Extended velocity overshoot in InGaAs collectors for high-speed heterojunction bipolar transistors

Abstract

As III-V device fabrication technology continues to improve and performance limiting parasitics are eliminated, the intrinsic speed of any given device structure and options for increasing that speed are of greater interest to device researchers and technologists. One approach to improving heterojunction bipolar transistor (HBT) high-frequency performance is to reduce the transit time across the collector space charge region by carefully tailoring the electric field in the collector in such a fashion as to extend or enhance velocity overshoot in the space charge region. This technique was first described1 and demonstrated2-3 for AlxGa1-x As/GaAs HBTs. Recently, structures composed of materials lattice matched to InP substrates4-5 have emerged as champions in the race for high-speed and optical applications honors. Because these devices have utilized conventional collector structures, further gains can be achieved by application of collectors designed for enhanced or extended velocity overshoot. This is easily seen by noting that the extended velocity overshoot condition is more easily achieved in In0.53Ga0.47 As than in GaAs, in large part due to the larger ? — L conduction band separation (?E?L — 0.55 eV for In0.53Ga0.47AS; ?E?L = 0.33 eV for GaAs). In this paper, we present Monte Carlo simulation results demonstrating the efficacy of the extended velocity overshoot approach for reducing transit times across the collector for a number of ln0.53Ga0.47As collector structures.