Complete 3 page APA formatted essay: Electric circuits and devices prototyping.
A BJT is a current controlled device and has three terminals i.e. base, collector and emitter. The base determines the current in the emitter and the collector output. Actually BJT is a piece of silicon with three regions that have two junctions namely n and p.  An NPN transistor and the PNP transistor are examples of the two types of BJTs. The charge carriers in these two types of BJTs differ i.e. a PNP has electrons as its primary carrier, while NPN has holes as their primary carriers. NPN and PNP transistors practically have identical operation principles with the only difference being in biasing and in the polarity of the power supply for each type. 
Typically, BJT has four distinct regions of operations. these are the forward active, reverse active, saturation and cutoff. Therefore, a BJT can operate in different modes depending on the junction bias. For instance, when base-emitter junction is forward biased and the base-collector junction is reversed biased, then the device is in the forward active region mode of operation.  The device is in reverse active region of operation when the base-collector junction is forward biased while base-emitter junction is reversed biased. The saturation mode occurs when there are forward bias potentials in both base-emitter and base-collector junctions. However, when both junctions are reverse biased then the device is in cutoff region of operation. 
Since a BJT is three terminal device, it can be connected in three possible ways with one terminal being common for both input and output. These three configurations include common base, common collector and common emitter configurations. The common base configuration has high voltage gain with no current gain while the common emitter has gain for both current and voltage. The common emitter configuration has a current gain with no voltage gain. 
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