SOLUTION

A Bipolar Junction Transistor (BJT) is a three-layer, two junction semiconductor device consisting of either two N-type and one P-type layer of material (NPN transistor) or two P-type and one N-type layer of material (PNP transistor).

There are three regions in which the transistors operate

• Active Region
• Saturation region
• Cutoff region

Active Region: The transistor is said to operate in an active region when the emitter junction is biased in forward direction and the collector junction in the reverse direction. The transistor is used as an amplifier when it is biased in an active region.

This active region of operation in a transistor is the most sensitive region of operation in a transistor. The variation of output collector current depends on the input emitter current variations. Even though the magnitude of an output current is small compared to that of the input current, the variation is very sensitive.

• In order to increase the value of current gain (α), the base region is lightly doped and also the thickness is very small compared to either the emitter region or collector region. Since the value of α is approximately equal to 1, the magnitude of the collector current and emitter current is almost the same.
• But since the emitter junction is forward biased, the potential drop across the junction is small (cutin voltage) while the collector junction is reverse biased and so the drop across the collector junction is very large.
• Due to this reason, the power dissipated in the emitter junction is small compared to the power dissipated in the collector junction.
• Hence, the collector should be designed to dissipate the unused energy efficiently. So, the area or, in other words, the volume of the collector is the maximum in a transistor.
• That is the reason why the metal can encapsulate of a general commercially available transistor is connected to the collector electrically. So, in general, the collector region has the maximum thickness and high doping levels and the emitter is slightly less doped and has a smaller area compared to the collector. The base is the least doped region in the transistor and also the thickness is at the minimum.