LOW EMITTER RESISTANCE GAAS BASED HBTS WITHOUT INGAAS CAPS

Low emitter resistance is demonstrated for AlGaAs/GaAs heterojunction bipolar transistors using Pd/Ge contacts on a GaAs contact layer. The contact resistivity to 2-10 x 10(18) cm-3 n-type GaAs is 4-1 x 10(-7) OMEGA-cm2. These are comparable to contact resistivities obtained with non-alloyed contacts on InGaAs layers. The non-spiking Pd/Ge contact demonstrates thermal stability and area independent resistivity suitable for scaled devices. The substitution of Pd/Ge for AuGe/Ni GaAs emitter and collector contacts reduced by an order of magnitude the emitter-base offset voltage at high current densities and increased f(t) by more than 15% with significantly improved uniformity for devices with 2 and 2.6 mum wide emitters having lengths two, four and six times the width.