What Is a Transistor?

Transistors amplify weak signals and switch electrical currents. They are compact in size and operate at high speeds. This has made them an essential part of electronic circuits.

Transistors are made of semiconductor materials like silicon and germanium. Semiconductors are neither conductors nor insulators, but somewhere in between. By applying small current or voltage to a transistor’s base, you can control the flow of electricity through the collector and emitter.

Basic Concepts

The transistor is a key component in electronic circuits. It acts like an electronic switch and can turn a current ON or OFF by applying a voltage on the third terminal called the base. It is made from semi-conductive materials and works by exploiting their properties.

The basic concept behind this device is to sandwich one semiconductor material between two other semiconductors to create a P-N or N-P-N junction. This gives the transistor its amplification and switching capabilities, allowing it to control larger amounts of current with small input signals and power.

When the PN junction is biased by a dc power supply, it can be in either of two states: forward bias or reverse bias. The state of the PN junction is determined by what type of doping was done on each chunk of the semiconductor. The N chunks are doped to be more positive and the P bits are doped to be more negative. This gives us the two transistor types, NPN and PNP.

A good way to test a transistor is to use a multimeter on continuity testing mode. This will tell you if the transistor is ON or OFF and can help you diagnose any problems with your circuits. When a transistor is ON it is in the active region and will have a low base-emitter voltage (VBE) with zero collector current. When the transistor is OFF, it will be in the cut-off or saturation regions.

Emitter

An emitter is the end of a transistor from which current enters the device. It can be used to control the flow of electrons and other electrical signals in a circuit. The term can also be applied to the part of an organism that releases chemicals or other substances for protection or signaling.

In the NPN (n-type and p-type) transistor, which has one p-type material between two n-type materials, electrons flow from the battery management emitter to the collector region when the base terminal is low. When a higher voltage is applied to the base terminal, the current from the emitter to the collector increases. This allows the device to amplify weak signals into strong ones, and acts as a switch between on and off conditions in a circuit.

The left side of the NPN transistor is the emitter, which emits electrons when the base terminal is not at a high voltage. The right side is the collector, which collects the electrons from the emitter. The collector is a bit larger than the emitter and base, and it is moderately doped.

Transistor types are standardized, but the assignment of functions to the different terminals can vary from one package to another. For example, the prefix 2N indicates a three-terminal device, while 3N is a four-terminal dual-gate field-effect transistor. The suffix letter of the transistor part number is often an indication of its function, and early devices typically have lower numbers than later versions.

Base

The base of the transistor is the region that, when connected to a positive battery voltage, causes current to pass from the emitter to the collector. It is very thin and moderately doped, allowing majority charge carriers (electrons in case of an NPN transistor and holes in case of a P-N-P transistor) to diffuse from the emitter through the base into the depletion region of the base-collector junction, where they can be collected by the strong electric field.

If the base were thicker, as in a pair of back to back diodes, most or all of the minority carrier recombination would take place within the base, and they programmable logic devices pld would not reach the collector at all. This magic only works because the base is so thin, and this is why the base is a very important part of the transistor.

For a transistor to be effective, it needs to have a current gain hFE that is at least five times the ratio of its base current Ib to its collector current Ic. A value of hFE of 100 to 300 is typical for small signal transistors. The base resistor (RB) should be chosen to allow Ib to reach this value, so that the transistor is fully saturated when switched on. The collector-base voltage Vc is then proportional to IC/Ib, as shown in the figure below.

Collector

A transistor is a semiconductor device that allows current to flow from one terminal to another, and can also control or amplify the current. It consists of three terminals, and is an important part of most electronic devices. Invented in 1947 by John Bardeen, Walter Brattain, and William Shockley of Bell Labs, it revolutionized the electronics industry because it was much smaller than the vacuum tubes that preceded it.

When the base terminal of a transistor is given a voltage, it opens up the barrier between the emitter and collector. The small current passing through the base controls the flow of a larger current between the two terminals, which is the basis for the transistor’s amplifier properties.

In a NPN transistor, the negative terminal of the battery (VEE) provides a potential that repels electrons from the emitter region and causes them to move into the base region. A very low percentage of these electrons recombine with free holes in the base, which constitutes the small base current IB. The rest escape from the base to become the collector current IC.

The collector region of a transistor is physically larger than the emitter and base regions, and is moderately doped to allow the carriers to pass through easily. The collector is often indicated on a transistor symbol with an arrow head, although other transistor types can use different symbols and assign different functions to their collector terminals.