Power gain singularities in transferred-substrate InAlAs-InGaAs-HBTs

Deep submicron transferred-substrate heterojunction bipolar transistors exhibit peaking and singularities in the unilateral power gain (U) at high frequencies. Unbounded U has been observed in some devices over a 20-110 GHz bandwidth. Associated with the effect are a strong decrease in collector-base capacitance with increased bias current, and negative conductance in the common-emitter output conductance G₂₂ and positive conductance in the reverse conductance G₁₂. Unbounded U is observed in devices operating at current densities as low as 0.56 mA/μm². A potential explanation of the observed characteristics is dynamic electron velocity modulation in the collector-base junction. A theoretical model for the dynamics of capacitance cancellation by electron velocity modulation is developed, and its correlation with experimental data examined.