A bipolar junction transisto

A bipolar junction transistor is formed by joining three sections of semiconductors with alternatively di erent dopings. The middle section (base) is narrow and one of the other two regions (emitter) is heavily doped. Two variants of BJT are possible: NPN and PNP.

We will focus on NPN BJTs. Operation of a PNP transistor is analogous to that of a NPN transistor except that the role of \majority" charge carries reversed. In NPN transistors, electron flow is dominant while PNP transistors rely mostly on the flow of \holes." Therefore, to zeroth order, NPN and PNP transistors behave similarly except the sign of current and voltages are reversed. i.e., PNP = -NPN. In practice, NPN transistors are much more popular than PNP transistors because electrons move faster in a semiconductor. As a results, a NPN transistor has a faster response time compared to a PNP transistor.

At the first glance, a BJT looks like 2 diodes placed back to back. Indeed this is the case if we apply voltage to only two of the three terminals, letting the third terminal

oat. This is also the way that we check if a transistor is working: use an ohm-meter to ensure both diodes are in working conditions. (One should also check the resistance between CE terminals and read a vary high resistance as one may have a burn through the base connecting collector and emitter.)

The behavior of the BJT is di erent, however, when voltage sources are attached to both BE and CE terminals. The BE junction acts like a diode. When this junction is forward biased, electrons

ow from emitter to the base (and a small current of holes from base to emitter). The base region is narrow and when a voltage is applied between collector and emitter, most of the electrons that were flowing from emitter to base, cross the narrow base region and are collected at the collector region. So while the BC junction is reversed biased, a large current can flow through that region and BC junction does not act as a diode. The amount of the current that crosses from emitter to collector region depends strongly on the voltage applied to the BE junction, vBE. (It also depends weakly on voltage applied between collector and emitter, vCE.) As such, small changes in vBE or iB controls a much larger collector current iC. Note that the transistor does not generate iC. It acts as a valve ncontrolling the current that can flow through it. The source of current (and power) is the power supply that feeds the CE terminals.

Several "models" available for a BJT. These are typically divided into two general categories:

"large-signal" models that apply to the entire range of values of current and voltages, and "small-signal" models that apply to AC signals with small amplitudes. \Low-frequency" and"high-frequency" models also exist (high-frequency models account for capacitance of each junction). Obviously, the simpler the model, the easier the circuit calculations are. More complex models describe the behavior of a BJT more accurately but analytical calculations become dificult. PSpice program uses a high-frequency, Eber-Mos large-signal model which is a quite accurate representation of BJT. For analytical calculations here, we will discuss a simple low-frequency, large-signal model (below) and a low-frequency, small-signal model in the context of BJT amplifiers later.

Large signal model (Charge control model)

The charge control model of a bipolar transistor is an extension of the charge control model of a p-n diode. Assuming the "short" diode model to be valid, one can express the device currents as a function of the charges in each region, divided by the corresponding transit or lifetime. In the general case one considers the forward bias charges as well as the reverse bias charges. This results in: