The Junction Diode

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The Junction Diode

A diode is a two-terminal electronic device consisting of a single p-n junction. This p-n junction is usually created on a single block of silicon by doping the block with donor and acceptor dopants at opposite ends. A diode is a rectifier, allowing current to pass in one direction but not in the opposite direction.

When the anode (p-type side) of the diode is connected to the positive terminal of a battery, the diode is said to be in forward bias, allowing current to pass through it. The diode is said to be in reverse bias if its cathode (n-type side) is the one connected to the positive terminal of the battery. A diode doesn�t conduct current in reverse bias.

Figure 1 shows a photo of a diode (left) and the circuit symbol for a diode (right). The cathode of a diode is usually indicated by a white band, as shown in the diode photo. The diode symbol consists of a triangle that indicates the direction of the flow of current when the diode is forward-biased, and a line perpendicular to this direction. This line indicates the cathode in the diode symbol.

A diode only becomes forward-biased when the potential at the anode is greater than the potential of the cathode by 0.7 V, the potential barrier. Under this condition, the potential barrier is effectively 'overcome' by the applied voltage, allowing the carriers of the diode to move across the junction. This means that the electrons from the n-type side can now go to the p-type side in the same way that the holes in the p-type side can now go to the n-type side.


Figure 1. Photo of an ordinary diode (left) and the symbol for a diode (right)

The current through the diode increases exponentially as the forward-bias voltage across the diode is increased. Thus, the increase in the current flowing through a diode is very abrupt once the diode starts to conduct. In physical terms, increasing the forward-bias voltage injects more electrons into the n-type side of the diode. These electrons immediately cross the junction in the absence of a potential barrier. Once these reach the p-type material, they are pulled back to the positive terminal of the battery again. The holes in the p-type side also move in the same manner under forward bias condition, although in the opposite direction as the electrons. This continuous flow of charges through the diode will go on as long as the diode is in forward bias.

When a diode is put under reverse bias, the holes of the p-type side are pulled toward the negative terminal of the battery while the electrons in the n-type side are pulled toward the positive terminal of the battery. In effect, the mobile charges are pulled away from the junction in opposite directions, inhibiting the flow of charges through the diode. This is also essentially widening the potential barrier of the diode, making it more difficult for the carriers to move across the junction.

In reality, however, a very small amount of current still flows through a reverse-biased diode. This current, known as reverse saturation current, is due to thermal generation of holes and electrons near the junction of the diode. This is therefore dependent only on temperature and not on the potential barrier of the diode.