Bipolar Junction Transistors:

• Emitter layer is heavily doped region.
• Collector layer is mediocre doped region.
• Base layer is lightly doped region.

The lower doping level decreases the conductivity (increasing the resistance) of the material by limiting the “free carriers”.

The term “BIPOLAR” reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material.

If only one carrier is employed (electron or hole) than it is called a “UNIPOLAR”device.

Transistor Operation For A PNP Transistor:

1. If we disconnect the base to collector bias we can compare the present state of PNP transistor to the forward bias diode.
   1. The depletion region, in this case, would have been reduced in width due to the applied bias;
   2. Resulting in heavy flow of majority charge carriers from p to n-type material in the transistor;
2. If we disconnect the emitter to base bias we can compare the present state of PNP transistor to the reversed bias diode.
   1. The depletion region in this case would have increased its width due to the applied reverse bias;
   2. Resulting in very less flow of minority charge carriers from n to p-type of the transistor.
3. Hence, we see that in proper DC biasing of the PNP transistor, one of the PN junction is forward biased and the other has to be reversed biased. 
4. When both the bias are applied for normal functioning of the PNP Transistor:
   1. In this case, emitter-base bias is forward biased and collector base bias is reversed so, the depletion region formed between emitter-base junctions would be of smaller width whereas, collector-base junction would have a large depletion region.
   2. As a result majority carriers would flow from the emitter (p region) to base (n region) and these carriers will flow into the collector (p region) directly as they experience large resistance at the base terminal due to its low doping level. This results in a very low base current (Ib) which is in the range of microamperes whereas, emitter and collector currents (Ie and Ic respectively) are in the range of milliamperes.
      Ie = Ic + Ib
   3. Collector currents comprises of two components:-
      1. Collector current due to majority charge carriers. (IC_majority);- this is also known as the leakage current. 
      2. Collector current due to minority charge carriers with emitter terminal open. (Ico_minority);- this is measured in nanoamperes and can be compared to reverse saturation current of a diode (Is).

 Ic = IC_majority   +  Ico_minority

Configurations:

1. Common Base Configurations: 
   1. Ic approximately equals to I­e   in active region.
   2. In cut-off region Ic = 0A; (cut off region is the region that develops as a result of the condition when both junctions are reversed biased).
   3. In saturation region (when both junctions are forward biased), Ve = 0.7V(always for all cases.) 
   4. Alpha(dc) = Ic / Ie ; 
   5. Value of alpha is always approaching 1 alpha = 0.996 in real life conditions (0.9). 
2. Common Emitter Configurations:
   

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