5.1 The Bipolar Transistor - Introduction

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In this section:

Introduction
Structure

5.1.1 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.

5.1.2 Structure of the bipolar transistor

The device structure, sign convention and circuit diagram are shown in figure 5.1. The device consists of three alternating n-type and p-type regions. A NPN structure is shown which will be used throughout this text to explain the device operation and to derive the device equations. Alternatively one can also construct and analyze a PNP structure.

bjt2.gif

Fig.5.1 Structure (left) and circuit symbol (upper right) of an NPN bipolar junction transistor (BJT). Also shown are the two p-n diodes (lower right) within the bipolar transistor.

The physical widths of the emitter, base, and collector are indicated on the figure with the associated symbols, w_E, w_B and w_C. Two p-n diodes exist in the three layer structure, namely the base-emitter diode and the base-collector diode. The diodes are shown in the lower right corner of figure 5.1. It should be noted that the two diodes do not represent a complete equivalent circuit of a bipolar transistor since the transport of minority carriers through the base is not included.

The two diodes are biased by the respective voltage sources. The sign convention indicated on the figure is such that the diodes are forward based when a positive voltage is applied. The collector and base currents are considered to be positive when current flows into the terminals while the emitter current is considered to be positive if the current flows out of the emitter terminal.

These "quasi-neutral" regions are neutral only in thermal equilibrium. However, when voltages are applied one finds that the charge densities and the electric field in these regions are significantly smaller than in the depletion regions. We will therefore treat these regions as if they were neutral and refer to them as being "quasi-neutral".

¹A p-n diode similarly acts as a current source when illuminated with light, provided that the diode is reversed biased. A p-n diode illuminated with light is therefore a light controlled current source. The combination of a light emitting diode - which is a current controlled light source - and a photodiode or solar cell yields a device with external characteristics which are very similar to that of a bipolar junction transistor.

5. ¬ ® 5.2