Planar, ion-implanted GaAs bipolar transistors

The high electron mobilities and wide bandgap of GaAs make it an attractive material for high speed, high temperature device applications. Integrated circuits employing MESFET devices have been fabricated. The speed, packing density, and drive capability advantages associated with bipolar devices make it attractive to investigate the feasibility of fabricating such devices in GaAs. In particular, if both bipolar and MESFET devices can be integrated into a planar IC process, the material properties of GaAs can be more fully exploited. In this paper we discuss the fabrication and properties of ion-implanted, planar bipolar n-p-n transistors in GaAs. The devices were fabricated by implanting the base and emitter regions into n-type epitaxial GaAs which acts as the collector. Using a thin evaporated metal layer as an implant mask, selective base regions were formed by Be-ion implantation. Silicon was then implanted to form the emitter region. The samples were annealed at 850°c for 30 minutes to electrically activate the impurities. A passivating dielectric layer (SixOyNz) was deposited by a plasma enhanced deposition process. Ohmic contacts to the collector and emitter regions were formed with Au/Ge:Ni while Ag:Mn ohmic contacts were made to the base region. Typical devices exhibited breakdown voltages in excess of 40V across the collector-base junctions and common emitter current gains of 8. Gains as high as 16 were observed. These results are extremely encouraging since structures were not designed for optimum performance. The leakage currents and the frequency response of these devices will be discussed.