This paper presents a design of emitter ledge that achieves thermal stability of the AlGaAs/GaAs heterojunction bipolar transistors (HBTs). The HBTs use a V-base layer to implement base ballast resistors, a fully-depleted AlGaAs ledge to implement input bypass capacitors, and boron ion implantation to reduce the base-collector parasitic capacitance. The minimum emitter ledge length for the experimental HBTs is estimated theoretically as 8.27 mu m, at which power density is 2.77 mW/mu m(2). Experimental results show that the HBTs were thermally stable at an emitter ledge length of 10 mu m, and their RF properties were degraded little from those of the HBTs without the emitter ledge when the n(-) collector underneath the emitter ledge was implanted with boron ions. (C) 2013 Elsevier Ltd. All rights reserved.