Ohmic Contacts

Abstract

Metal contacts on moderately doped semiconductor are generally rectifying. This behavior is due to the fact that the branch points of almost all semiconductors are well within their fundamental band gaps. For sufficiently large barrier heights the current transport is then dominated by thermionic emission over the barrier. However, the I/V characteristics will become apparently ohmic if the barrier height drops to below approximately 0.3 eV. The I/V characteristics displayed in Fig. 11.1 a,b illustrate this behavior and also indicate the influence of the series resistance. When the doping level of the semiconductor is increased and, as a consequence of this, the depletion layer narrows, tunneling or field emission through the barrier becomes increasingly important. As a consequence of this, the I/V characteristics of metal contacts on highly doped semiconductors show ohmic behavior. There are, however, also metal—semiconductor contacts that are naturally ohmic.