In one line: By adding tiny amounts of impurity to silicon, engineers make devices that conduct in one direction (diodes) or that let a small signal control a large one (transistors). Every amplifier and every logic gate is built from them.
Core idea
Pure silicon conducts poorly. Doping it with small amounts of other elements adds free charge carriers:
- n-type silicon has extra electrons.
- p-type silicon has “holes,” places where an electron is missing.
Where n-type and p-type regions meet, a pn junction forms. It conducts easily in one direction and blocks the other, and that one-way behavior is the basis of all semiconductor devices.
How it works
Diodes:
- A silicon diode conducts once forward-biased by about 0.7 V, and blocks reverse voltage.
- Rectifiers use diodes to turn AC into DC. A half-wave rectifier passes one polarity. A full-wave bridge of four diodes flips the negative half-cycles positive.
- Zener diodes are designed to operate in reverse breakdown at a precise voltage, and are used as simple voltage regulators.
- Light-emitting diodes and photodiodes convert between current and light.
Transistors are three-terminal devices in which a small input controls a large current:
- Bipolar junction transistors (BJTs) are current-controlled. The collector current is roughly β times the base current, with β often around 100.
- MOSFETs are voltage-controlled. The gate voltage opens or closes a conducting channel, and almost no current flows into the gate. Complementary pairs of n-channel and p-channel MOSFETs make up CMOS, the technology of nearly all digital chips (see Digital Logic).
Two ways to use a transistor:
- As a switch, fully on or fully off: logic, power control, driving motors and LEDs.
- As an amplifier, biased in its active region. A common-emitter stage gives large voltage gain, with inversion. A common-collector stage, the “emitter follower,” gives no voltage gain but drives heavy loads from weak sources, acting as a buffer.
In practice
For non-specialists, the useful questions concern operating limits: maximum voltage, current, power dissipation and temperature. Datasheets give all of them, and exceeding any one destroys the part. The history is also worth knowing. The transistor, invented at Bell Labs in 1947, and the integrated circuit a decade later turned electronics from a craft of discrete components into an industry that doubled its capability every couple of years. Many engineering careers, such as that of Bernard M. Gordon, rode that wave.
Questions to ask the experts
- Is this device used as a switch or as an amplifier, and what limits its speed?
- How close is it running to its voltage, current and thermal limits?
- What happens to the circuit if the device’s gain or threshold varies from part to part?
Connections
- The discipline: Electrical and Computer Engineering. Built from them: Operational Amplifiers, Digital Logic.
- An engineer whose career they shaped: Bernard M. Gordon.
- Capabilities exercised: Inquiry, Connecting Across Disciplines, Vision.
Sources
- Course lecture on semiconductor devices; no public source.