This a classic question of semiconductor device physics. See e.g.:

Sze and Ng - Physics of semiconductor devices (ISBN 0471143235)

Generally it will be necessary to get the workfunction of the metal aligned to the transport band of your semiconductor. Amongst others there are the following approaches:

1) select a suitable metal, such that:

(metal workfunction) == (Etransportband - Evacuum)

This means usually that you need a high workfunction material (e.g. Au) for p-type semiconductors and a low workfunction material (e.g. Al, Mg, Ca) for n-type semiconductors

2) remove the schotty interface by degenerately doping the semiconductor close to the metal interface. By that the space charge region becomes so small that the contact becomes ohmic. This is often times written as n++ or p++ in semiconductor books.

I hope this helps,

Best regards,

Deniz Bozyigit

Good reviews:

1. S. Noor Mohammad, Contact mechanisms and design principles for alloyed Ohmic contacts to p-type GaN, PHILOSOPHICAL MAGAZINE, 21 August 2004

VOL. 84, NO. 24, 2559–2578

2. S. Noor Mohammad, Contact mechanisms and design principles for alloyed ohmic contacts to n-GaN, JOURNAL OF APPLIED PHYSICS VOLUME 95, NUMBER 12, 15 JUNE 2004

3. T.V. Blank, Yu.A. Goldberg, The current flow mechanism in metal -semiconductor ohmic contacs, Semicoductors, November 2007, Volume 41, Issue 11, pp 1263-1292