A new effect at high currents in heterostructure bipolar transistors

Large current densities in heterostructure bipolar transistors with heterostructure collectors are shown to cause an excess electron barrier leading to an increase in minority-carrier charge storage in the base and a decrease in current gain of the device. This effect occurs at current densities where the mobile charge in the collector depletion region significantly reduces the electrostatic field, thus exposing an electron chemical potential barrier due to bandgap grading at the junction. The effect appears at lower current densities than the Kirk effect and should occur in wide-gap heterostructure collector devices. The effect is demonstrated using experimental data and analyzed using device modeling; solutions are suggested for its elimination.<>