each n-region (collector) positive ion produces two p-region (base) negative ions.
Therefore, the depletion region at the base-collector junction contains more negative ions than the positive ions. The negative ions reside at the p-region (base) near the junction and the positive ions reside at the n-region (collector) near the junction.
Therefore, the depletion region is penetrated more towards the p-region (base) than the n-region (collector).
However, the depletion layer width at the collector side is more than the depletion layer width at emitter side. This is because the collector region is lightly doped than the emitter region.
Biased npn transistor
When external voltage is applied to an npn transistor, it is said to be a biased npn transistor. Depending on the polarity of the applied voltage, the npn transistor can be operated in three modes: active mode, cutoff mode and saturation mode.
The npn transistor is often operated in active mode because in active mode the npn transistor amplifies the electric current.
So let’s see how an npn transistor works in active mode.
Let us consider an npn 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 free electrons in the left side n-region (emitter) experience a repulsive force from the negative terminal of the DC battery and also they experience an attractive force from the positive terminal of the battery. As a result, the free electrons start flowing from emitter to base. In the similar way, holes in base experience a repulsive force from the positive terminal of the battery and also experience an attractive force from the negative terminal of the battery. As a result, the holes start flowing from base to emitter.
Due to the applied external voltage, each emitter atom has more than one or two free electrons. Hence, each emitter atom donates more than one or two free electrons to more positive ions. As a result, the positive ions become neutral. Similarly, each base atom accepts more number of electrons from more negative ions. As a result, the negative ions become neutral. We know that depletion region is nothing but combination of positive ions and negative ions.
Thus, the depletion width at the emitter-base junction reduces by applying the forward bias voltage.
We know that electric current means flow of charge carriers. The free electrons (negative charge carriers) flow from emitter to base whereas holes (positive charge carriers) flow from base to emitter. These charge carriers conduct electric current. However, the conventional current direction is same as the direction of holes.
Thus, the electric current flows from base to emitter.
Base-collector junction:
Due to the reverse bias, a large number of free electrons in the right side n-region (collector) 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 collector atoms loses electrons and becomes positive ions. On the other hand, holes in the p-region (base) 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 base atoms gains electrons and becomes negative 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.
Collector-base-emitter current:
The free electrons that are flowing from emitter to base due to forward bias will combine with the holes in the base. However, the base is very thin and lightly doped. So only, a small percentage of emitter free electrons combines with the holes in the base region. The remaining large number of free electrons will cross the base region and reaches to the collector region. This is due to the positive supply voltage applied at collector. Hence, free electrons flow from emitter to collector. At collector, both the emitter free electrons and collector free electrons produces current by flowing towards the positive terminal of the battery. Therefore, an amplified current is produced at the output.
In npn transistor, the electric current is majorly conducted by free electrons.
