BJT.1 Basic Operation

A bipolar junction transistor is a three-terminal device that, in most logic circuits, acts like a current-controlled switch. If we put a small current into one of the terminals, called the base, then the switch is “on”—current may flow between the other two terminals, called the emitter and the collector. If no current is put into the base, then the switch is “off”—no current flows between he emitter and the collector.

To study the operation of a transistor, we first consider the operation of a pair of diodes connected as shown in Figure BJT-1(a). In this circuit, current can flow from node B to node C or node E, when the appropriate diode is forward biased. However, no current can flow from C to E, or vice versa, since for any choice of voltages on nodes B, C, and E, one or both diodes will be reverse biased. The pn junctions of the two diodes in this circuit are shown in (b). Now suppose that we fabricate the back-to-back diodes so that they share a common p-type region, as shown in Figure BJT-1(c). The resulting structure is called an npn transistor and has an amazing property. (At least, the physicists working on transistors back in the 1950s thought it was amazing!) If we put current across the base-to-emitter pn junction, then current is also enabled to flow across the collector-to-base np junction (which is normally impossible) and
from there to the emitter.

The circuit symbol for the npn transistor is shown in Figure BJT-1(d).
Notice that the symbol contains a subtle arrow in the direction of positive current flow. This also reminds us that the base-to-emitter junction is a pn junction, the same as a diode whose symbol has an arrow pointing in the same direction.

It is also possible to fabricate a pnp transistor, as shown in Figure BJT-2.
However, pnp transistors are seldom used in digital circuits, so we won’t discuss them any further.

The current Ie flowing out of the emitter of an npn transistor is the sum of the currents Ib and Ic flowing into the base and the collector. A transistor is often used as a signal amplifier, because over a certain operating range (the active region) the collector current is equal to a fixed constant times the base current.

However, in digital circuits, we normally use a transistor as a simple switch that’s always fully “on” or fully “off,” as explained next. Figure BJT-3 shows the common-emitter configuration of an npn transistor, which is most often used in digital switching applications. This configuration uses two discrete resistors, R1 and R2, in addition to a single npn transistor. In this circuit, if VIN is 0 or negative, then the base-to-emitter diode junction is reverse biased, and no base current (Ib) can flow. If no base current flows, then no collector current (Ic) can flow, and the transistor is said to be cut off (OFF).