Current density limits in InP DHBTs: collector current spreading and effective electron velocity

To minimize the dominant delay term in emitter-coupled logic, ΔV_logicC_cb/I_c, HBTs must operate at high current densities. Current density is limited by device thermal failure and by the Kirk effect. We here experimentally determine two key factors influencing the Kirk-effect limit. The collector current spreads laterally away from each side the emitter stripe over a distance Δ approximately equal to the collector depletion thickness. This effect substantially increases the achievable current in submicron-emitter HBTs. Further, the variation of the Kirk-effect-limited current density with bias voltage indicates a 3.2(10⁵) m/s effective collector electron velocity, consistent with that extracted from the measured transistor f_τ.