- Diode Definition: A diode is defined as a two-terminal electronic component that allows current to flow in one direction only.
- Forward Biasing: In forward biasing, the P-terminal connects to the positive battery terminal and the N-terminal to the negative, allowing current to flow.
- Reverse Biasing: Reverse biasing occurs when the P-terminal connects to the negative battery terminal and the N-terminal to the positive, blocking current flow.
- Avalanche Breakdown: At high reverse voltages, avalanche breakdown happens when minority carriers gain enough energy to knock electrons from bonds, leading to a large current flow.
- Zener Effect: The Zener effect occurs at lower reverse voltages, where a high electric field breaks covalent bonds, causing a sudden increase in current and junction breakdown.
semiconductor materials such as silicon and germanium form many electronic devices. A diode is a two-terminal device, and a common form contains a single p-n junction. A PN junction forms where p-type and n-type regions meet in one crystal. Electrons and holes diffuse across the junction and recombine, leaving fixed ionised dopants. This region has few mobile charge carriers and is called the depletion region. Its internal electric field creates a potential barrier. An external voltage changes that barrier and therefore the current. The image below shows the p-n junction diode symbol.
A diode conducts current much more readily from anode to cathode than in the reverse direction. It is therefore described as unidirectional, although a real reverse-biased diode still has leakage current and can conduct heavily in breakdown.
Forward Biasing Characteristic of Diode
When the p-side connects to the positive terminal of a battery and the n-side connects to the negative terminal, the diode is forward biased.
Forward bias lowers the junction barrier and narrows the depletion region. Majority charge carriers can then diffuse across the junction and recombine. In the ideal low-current model, forward current rises exponentially with applied voltage. At higher current, bulk and contact resistance cause the measured curve to depart from the ideal exponential law.
Reverse Biasing Characteristic of Diode
In reverse bias, the p-side connects to the negative terminal of the battery and the n-side connects to the positive terminal. The applied voltage therefore makes the n-side more positive than the p-side. 
The electric field pulls majority carriers away from the junction, so the depletion region widens. A small current still flows because of thermally generated minority carriers and surface leakage. This reverse leakage depends strongly on temperature, device construction and reverse voltage. A real diode therefore does not become a perfect open circuit. Its I-V diode characteristics show small reverse current before breakdown and rapidly increasing forward current.
Before breakdown, the ideal reverse-bias approximation is 
Here, V is diode voltage.
ID is diode current.
IS is reverse saturation current.
For forward bias, the ideal diode equation is 
Here, VT is thermal voltage, kT/q, approximately T/11600 volts when T is in kelvins.
Q is the magnitude of electronic charge: 
K is the Boltzmann constant: 
N is the ideality factor, usually between 1 and 2 depending on the device, current range and temperature.
The exact value comes from the device datasheet.
As reverse voltage reaches the specified breakdown region, reverse current can rise sharply. Current must be limited so power dissipation remains within the device rating. In an ordinary rectifier, breakdown can be destructive; a breakdown-rated device is designed for controlled operation. Two physical mechanisms shape this part of the diode characteristics curve.
Avalanche Breakdown
In a sufficiently wide depletion region, carriers accelerated by the reverse electric field can free additional electron-hole pairs through impact ionisation. The new carriers accelerate and repeat the process, producing avalanche breakdown. This mechanism dominates higher-voltage junctions and is used deliberately in rated avalanche diodes.
Zener Effect
In a heavily doped junction, the thin depletion region and strong reverse electric field can produce quantum-mechanical tunnelling, known as Zener breakdown. The Zener mechanism tends to dominate lower breakdown voltages, avalanche tends to dominate higher voltages and both can contribute in the transition range around 5 to 8 V. Devices sold as Zener diodes may therefore operate through either mechanism or a combination.
