P-N Junction and Biasing | Electronic Devices | GATE Preparation | EC

by / Monday, 26 June 2017 / Published in Blog

Real Life Application
The p-n junction is a versatile element, which can be used as a rectifier, as an isolation structure and as a voltage-dependent capacitor. In addition, they can be used as solar cells, photodiodes, light emitting diodes and even laser diodes. They are also an essential part of Metal-Oxide-Silicon Field Effect Transistors (MOSFETs) and Bipolar Junction Transistors (BJTs).

Explanation
P-n junctions consist of two semiconductor regions of opposite type. They are called p-n diodes in analogy with vacuum diodes.
A p-n junction consists of two semiconductor regions with opposite doping type. One region is p-type with an acceptor density NA, while the other is n-type with a donor density ND. The dopants are assumed to be shallow, so that the electron (hole) density in the n-type (p-type) region is approximately equal to the donor (acceptor) density.
We will assume, unless stated otherwise, that the doped regions are uniformly doped and that the transition between the two regions is abrupt. We will refer to this structure as an abrupt p-n junction or step graded.
We will assume, unless stated otherwise, that the doped regions are uniformly doped and that the transition between the two regions is abrupt. We will refer to this structure as an abrupt p-n junction.
The junction is biased with a voltage. We call the junction forward-biased if a positive voltage is applied to the p-doped region and reversed-biased if a negative voltage is applied to the p-doped region. The contact to the p-type region is also called the anode, while the contact to the n-type region is called the cathode, in reference to the anions or positive carriers and cations or negative carriers in each of these regions.
The built-in potential in a semiconductor equals the potential across the depletion region in thermal equilibrium.

Forward Bias:
When an external voltage VD is applied, with negative terminal to n-side and positive terminal to p-side, it forms a forward bias configuration. In this setup, electrons and holes will be pressured to recombined with the ions near the boundary, effectively reducing the width and causing a heavy majority carrier flow across the junction. As VD increases, the depletion width decrease until a flood of majority carriers start passing through.

Reverse Bias
When an external voltage VD is applied, with positive terminal to n-side and negative terminal to p-side, the free charge carriers will be attracted away by the voltage source. This will effectively increase the depletion region within the diode. This widening of the depletion region will create too great a barrier for the majority carriers to overcome, effectively reducing the carrier flow to zero. The number of minority carriers will not be affected. This configuration is called reverse Bias. This small current flow during reverse bias is called the reverse saturation current, Is.

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