New Cu/Mo/Ge/Pd ohmic contacts on highly doped n-GaAs for InGaP/GaAs heterojunction bipolar transistors

The feasibility of using novel Cu/Mo/Ge/Pd ohmic contacts on n+-GaAs for heterojunction bipolar transistors (HBTs) is investigated. The electrical and material characteristics of the Cu/Mo/Ge/Pd/n(+)-GaAs structure were studied. After thermal annealing at 350 degrees C, the specific contact resistances of the copper ohmic contacts Cu/Mo/Ge/Pd were measured to be 2.8 x 10(-7) Omega cm(2). Judging from the data of sheet resistance, X-ray diffraction analysis, Auger electron spectroscopy, and transmission electron microscopy, the Cu/Mo/Ge/Pd contact structure was very stable after annealing at 350 degrees C. However, after 400 degrees C annealing, the reaction of copper with the underneath layers started to occur and formed MoGe2, Cu3Ga and Ge3CU phases. An InGaP/GaAs HBT with C'u/Mo/Ge/Pd contact metals was fabricated and compared with an HBT fabricated with traditional Au/Ni/Ge/Au contact metals. These two kinds of HBTs showed similar device characteristics. After reaching thermal stability and per-forming a high current-accelerated stress test at a current density of 120 kA/cm(2) for 24h, the device with the Cu/Mo/Ge/Pd ohmic contacts still exhibits excellent electrical characteristics.