High-frequency performance limitations of millimeter-wave heterojunction bipolar transistors

Heterojunction bipolar transistor structure (HBTs) with 0.25-, 0.4-, and 0.6-μm emitter stripe widths and ultrasubmicrometer base widths, which are designed to achieve minimum transit time and low parasitic effects, are examined for their millimeter-wave performance. In particular,the dependence of the unity current gain frequency (f _τ), the maximum oscillation frequency (f _max), and the stability of power gains on the device structure and material parameters are critically analyzed. It is shown that the classical f_max expression commonly used for bipolar transistors, involving the effective carrier transit time and the collector-based RC time constant does not adequately represent the performance of ultrasubmicrometer-based-width HBTs, where the transadmittance phase delay associated with the collector-base depletion layer transit time and the parasitic collector-based capacitance are significant. The expected ballistic and quasiballistic behaviour of electron in these ultrasubmicrometer structures, if properly designed, minimizes the effective carrier transit time effect, but its impact on the f_max by the excess transadmittance phase delay poses a more fundamental and serious high-frequency limiting factor for the realization of millimeter-wave HBTs than has been hitherto recognized. The accuracy and usefulness of the proposed analytical approach is demonstrated for a practical HBT structure with 1.2-μm emitter stripe design, giving results that agree well with measurements