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Transistors 101: The Junction Transistor

To understand how a transistor works, first consider the lowly diode. It is a simple union of the two most fundamental kinds of semiconductor, known as n-type and p-type. Both conduct current, but the n-type does it with electrons, while the p-type depends on electron deficiencies, better known as holes. Joining these two types of semiconductors forms what is known as a pn junction at their boundary. This is the core of a semiconductor diode, which conducts current in one direction.

Connect a battery's positive terminal to the n-type material [figure A, top] and electrons are attracted to that terminal, while holes in the p-type material move toward the negative terminal. In other words, charge carriers stream away from the junction, expanding a barren volume, aptly called the depletion region. The diode is said to be reverse-biased, and hardly any current flows.

Now reverse the battery connections [figure A, bottom]. Electrons in the n-type material move toward the junction and are constantly replenished by the battery. Meanwhile, holes in the p-type material stream toward the junction, repelled by the positive battery terminal. The depletion region shrinks tremendously as holes and electrons combine at the junction, neutralizing one another, as more stream in on either side from the battery. The diode is said to be forward-biased; current flows easily. Thus, a diode can control the direction of current, but not how large it is.

 

A transistor, on the other hand, can control how much current goes through it and also act as an amplifier. The simplest transistor has three parts: an emitter, a base, and a collector. Think of the transistor as a sandwich of two pn junctions back to back [figure B] in either npn or pnp order; they operate similarly.

In an npn transistor, for example, the n-type emitter has many extra electrons, the relatively thin p-type base has a small number of holes, and the n-type collector has a moderate number of electrons. (Junction transistors are also known as bipolar devices because, in the emitter, holes and electrons flow in opposite directions.) A transistor amplifier takes a small, varying voltage—an input signal—between the base and the emitter, and uses it to control a larger current flowing from the emitter to the collector. That's the output. The key agents in this amplification are the depletion regions. With two pn junctions in the device, there are two depletion regions: one between the emitter and the base, the other between the base and the collector.

First, the emitter-base diode is forward-biased by a voltage source [left in figure B]. Electrons flow from the emitter into the base. The base-collector diode, on the other hand, is reverse-biased, so that holes will not flow into the base, which would intercept any electrons coming across from the emitter and therefore block current from flowing through the device.

 

With this setup, the current through the transistor, from emitter to collector, is controlled by the depletion region around the emitter-base junction. When it is thick, the current is choked off; when it is thin, lots of current flows through the device. But hold on—when it is thin, and electrons shoot across the emitter-base junction, aren't they blocked by the fat depletion region around the base-collector junction? No—the base is narrow, so the momentum of the electrons pouring in from the emitter brings them close to that junction. From there, the positive voltage at the junction then sweeps most of the electrons into the collector. Only a few are lost in the base as they move into the vacant holes.

The transistor is designed so that the flow of electrons from emitter to collector is very sensitive to the current into the base. This is done by making the base very thin (so electrons don't have far to go before reaching the collector) and using low doping (electrons cannot easily find vacant holes to fill). The voltage across the base-emitter junction provides the electric field that drives electrons from the base into the collector.

With the emitter-base junction forward-biased, a varying voltage put on top of it—an input signal—varies the depletion region, which in turn varies a relatively large current flowing through the device. Add a load resistor in the collector circuit, and that small varying input produces a much larger varying collector voltage: the transistor amplifies the signal at the base. Depending on the circuit, the result will be current, voltage, or power amplification.

Although bipolar junction transistors have been surpassed for many applications by various forms of field-effect transistors, bipolars remain popular for applications involving high-frequency signals. They're found in countless modern electronic devices, including broadband Internet modems, set-top boxes, DVD players, and CD-ROMs.

By Alfred Rosenblatt