Extension of high fT operation bias range for an AlInAs/InGaAs HBT

The authors report on the fabrication and characteristics of AlInAs/InGaAs HBTs (heterojunction bipolar transistors) with a base-collector structure that results in high-f_T operation over a wide range of collector bias voltage variation. This feature results in high-speed switching performance for digital circuits, where the device operating bias point varies widely due to logic swing. The fabricated device exhibited an f_T- V_CE characteristic with a broad peak at around 2.2 V (V_BE=1.0 V). It was found that the electron transit time t_B+t_C is insensitive to external voltages. A realistic flat f_T-V _CE characteristic can be obtained by reducing extrinsic delay time