The transistor is a multifunction semiconductor device that, when used with other circuit elements has the ability to produce a current gain, voltage gain and signal-power gain. The transistor is referred to as a passive device, while the diode is passive. The three basic types of transistor technologies are the bipolar transistor, the metal-oxide-semiconductor field effect transistor (MOSFET) and the junction field effect transistor (JFET). The bipolar transistor most often functions as a voltage-controlled current source.
The Bipolar Junction Transistor
The BJT has three separately doped regions and two pn-junctions, which are close enough to interact between each other. The BJT can either be constructed as an NPN or PNP transistor, which stands for the arrangement of positive and negatively doped regions.
The main connections of a BJT transistor are referred to as the collector, base and emitter. Generally, the emitter side is doped to a higher level than the collector. The result of this is that when a supplied a voltage, the electrons will flow in the direction from the emitter to the collector. The direction of current then will be from the collector to the emitter.
BJT Modes of Operation
There exist three modes of operation for the BJT transistor. In reference to the diagram below, when the Base-Emitter voltage is zero or reverse biased, the majority of carrier electrons from the emitter will not be injected into the base. This mode where all currents in the transistor are zero is referred to as cut-off. When the Base Emitter voltage is positive (forward biasing), an emitter current is generated. As the Base Emitter voltage increases, the collector current will continue to increase until a certain point at which both the Base Emitter and Base Collector junctions become forward biased. This mode is called saturation.