Stability and noise of Pd-Ge-Ag-Au ohmic contacts to InGaAs-InAlAs high electron mobility transistors

This paper discusses the electrical characteristics of Pd-Ge-Ag-Au ohmic contacts for InAlAs-InGaAs HEMT structures. A comparison with different Au-Ge based metallizations is made. Both types of metallization appear to provide low contact resistances. The Pd-Ge system, however, exhibited an excellent thermal stability not achieved with Au-Ge. No contact degradation was observed following heat treatment at 325 degrees C for more than 100 h plus 425 degrees C for 24 h. The beneficial effect of the Ag barrier layer is evident from the surface morphology and thermal stability of the contact. Low frequency noise measurements on ungated devices with various values of source-to-drain distance enable determination of the channel and contact contributions to the current noise. The contact noise was observed to be essentially controlled by the contact resistance, independently of the nature and process conditions of the metallization.