The Junction Transistor

	A bipolar junction transistor consists of three regions of doped semiconductors. A small current in the center or base region can be used to control a larger current flowing between the end regions (emitter and collector). The device can be characterized as a current amplifier, having many applications for amplification and switching.

Transistor as Current Amplifier

The larger collector current I_C is proportional to the base current I_B according to the relationship I_C =βI_B , or more precisely it is proportional to the base-emitter voltage V_BE . The smaller base current controls the larger collector current, achieving current amplification.

The analogy to a valve is sometimes helpful. The smaller current in the base acts as a "valve", controlling the larger current from collector to emitter. A "signal" in the form of a variation in the base current is reproduced as a larger variation in the collector-to-emitter current, achieving an amplification of that signal.

Transistor Structure

Transistor as Current Amplifier

The larger collector current I_C is proportional to the base current I_B according to the relationship I_C =βI_B , or more precisely it is proportional to the base-emitter voltage V_BE . The smaller base current controls the larger collector current, achieving current amplification.

Constraints on Transistor Operation

Transistor Maximum Values

Part of the manufacturer's data for transistors is a set of maximum values which must not be exceeded in its operation. These form some of the constraints on transistor operation which are a part of the design of any circuit. A typical set, for the silicon transistor 2N2222: