The Bipolar Junction Transistor
The transistor was invented by John Bardeen and W.H. Brattain in 1948. A transistor often called as Bipolar Junction Transistor (BJT) conducts due to both electrons and holes. BJT have two junctions and three terminals.
There are two types of transistor:
1. N-P-N Transistor
2. P-N-P Transistor
A transistor consist of three regions with three pins named as emitter, collector and base.
- EMITTER: The doping concentration in this region is higher then the rest two. This region transfer majority charge carriers to base.
- BASE: This region is very lightly doped as compared to emitter and collector. This act as a path for majority charge carriers to reach the collector.
- COLLECTOR: This region is moderately doped and its function is to collect majority charge carriers from base.
In the absence of external power supply the transistor is called as in unbiased condition.
The transistor is just a combination of two diodes. The junction between emitter and base is called emitter -base junction and between base and collector is called collector-base junction. In unbiased condition, there is no battery connected. The width of the two junctions will be different because the regions are doped at different ratios.
The depletion region penetrates more to lightly doped side in order to include equal no. of impurity atom. Therefore, the depletion region at emitter junction penetrate more to base side.
The depletion layer formed at collector region is much larger then formed at emitter region.
In active region, the emitter-base junction is forward biased and collector-base junction is reverse biased.
The potential at emitter-base side is VEE and between collector and base is VCC.
The forward-biased emitter has low resistance and reverse-biased collector has high resistance. So, a transistor transfer a signal from low resistance to high resistance.
Current Component in a Transistor
The emitter current consist of:
- Hole current IpE constituted by holes.
- Electron current InE constituted by electrons.
So, total emitter current:
IE=IpE + InE
Thus for a P-N-P Transistor
IE = IB + IC