What is a Semiconductor?

by Reggie Huff

Electronic systems are built by connecting electronic components together on some kind of foundation. Typically, this foundation-or substrate-is a board with conductive paths painted on it to connect the components together. The function performed by the system is determined by which components are used and how those components are interconnected.

The components may be selected from three primary product groups:
• Passive Components - resistors, capacitors and inductors
• Electro-Mechanical Components - switches, solenoids, speakers, microphones, connectors, etc.
• Active Components - vacuum tubes and semiconductors

So...what IS a semiconductor anyway?
Today, the term "semiconductor" is used in several contexts, each with a slightly different meaning yet, ultimately, referring to the same underlying distinction. We might say "XYZ Co. is a semiconductor manufacturer," by which we mean that XYZ makes stuff that utilizes the semiconductor property. We might also say "I just got an order for most of the semis on their board!" In this context, we are referring to products which utilize the semiconductor property, and, most commonly, also have areas which are richly conductive and areas which are poorly conductive (resistive). In either case, we are distinguishing that the subject involves the semiconductor property.

What is the Semi-Conductive property?
All materials have some ability to conduct electricity. Some are very bad at this and we categorize these materials as Insulators. Wires are coated with insulator material to isolate the wire from other stuff which we don't want current passing to. Other materials are very good electrical conduits of electricity and these we categorize as Conductors (though this is a bit misleading since all materials conduct at least a little bit).

Conductors include copper, aluminum, impure water, conductive plastics, and folks sitting in an electric chair. Super Conductors are special materials that pose almost zero resistance to current flow. The Semi-Conductive property refers to the ability of something to act as a good conductor under certain conditions, and, under other conditions, to act as a good insulator. Your standard wall switch could be called a mechanical semiconductor in that it performs as a good conductor when pushed into the UP (on) position and performs as an excellent insulator when pushed into the DOWN (off) position.

A water solution might be called a liquid semiconductor since, when it is very, very pure (like DI Water), it is a good electrical insulator. As solutes are dissolved into the water, the solution becomes a good electrical conductor.

A brick wall is an excellent sun light insulator. It consistently and reliably prevents sunlight from passing through it. (Fig. A) A clean, clear glass window is an excellent sunlight conductor. Such a window consistently and reliably allows sun light to pass through it. (Fig. B)

Sun Light Insulator (Figure A)

Sun Light Conductor (Figure B)

So what would be the analogy for a sunlight semiconductor. That would be a venetian blind (Fig. C). These blinds provide sunlight conduction under certain conditions (OPEN) and sunlight insulation under other conditions (CLOSED).

Sun Light Semi-Conductor (Figure C)

While we have considered several examples of the semiconductive property, I have never heard anyone using the term "semiconductor" in such a context. When the word semiconductor is used we are almost exclusively referring to non-mechanical, Solid State products.

(Refresher for the interested reader: Solid state is a term that distinguishes a material in its SOLID STATE from one in its LIQUID STATE or GASEOUS STATE. Something in its solid state retains its shape, in its liquid state it takes the shape of its container, and in its gaseous state it fill its container.)

Germanium is a semiconductive material that acts as an electrical Insulator when it is below its threshold temperature. (Fig. D) When it is heated above that threshold temperature, it suddenly transforms into an electrical conductor. This might provide a simple means to control the engine overheating light in your automobile.

A Photo Cell is a semiconductive material that acts as an electrical Insulator when exposed to light. (Fig. E) In the dark it behaves as an electrical conductor. This is exactly how those little night-lites which you plug in the bathroom or kids' room are controlled.

In summary then, we can conclude that a semiconductor is a monolith in its solid state (they don't work so well when you melt or vaporize them) that acts as a good electrical conductor under some conditions and as an electrical insulator otherwise. Hence, we often use this stuff to make solid state switches that can be turned ON or OFF by changing some influential condition. Transistors are simply semiconductor devices that are turned ON or OFF by applying or removing a voltage or current.

"What is a Semiconductor?" first appeared on EBNONLINE.COM on August 30, 1999.

© Copyright 1999, 2000, 2001 Huff Communications. All Rights Reserved.