- 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.
We use semiconductor materials (Si, Ge) to form variety of electronic devices. The most basic device is diode. Diode is a two terminal PN junction device. PN junction is formed by bringing a P type material in contact with N type material. When a P-type material is brought in contact with N- type material electrons and holes start recombining near the junction. This results in lack of charge carriers at the junction and thus the junction is called depletion region. When we apply voltage across the terminals of PN junction, we call it as diode. The picture below shows the symbol of PN junction diode.
A diode is a unidirectional device that allows current to flow in only one direction, depending on how it is biased.
Forward Biasing Characteristic of Diode
When the P-terminal is connected to the positive end of the battery and the N-terminal to the negative, the diode is forward biased.
In forward bias, the positive battery terminal repels holes in the P-region and the negative terminal repels electrons in the N-region, pushing them toward the junction. This increases the concentration of charge carriers near the junction, causing recombination and reducing the width of the depletion region. As the forward bias voltage increases, the depletion region narrows further, and current rises exponentially.
Reverse Biasing Characteristic of Diode
In reverse biasing P- terminal is connected to negative terminal of the battery and N- terminal to positive terminal of battery. Thus applied voltage makes N side more positive than P side. 
Negative terminal of the battery attracts majority carriers, holes, in P-region and positive terminal attracts electrons in the N-region and pull them away from the junction. This result in decrease in concentration of charge carriers near junction and width of depletion region increases. A small amount of current flow due to minority carriers, called as reverse bias current or leakage current. As reverse bias voltage is raised depletion region continues to increase in width and no current flows. It can be concluded that diode acts only when forward biased. Operation of diode can be summarized in form of I-V diode characteristics graph.
For reverse bias diode, 
Where, V = supply voltage
ID = diode current
IS = reverse saturation current
For forward bias, 
Where, VT = volt’s equivalent of temperature = KT/Q = T/11600
Q = electronic charge = 
K = Boltzmann’s constant = 
N = 1, for Ge
= 2, for Si
As reverse bias voltage is further raised, depletion region width increases and a point comes when junction breaks down. This results in large flow of current. Breakdown is the knee of diode characteristics curve. Junction breakdown takes place due to two phenomena.
Avalanche Breakdown (for V > 5V)
Under very high reverse bias voltage, the kinetic energy of minority carriers becomes so large that they knock electrons out of covalent bonds, starting a chain reaction. This process continues until the junction breaks down, known as avalanche breakdown, which is central to avalanche diodes.
Zener Effect (for V < 5V)
Under reverse bias voltage junction barrier tends to increase with increase in bias voltage. This results in very high static electric field at the junction. This static electric field breaks covalent bond and set minority carriers free which contributes to reverse current. Current increases abruptly and junction breaks down. This is known as Zener breakdown, and is a phenomenom that is cental to Zener diodes.
