Bipolar-Junction Transistors

Bipolar-junction transistors (BJTs) are extremely common, versatile devices. They can make excellent oscillators, amplifiers, and switches, and are an important component of most modern circuits. They are quite different from Field Effect Transistors,or FETs.

Most BJTs today are made of silicon, but they have been manufactured using germanium, gallium arsenide, and other semiconducting materials.

The P-N junction
The BJT is based on the P-N junction, a joint between doped silicon which has an excess of free electrons (N for negative) and doped silicon with an excess of holes, locations in the crystal lattice where electrons can fit and be absorbed (P for positive).

A diode consists of a single P-N junction. When the P material is connected to a positive voltage and N a negative voltage, current flows through the junction with a voltage drop of about 0.6 volts. When P is connected to negative and N to positive, no current flows.

NPN vs. PNP
A BJT consists of two P-N junctions immediately adjacent to each other, close enough that electronics crossing one junction can interact with the other. It can either be an NPN transistor, with a piece of P material between two pieces of N material, or a PNP transistor, with a piece of N between two pieces of P. The two types of device behave exactly the same except for the reversal of polarity.

NPN devices are used somewhat more often than PNP devices.

Terminals
There are three terminals on each transistor, known as Emitter, Base, and Collector. A small current flowing between base and emitter can effectively control a much larger current flowing between collector and emitter. (Note that emitter and collector are not interchangable, although the symmetry of NPN or PNP might lead one to think that they are.)

Operating Regions
A BJT can operate in one of three regions: the active region, the cutoff region, and the saturated region.

In the cutoff region, no current flows into the base. The voltage at the base is typically close to that at the emitter. The transistor can be viewed as turned off.

In the active region, the base/emitter junction is forward biased and the collector/base junction is reverse biased. The current flowing from collector to emitter is given by


 * $$I_{ce} = \beta I_{be}\,$$

where $$I_{ce}\,$$ is the collector/emitter current, $$I_{be}\,$$ is the base/emitter current, and $$\beta\,$$ is a constant that depends on the design of the transistor. $$\beta\,$$ is also known as $$h_{fe}\,$$, and typically has a value in the range of 100 to 200.

In the saturated region, the voltage on the base is high enough that the collector/base junction loses its reverse bias. In full saturation, a silicon BJT will have a base voltage about 0.2 volts below the collector voltage. The collector/emitter current will be at a maximum, and the transistor is fully turned on. $$\beta\,$$ no longer predicts the relationship between $$I_{ce}\,$$ and $$I_{be}\,$$ in this region.

When a BJT is used as a switch or oscillator, it moves back and forth between cutoff (off) and saturated (on). Most amplifier designs attempt to keep the transistor in the active region, in order to keep the response linear.