What is a Diode?

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The diode is fabricated of a semiconductor material, usually silicon, which is doped with two impurities. One side is doped with a donor or n-type impurity which releases electrons into the semiconductor lattice. These electrons are not bound and are free to move about. Because there is no net charge in the donor impurity, the n-type semiconductor is electrically neutral. The other side is doped with an acceptor or p-type impurity which imparts free holes into the lattice. A hole is the absence of an electron which acts as a positive charge. The p-type semiconductor is also electrically neutral because the acceptor material adds no net charge. When a P-type semiconductor material is combined with an n-type semiconductor material, a p-n junction is formed. This p-n junction is called a diode.

Thus the diode has two terminals or electrodes (di-ode),that act like an on-off switch. When the diode is “on”, it acts as a short circuit and passes all current. When it is “off”, it behaves like an open circuit and passes no current. The two terminals are different and are marked as plus(+) and minus(-) in the schematic below:

The positive electrode is called the Anode and the negative electrode is called the Cathode. If the polarity of the applied voltage matches that of the diode (forward bias), then the diode turns “on”. When the applied voltage polarity is opposite (reverse bias), it turns “off”. This is just the theoretical behaviour of an ideal diode, but it can be seen as a good approximation for a real diode which will have some reverse current when reverse biased.

Basic Characteristics of the Semiconductor Diode
A diode has the following basic characteristics:
(1) When forward bias, the diode needs a small voltage to conduct electricity. This voltage is maintained across the diode during conduction
(2) The maximum forward current a diode can carry is limited by the heat dissipation capacity of the diode.
(3) There is a small reverse current flowing even when the diode is reversed bias
(4) Every diode has a maximum reverse voltage, called the breakdown voltage, which cannot be exceeded without damage to the diode.

Technical Specifications of a Diode
There are four diode ratings that apply to one or other diodes used in various applications. These include:

Forward Voltage Drop
This is the forward-conducting junction voltage drop (0.7 V for Silicon diodes
and 0.3 V for Germanium diodes).

Average Forward Current
This is the maximum amount of forward current that the diode can carry for an indefinite period. If the average current exceeds this value, the diode will overheat and, eventually, will be destroyed.

Peak Reverse Voltage or Reverse Breakdown Voltage
This is the largest amount of reverse-bias voltage the diode’s junction can withstand for an indefinite period of time. If a reverse voltage exceeds this level, the voltage will punch through the depletion layer and allow current to flow backwards through the diode, which is a destructive operation (except for the case of a Zener diode).

Maximum Power Dissipation
The actual diode power dissipation is determined by multiplying the forward voltage drop and the forward current. Exceeding the maximum power dissipation will result in thermal breakdown of the diode.

In practical diode applications, excessive forward current and reverse breakdown voltage are the most common causes of diode failure. In both cases the diode gets very hot, resulting in the destruction of the p-n junction. Occasional peaks of voltage or current exceeding these rates for very short times (few milliseconds) may not overheat the junction, but repeated peaks may fatigue the junction.
When used in most applications, diodes are selected with ratings that exceed two or three times the expected peaks in the circuit where they operate.

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