There are two bipolar transistor types, NPN and PNP. Bipolar is the description given to a transistor, which uses both electrons and holes (absence of electrons), as current carriers. The operating conditions are considered for the more popular NPN type, which consists of two PN junctions fused together to form one continuous piece of semiconductor material, with the connections as shown. Note the emitter arrow direction in the transistor symbol, this indicates the direction of base emitter conventional current flow. A transistor can be represented as two back-to-back diodes. The applied voltage polarities show the base-emitter junction to be a forward biased diode (low resistance path), whereas the collector-base is reverse biased (high resistance path). Connecting two diodes as shown will not perform the action of a transistor as the junctions must be fused together. In practice the piece of semiconductor is made extremely small with the base region measured as just a few microns in width. This is to enable electrons to pass from the emitter to the collector (current flow), attracted by the higher collector potential. TEST YOUR UNDERSTANDING: Taking each transistor theory topic in turn compile a list of the parameters you need to consider when designing a transistor circuit. Search online for a suitable transistor that could be used as an output stage for a portable audio amplifier.
