What is metal-semiconductor contact? Physics behind it.

 Introduction:

In semiconductor technology, there are two types of contact one is rectifying(Schottky), and another is ohmic contact. So, the type of contact depends on the difference in the work function. The Schottky contact has applications in semiconductor physics and Ohmic contact has an influence on the performance of the device specifically power devices.

Physics behind metal-semiconductor contact

The physics behind the contact nature mainly depends on the doping concentration (Nd) of the semiconductor, temperature (T), and carrier transport mechanism at the metal-semiconductor interface (M-S). There are three carrier transport mechanisms that exist in the M-S interface: Firstly, in the case of low doping concentration, the thermionic emission (TE) model dominates where the temperature plays the main role for carrier transport over barrier height between M-S. Secondly, in the case of intermediate doping concentration thermionic field emission dominates (TFE), and the tunnel barrier also makes an additional contribution. Thirdly, the high doping reduces the tunnel barrier more than the field emission dominates the carrier transport. The following figures explain the phenomenon in detail. In Ohmic contact the thermionic dominant which means non-rectifying M-S contact with negligible resistance which means no disturbance for the current-voltage (I-V) characteristics. 

The contact area mainly determined the total contact resistance, Rc(Ω). It is mainly dependent on the contact area, contact geometry, and obviously the quality of the interface. The quality of ohmic contact is determined by the specific contact resistance ρc (Ω cm² ) and is independent of geometry. ρc depends on Schottky barrier height (SBH), semiconductor doping, quality of the interface, semiconductor surface, metal deposition chamber, etc.

On the other hand, the rectifying M-S contact with asymmetrical and nonlinear (I-V) behaviors is said to be a Schottky contact. This is activated if the metal work function is higher than that of the semiconductor electron affinity. Usually, SBH is varied by surface states, metal-induced gap states, defects, and a thin interfacial layer. The interfacial layer having atomic layer thickness allows tunneling of charge carriers. The potential drops across the interfacial layer result in the lowering of SBH. The surface sometimes acts like an acceptor i.e neutral when empty and negative when occupied and the opposite is true. These acceptors are distributed within a forbidden gap resulting in control of the fermi level. The SBD may also be lowered by the image force called the Schottky effect. The image force is nothing but coulombic force from interface electron to far away positive charge.

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