ELECTRICAL AND MICROSTRUCTURAL INVESTIGATION OF AU PD/TI OHMIC CONTACTS FOR ALGAAS/GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS/

The microstructure and electrical properties of as deposited and annea led Au (400 nm)/Pd (75 nm)/Ti (10 nm) contact structures to p type GaA s, C doped with a concentration of 5 x 10(18) and 5 x 10(19) cm(-3), h ave been investigated using transmission electron microscopy, and curr ent-voltage measurements as a function of temperature in the range 198 -348 K. The specific contact resistivities have also been measured usi ng the transmission line method. It was found that increasing the epil ayer doping level by an order of magnitude, from 5 x 10(18) to 5 x 10( 19) cm(-3), caused the dominant current transport mechanism to change from thermionic field emission to field emission. For the lower level doped epilayers generation-recombination within the depletion region w as found to be the dominant current transport mechanism for temperatur es below 298 K. The contacts to the more highly doped epilayers (C dop ed, 5 x 10(19) cm(-3)) had specific contact resistivities of 0.08 +/- 0.03 Ohm mm and 0.05 +/- 0.06 Ohm mm, respectively. These values, toge ther with a minimal metal penetration in the semiconductor of <15 nm, indicate that these contacts are suitable for heterojunction bipolar d evice applications.