Transistor is a semiconductor device with three terminals viz., Emitter (E), Base (B) and Collector (C) and thus has two junctions viz., Base-Emitter (BE) junction and Base-Collector (BC) junction as shown by Figure 1a. Such a device can operate in three different regions viz., cutoff, active and saturation. Transistors are fully-off in cut-off region while fully-on when operating in saturation region. However, while they operate in active region, they act as amplifiers i.e. they can be used to increase the strength of the input signal without altering it significantly.The reason behind such a behavior can be understood by analyzing the working of transistor in terms of charge carriers. For this, let us consider a npn bipolar junction transistor(BJT) biased to operate in active region (BE junction is forward biased while BC junction is reverse biased) as shown by Figure 1b.

Here, in general, the emitter will be heavily doped, the base will be lightly doped and the collector will be moderately doped. Further the base will be narrow, the emitter will be broader and the collector will be much broader.

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