Thermal performance of metamorphic double heterojunction bipolar transistors with InP and InAlP buffer layers

InP-based double heterojunction bipolar transistors (DHBT) are a key technology for high-speed optical fiber transmission. InP substrates are expensive and are not available in as large sizes as GaAs substrates. Moreover, InP substrates are fragile and are easily broken in fabrication. This motivates development of metamorphic InP DHBT (MDHBT) on GaAs. Thermal performance is of critical importance, as high speed DHBTs must operate at emitter power densities exceeding 250 kW/cm/sup 2/. In the case of MDHBTs, thermal resistance is dominated by the poor thermal conductivity of the thick (1-1.5 μm) metamorphic buffer layer lying immediately below the collector. Here we compare the thermal characteristics of MDHBTs grown on InP and InAlP buffer layers to those of lattice-matched devices. In the case of InP buffer layers and narrow emitter geometries, MDHBT thermal resistance is comparable to that of lattice-matched HBTs. A low 32 °C junction-ambient temperature rise is obtained in a device operating at 235 kW/cm/sup 2/.