Improved thermal reliability in base contact of full 3-inch InP Double-HBTs with f_T and f_max in excess of 300 GHz

In this paper, we have structurally and electrically examined the thermal degradation of platinum-based ohmic contacts for base layers of InP DHBTs. The results show that the degradation of the base contact is caused by the formation of an inhomogeneous Au-InGaAs alloy. It was also found that the contact resistance degradation could be effectively suppressed by inserting a molybdenum diffusion barrier. We demonstrate an InP DHBT on a 3-inch InP substrate via stepper-based photolithography with an improved base ohmic contact. The fabricated DHBTs (W_E = 1.0 μm and L_E = 15 μm) exhibit high current gain (β) = 57 at V_CE = 1.0 V. Excellent f_T of 331 GHz and f_max of 310 GHz are obtained at J_C = 3.2 mA/μm² and V_CE = 1.4 V.