UNDERSTANDING THE SHEET RESISTANCE PARAMETER OF ALLOYED OHMIC CONTACTS USING A TRANSMISSION-LINE MODEL

The electrical characterization of alloyed ohmic contacts is commonly undertaken using a Transmission Line Model (TLM) network to model and experimentally determine two parameters-the specific contact resistance rho(c) and the sheet resistance R(sk) beneath a planar ohmic contact. This paper describes the use of a recently reported modification to the TLM network [the Tri-Layer Transmission Line Model (TLTLM)] to interpret measurements of the sheet resistance parameter. The TLTLM network models the composite alloyed ohmic contact as three layers (metal layer, alloyed semiconductor layer and the unalloyed semiconductor layer) and two interfaces between the three layers. By assigning appropriate parameters to the TLTLM network, it is possible to calculate a value for the sheet resistance R(sk) that has been experimentally derived using the standard TLM. The new TLTLM model predicts that values of R(sk) greater and less than R(sh) (the unmodified sheet resistance of the epitaxial layer) are possible, in agreement with experimentally reported observations.