Electronics Components: Transistor Specifications - dummies

Electronics Components: Transistor Specifications

By Doug Lowe

There are many different kinds of transistors. The most basic kind is called a bipolar transistor. Another variety of transistor, called a field-effect transistor (FET), has become extremely popular in recent years, especially as the building blocks for integrated circuits (also known as ICs). Field-effect transistors can be made much smaller than bipolar transistors, and they use much less current.

Bipolar transistors for the electronics hobbyist

Bipolar transistors are the easiest to understand, and they’re the ones you’re most likely to work with as a hobbyist.

There are three leads on a bipolar transistor and each of these leads is given a name:

  • Collector: This lead is attached to the largest of the semiconductor regions. Current flows through the collector to the emitter as controlled by the base.

  • Emitter: Attached to the second largest of the semiconductor regions. When the base voltage allows, current flows through the collector to the emitter.

  • Base: Attached to the middle semiconductor region. This region serves as the gatekeeper that determines how much current is allowed to flow through the collector-emitter circuit. When voltage is applied to the base, current is allowed to flow.

These two current paths are important in a transistor:

  • Collector-emitter: The main current that flows through the transistor. Voltage placed across the collector and emitter is often referred to as Vce, and current flowing through the collector-emitter path is called Ice.

  • Base-emitter: The current path that controls the flow of current through the collector-emitter path. Voltage across the base-emitter path is referred to as VBE and is also sometimes called bias voltage. Current through the base-emitter path is called IBE.

Field-effect transistors for integrated circuits

Field-effect transistors behave much like bipolar transistors, but they have a nomenclature all their own: Instead of base, emitter, and collector, the terminals in a field-effect transistor are called the gate, drain, and source.

Internally, a field-effect transistor is very different from a bipolar transistor. Instead of using a pair of p-n junctions, a field-effect transistor consists of a single piece of n- or p-type semiconductor with a special substance placed on it that can control the current flow through the semiconductor.

There are a dozen or so different types of field-effect transistors in existence, but the most commonly used are called MOSFET (for metal-oxide-semiconductor field-effect transistor) and JFET (for junction field-effect transistor).

Be warned that field-effect transistors are quite susceptible to accidental static discharge. If you touch one and hear a little pop as static in your skin discharges through the FET, you may as well throw it away. You should always take precautions against static discharge whenever you handle a field-effect transistor or an integrated circuit that contains field-effect transistors.