March 25, 2011

The Diode

          Both P-type and N-type silicon conduct electricity. The resistance of both type is determined by the proportion of holes or surplus electrons. Therefore both types can function as resistors. And they will conduct electricity in any direction.

By forming some P-type silicon in a chip of N-type silicon, electrons will flow through the silicon in only one direction. This is the principle of the diode. The P-N interface is called the PN juction.

How Diode Works

Here's a simplified explanation of how a diode conducts electricity in one direction (forward) while blocking the flow of current in the opposite direction (reverse).

Here the charge from the battery repels holes and electrons toward the junction. If the voltage exceeds 0.6-volts (silicon), then electrons will cross the junction and combine with holes. A current then flows.

Here the charge from the battery attracts holes and electrons away from the junction. Therefore, no current can flow.

A Typical Diode

Diode are commonly enclosed in small glass cylinders. A dark band marks the Cathode terminal. The opposite terminal is the Anode.

Diode Operation

We already know a diode is like an electronic one-way valve. It's important to understand some additional aspects of diode operation. Here some key ones:

1.   A diode will not conduct until the forward voltage reaches a certain threshold point. For silicon diode this voltage is about 0.6-volt.


2.   If the forward current becomes excessive, the semiconductor chip may crack or melt! And the contacts may separate. If the chip melts, the diode may suddently conduct in both directions. The resulting heat may vaporize the chip!

3.   Too much reverse voltage will cause a diode to conduct in the wrong direction. Since this voltage is fairly high, the sudden current surge may zap the diode.

Summing Up Diode Operation

This graph sums up diode operation. (it's approximate.)

VF = Forward Voltage
VR = Reverse Voltage
IF   = Forward Current
IR   = Reverse Current

The other famous semiconductor product is the transistor.

See also:

Types of Diodes

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