How does a transistor work?

Answer 1

There are two kinds of transistor: Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). These two transistors work in two different ways. However, both types of transistors are based on the fact that when a voltage is applied to impure semiconductor such as Silicon, it changes from a conductor to an insulator or vice versa.

Field Effect Transistors contain a narrow conductive channel which passes near a "Gate" electrode. The two ends of this channel are connected to terminals called "Source" and "Drain." When a voltage of the correct polarity is applied between the Gate and the transistor channel, the channel becomes wider, and if this polarity is reversed, the conductive channel narrows, or it even vanishes entirely. By changing the size of this conductive channel, the FET behaves as a voltage-controlled valve or switch. Since the Gate does not require any continuing current, the FET operation can be improved by including a layer of insulating glass (Silicon oxide) between the gate electrode and the rest of the transistor. This type of FET is called a "MOSFET," for Metal-Oxide-Semiconductor layers. Transistors without the glass layer are called JFETs or "Junction-FETs."

Bipolar transistors are composed of three segments called "Collector," "Base," and "Emitter." A thin insulating layer arises naturally between Base and Emitter. If a voltage of the correct polarity is applied to the Base and Emitter terminals, the insulating layer becomes so thin that it behaves as a conductor. If this voltage polarity is reversed, the insulating layer becomes wide. By changing the thickness of this insulating layer, the BJT behaves as a voltage-controlled valve or switch. HOWEVER, whenever the Base-Emitter voltage is causing the insulator layer to become thinner, there also is a leakage current in the base terminal. This tiny current is proportional to any larger current passing through the entire transistor. Although the BJT is controlled by the voltage between Base and Emitter, designers usually ignore the base-emitter voltage, and the BJT is treated as a current-controlled valve or switch.

All transistors are made of a "doped" semiconductor, typically silicon. Pure silicon contains almost no movable charges, so it behaves as an insulator. To create transistors, the silicon crystal has impurities deliberately introduced during manufacture. Each impurity atom will "donate" a movable charged particle in the silicon, which changes the silicon into a conductor. Doped silicon is very different than metal conductors. The charges within metals behave like a dense liquid, while the charges in doped silicon behave as a highly compressible gas. Doped semiconductor is a special kind of conductor where an externally-applied voltage can easily "compress" or sweep the charges away. By sweeping the charges away, the semiconductor is changed from a conductor back into an insulator. Semiconductors are like electric switches or valves, but with no moving parts.