A Cause and Cure of Hfe Instability in Shallow Junction Bipolar Microwave Transistors

Bipolar microwave transistors with Ti/Pt/Au contact metalization have shown a reversible degradation in current gain (Hfe) due to high temperature storage (HTS) stress at 240 degrees Celcius for 24 to 48 hours. A model for current gain in shallow junction bipolar transistors is described. The model shows that Hfe of such devices is dependent on the properties of the metal-silicon interface at the emitter. The cause of the reversible Hfe degradation has been found to be hydrogen absorption and desorption in the Ti contact metallization which causes shifts in the Schottky barrier potential of the Ti/Si contact with consequent changes in the surface recombination velocity at the emitter. A new contact metallization of TiW/Au has been developed which exhibits no drift in Hfe.