Perhaps the most popular type of transistor is the bipolar junction transistor (BJT). Bipolar junction transistors are three-terminal devices, called the emitter, base, and collector. In an NPN transistor, the emitter and collector are N-type material and the base is P-type material. In a PNP transistor, the emitter and collector are P-type, while the base is N-type. The base is sandwiched between the base and emitter, so there is a diode junction between the base and the collector and the base and emitter.
Refer to Figure E6-1 above. In Figure E6-1, the schematic symbol for a PNP transistor is #1. (E6A07) #2 is the schematic symbol for an NPN transistor. The arrow in both symbols shows the direction of the current flow.
When the base-emitter diode is forward-biased, a current, called the base current will flow. If there is an appropriate voltage between the collector and emitter, this small base current will cause a much larger current to flow between the collector, through the base to the emitter. The amount of base current controls how much collector current flows. This is how transistors amplify signals.
The change in collector current with respect to base current is the beta of a bipolar junction transistor. (E6A06) This is also sometimes called the hfe or current gain of a transistor. The change of collector current with respect to emitter current is the alpha of a bipolar junction transistor. (E6A05)
Another important characteristic of a bipolar transistor is the alpha cutoff frequency. This is a measure of how high in frequency a transistor will operate. Alpha cutoff frequency is the frequency at which the grounded-base current gain of a transistor has decreased to 0.7 of the gain obtainable at 1 kHz. (E6A08)
What resources have you used for learning how transistors work?