Current Transport Process in PN Junction

Introduction: The current transport in contacts of metal-semi conductor is mainly due to majority carriers, in comparison to p-n junctions where minority carriers are responsible.

Current Transport Processes: Figure provides below demonstrates five basic processes under forward bias (the inverse processes occur under reverse bias).
These five processes are
(1 )Electrons emission from the semi conductor over the potential barrier in the metal( the dominant process for Schottky diodes with moderately doped semiconductors (e.g., Si with ND < 1017cm-3) operated at moderate temperatures (e.g., 300 K)],
(2) Quantum mechanical electrons tunneling through the barriers (crucial from heavily doped semiconductors and is accountable for most ohmic contacts) 
 (3) recombination in the space-cahreg region( similar to the process of recombination in a p-n junction)
(4) electron diffusion in the depletion region, and
(5) holes injected from the metal which diffuse into the semiconductor which is similar to the recombination in the neutral region).

Apart from this, we also have edge leakage current because of high electric field at the metal-contact interface or periphery current due to traps at the interface of metal semiconductor. Several methods are used for enhancing the quality of interface and various device structures have been proposed to minimize or remove the edge leakage current.

For typical high-mobility semiconductors (eg. Si and GaAs) the transport can be described adequately with the help of this theory of thermionic-emission.

We will also consider the theory of diffusion which is applicable to low-mobility semiconductors and a generalized thermionic-emission-diffusion theory which is actually the synthesis of the two theories discussed earlier.

The behavior of Schottky diode is electrically similar to single-sided abrupt p-n junction to some extent, and yet we can operate the Schottky diode as a majority-carrier device having inherent quick response.

Therefore, the terminal functions of a p-n junction in general can be performed with the help of a Schottky diode with single exception as charge storage diode.

This is because the time of charge storage in majority-carrier device is very small.

One more difference is the greater density of current in a Schottky diode because of small built-in-potential with nature of thermionic emission in comparison to diffusion. It results in a quite smaller forward voltage drop.

With same token, the demerit is the lower breakdown voltage and larger reverse current in the Schottky diode.

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