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

Transistor Structure

Transistor as Current Amplifier

The larger collector current is proportional to the base current according to the relationship , or more precisely it is proportional to the base-emitter voltage . 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: