# Diode1

## What is a PN junction diode?

The name PN junction diode stands for a junction termed as PN junction formed by injecting into one side of intrinsic semiconductor crystal acceptor impurities (forming a p-type semiconductor )and donor impurities from the other side(forming an n-type semiconductor) with two active terminals (di means two) of one terminal acts as anode (p-type) and other as cathode (n-type).

## What is a Junction capacitance of a diode?

The increase or decrease of uncovered charge at the junction in depletion layer region can be considered a capacitive effect and is termed as junction capacitance. On the either side of depletion region the combination of charged ions (acceptor atoms in p-region by accepting diffused electrons from n-region becomes positively charged and the donor atoms by accepting the diffused holes from p-type region becomes positively charged) acts as a parallel plate capacitor. Any variation in the applied voltage causes a corresponding change in charge stored in the depletion region, Hence the junction capacitance is given as Cj = dqj / dvj where dqj is variation in charge stored in depletion region due to variation in voltage applied across junction dvj.

## What is diffusion capacitance or Transit time capacitance?

The rate of change of injected charge stored near the junction outside depletion region with applied voltage is called diffusion or Transit time or storage capacitance.

Cd = dQ/dV

dQ is the change in the injected minority charge store near the junction outside depletion region. Assume for simplicity p material is heavily doped compared to n material. In such a case the injected electron current in to p side is negligible compared to injected hole current to n side across the junction. Therefore the hole current in terms of total injected charge is given by

I = Q/t where t is life time of holes

Hence the diffusion capacitance of diode is given by

Cd = dQ/dV = t*(dI/dV) = t/rd

Cd = t*I / (η*Vt)

## What is a diode static and dynamic resistance?

Static resistance of a diode is the ratio of voltage across the diode to the current through the diode. It varies with bias point on the V-I characteristic curve of diode.

Static resistance rs = V/I

Dynamic resistance of a diode at a particular bias point (V, I) is defined as inverse of slope of V-I characteristics of diode corresponding to that voltage across the diode and current through the diode.

Dynamic resistance rd = dV/dI

Static resistance of the diode varies with the bias point whereas Dynamic resistance of a diode is constant over a wide range of bias points.

Dynamic resistance of a diode

## What is Reverse Break down (VBD) voltage of a diode?

The Reverse Break down (VBD) voltage of a diode is defined as the reverse bias voltage across the diode at which it starts conducting currents heavily. At voltages greater than Reverse Break down voltage the diode currents increase for even a small change in reverse bias voltage. Sufficient heat sink has to be provided to remove the heat dissipated at the diode junction for voltages greater than Reverse Break down voltage; otherwise it will lead to permanent damage of diode

## What are Reverse break down mechanisms in a diode?

There are two Reverse break down mechanisms in a diode. They are

• Avalanche Breakdown
• Zener breakdown

## What are high frequency diodes?

Diodes which support high frequency switching are called as high frequency diodes. The following diodes are some of high frequency diodes:

•  PIN diode
•  Schottky diode
• Tunnel diode.

## Why direct band gap semiconductors are used in LED?.

In direct band gap semiconductors most of the energy released after an electron in conduction band recombines with a hole in valence band will be in the form of light energy whereas In indirect band gap semiconductors most of the energy released after an electron in conduction band recombines with a hole in valence band will be in the form of heat energy Hence direct band gap semiconductors are used in Light emitting diodes. Some of the direct band gap semiconductors used in the manufacture of LED’s are direct band gap semiconductors gallium antimonide (GaSb), gallium arsenide (GaAs), indium phosphide (InP) e.t.c

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