The Bipolar Transistor - Introduction

The bipolar junction transistor was invented in 1948 by Bardeen, Brittain and Shockley. It consists of a three-layer structure with alternating n-type and p-type regions as shown below. The three regions are referred to as the emitter, the base and the collector region and are contacted with an ohmic contact.

A first examination of the structure reveals that it contains two p-n diodes, one between the base and the emitter, and one between the base and the collector.

However, the thin base region between the two p-n diodes enables the transport of minority carriers through this region. It is the transport of minority carriers - injected from the emitter into the base - which results in a collector current. This collector current is almost independent of the voltage applied between the base and the collector, V_BC, as long as the base-collector diode is reversed biased. This causes the base-collector diode to act as a current source ¹ whose value is controlled by the base-emitter voltage, V_BE.

Proper design of the 3-layer structure also yields a collector current, which is very close in value to the emitter current. The current which needs to be supplied to the base, I_B = I_E - I_C - as obtained using Kirchoff's current law and the sign convention provided below - can therefore be significantly smaller than either the emitter or collector current. This leads to current amplification between the collector and the base: a small variation of the base current causes a larger variation of the collector current. The ratio of the two, dI_C/dI_B, is refered to as the current gain, b.

The reader should note that derivation of the current gain starts by calculating the collector and emitter current as a function of the base-emitter voltage. A direct calculation of the collector current as a function of an applied base current is not possible even though such approach may seem more logical, especially to anyone who has used a bipolar junction transistor as a circuit element.