Conductors versus Semiconductors
|The comparison between the behavior of a normal electric circuit and a semiconductor graphically illustrates the differences that make a semiconductor so useful and unique.
The circuit on the top left is very similar in layout to the field effect transistor circuit on the top right. The difference is that copper wire (which is a conductor) now replaces the n-type semiconductor. Current flows between A and B in one case through the copper wire in the other through the n-type semiconductor.
In the lower images C is charged negative and placed near but not touching the copper wire. Its counterpart, the gate of the transistor, is charged negative and is separated from the n-type semiconductor layer by a layer of insulation. Terminals A and B in the circuit (lower left) are charged in the same way as the source and drain of the semiconductor (lower right).
When the gate is charged negative, the current in the FET switches off immediately. This is because the negative charge on the gate repells the electrons flowing through the channel and stops the current flow.
What happens to the current flowing in the copper wire (a conductor) when a large negative charge is placed near it? Does it switch off like the current in the semiconductor?
The answer is no. In striking contrast to the behavior of the transistor, the current in the copper wire is unaffected by the large negative charge place near it. There are so many free electrons moving around in the copper wire that they easily shield the circuit. Positive charges move toward the outer edge of the wire facing the large negative charge. The positive and negative charges will continue to move until there is no electric field in the wire. Now if a voltage is added across A and B, the same current will flow with or without the negative charge on C.
This graphically illustrates that switch-like behavior is made possible only through the use of semiconductors.