Bipolar Junction Transistor (BJT)

BIPOLAR JUNCTION TRANSISTOR (BJT)

INTRODUCTION

A Bipolar Junction Transistor (BJT) has three layers of semiconductor material. These are arranged either in npn sequence or in pnp sequence and each of the three layers has a terminal. A small current at the central regional terminal controls the much larger total current flow through the device. This means that the transistor can be used for current amplification. It can also perform voltage amplification.

CONSTRUCTION

PNP and NPN Transistors

A bipolar junction transistor is simply a sandwich of one type of semiconductor material (p-type or n-type) between two layers of the opposite type.

A block representation of a layer of p-type material between two layers of n-type is shown in Fig. 1.33(a). This is described as an npn transistor. Fig. 1.33(b) shows a pnp transistor, consisting of a layer of n-type material between two layers of p-type.

The centre layer is called the base, one of the outer layers is termed the emitter, and the other outer layer is referred to as the collector.

The emitter, base, and collector are provided with terminals, which are appropriately labeled E, B and C.

Two pn-junctions exist in each transistor.

i. The collector base junction and

ii. The emitter base junction

Circuit symbols for pnp and npn transistors are shown in Fig. 1.34. The arrow head on each symbol identifies the transistor either terminal and indicates the conventional direction of current flow. For an npn transistor, the arrowhead points from the p-type base to the n-type emitter. For a pnp device the arrowhead points from the p-type emitter to the n-type base. Thus the arrowhead always points from p to n.

Two transistors packages are there

i. Low power transistor 1 mA to 20 mA current levels

ii. High power transistor - 100 mA to several amps range.