Diodes:

The simplest semiconductor device is the diode. It is made of one layer of P-type material and one of N-type material. Diodes allow current flow in one direction only. They act like a check-valve for current flow. Figure 4-7 illustrates a diode package, the P-N structure and the schematic symbol. On the diode package (the actual diode), the band painted near one end indicates the cathode, or negative (N-type) terminal. The opposite end is the Anode, or positive terminal. The schematic symbol is an arrow, representing the direction of conventional current flow (positive to negative), with a line perpendiculat to the tip of the arrow. This line represents indicates the cathode end (thus the band on the diode package).

Fig. 4-7   -  Diode

Diodes have many uses in automotive circuits, including illumination, rectification and voltage spike suppression.

Diode Bias:

The term "bias" is used to refer to a diode's ability to allow or prevent the flow of current in a circuit. A "forward biased" diode is connected to a circuit in such a way as to allow the flow of electricity (see figure 4-8). This is done by connecting the cathode (N side) of the diode to the negative voltage, and the anode (P side) to the positive voltage. When the diode is connected in this way, both electrons and holes are being forced into the depletion zone, connecting the circuit.

Fig. 4-8   -  Forward Biased Diode

When a forward biased diode is connected to a voltage source, it acts as a switch closing a circuit. You can think of the voltage as forcing both electrons and holes into the depletion zone, allowing current to flow.

A diode connected to voltage so that current cannot flow is reverse biased. This means that the anode (positive terminal / P side) is connected to the negative voltage and the cathode (negative terminal / N side) is connected to the positive voltage (see Figure 4-9).

Fig. 4-9  -  Reverse Biased Diode

When voltage is applied to this circuit, the electrons from the negative voltage terminal combine with the holes in the P-type material. The electrons in the N-type material are attracted towards the positive voltage terminal. This enlarges the depletion zone. Since the holes and electrons in the depletion zone don't combine, current can't flow across it.

When a diode is reverse biased, the depletion zone acts like an open switch, preventing current flow. With the positive voltage connected to the N material, electrons are attracted away from the depletion zone. With the negative voltage connected to the P material, holes are attracted away from the depletion zone. The result is a wider depletion zone that contains neither hole or electrons and, therefore cannot support current flow.

Diode Leakage Current:

In reality, a minute amount of current can flow through a reverse biased diode. If the supply voltage becomes high enough, the atomic structure inside the diode will break down, and the amount of current that flows through it will increase sharply. If the reverse current is large enough and lasts long enough, the diode will be damaged by the heat.

Zener Point:

The applied reverse bias voltage at which the diode fails is called the maximum reverse voltage or Zener Point. Diodes are rated according to this voltage. Circuits must be designed to include diodes with a rating high enough to protect the diode and the circuit during normal operation.

Applications:

Common uses for diodes in automotive electrical circuits include:

• voltage regulation (using Zener diodes)
• indicators (using Light Emitting Diodes or LED's)
• rectification (changing AC current to DC current)
• clamping to control voltage spikes and surges that could damage solid-state circuits (acting as circuit protectors)