Contacts must be made between semiconductor and metal lines to connect semiconductor device to outside world. Two types of contacts are often made: Schottky contact and ohmic contact.

1. Schottky or rectifying contact

Schottky barriers have rectifying characteristics, suitable for use a diode l. The contacts have very low resistance in forward direction and infinite resistance in reverse direction

They should have well defined and perfectly reproducible 'on voltage'. Schottky barriers contacts have variety of applications related to voltage clamping and controlled diode drops.

One of the primary characteristics of a Schottky barrier height, denoted by ϕB.

In most processes that make Schottky diode, metal is not deposited under ultrahigh vacuum condition.

Rather an active region of a semiconductor is first exposed, for example, by etching a hole in insulting layer. Then the water is placed in the vacuum to deposit the metal.

During this time, surface of water will get oxidized and receive a coating of carbon and other contaminants from residual vapor in the chamber and from the air in the room.

To improve the process, we can pre-sputter the surface deposition however it does not give perfect surface. Both contamination and damage have an impact on Schottky diode.

PtSi is most commonly used Schottky contact fir lightly doped n-type silicon.

GaAs technologies use Schottky diodes since GaAs doesn't have diode similar to silicon. GaAs field effect device is MESFET. For MESFET to work well, the gate electrode must make Schottky contact with channel. Pinch off voltage of FET depends on barrier voltage of Schottky diode.

The Schottky barrier also strongly depends in the interface quality. A poor interface results in reduced barrier height. Sputter cleaning of interface cab greatly reduce the non - uniformly of pinch - off voltage of GaAs MESFET.

2. Ohmic contacts

In ideal ohmic contact, the current varies linearly with applied voltage (as with Ohm's Law). Ohmic contacts have low resistance.

Fir heavily doped substrate; width of depletion region becomes sufficiently small so that the carrier can tunnel through the barrier. The specific contact resistance this region can be approximated as

$R_C=A_0 exp \bigg[\frac{C_2ϕ_b}{\sqrt{N_D}}\bigg]$

Equation shows that large substrate doping can reduce the contact resistance. Most silicon technologies achieve high doping by implantation.

To form an excellent ohmic contact (low resistance), the barrier height should be small everywhere and furthermore the interface should not reflect electrons.

Higher the concentration, lower the specific contact resistance. The values as low as $10^{-7} Ω cm^2$ are often obtained in semiconductor fabrication.

Aluminum is mostly used for silicon ohmic contact. To form low resistance contact with silicon, moat aluminum metallization include a low temperature anneal or sinter as the last step.

Formation of contacts to compounds semiconductors is considerably more difficult than with silicon.

For example, GaAs surfaces tend to lose arsenic and trends towards as loss can be considerably exacerbated by deposition of metal.

Self-aligned silicides (salicides) have been developed to reduce the series resistance of shallow junction in silicon.