Aluminum graded-base AlGaAs/GaAs PNp HBT with 37 GHz cut-off frequency

A PNp AlGaAs/GaAs heterojunction bipolar transistor (HBT) with Al-alloy graded base structure is described. In order to reduce the base transit time (/spl tau/b) and thus to obtain high performance pnp-type AlGaAs/GaAs HBTs, Al-alloy grading is adopted to form a high quasi-electric field in the base. The effectiveness of Al-alloy grading in the base is demonstrated by the RF performance of the fabricated device which shows a cut off frequency (f/sub t/) of 37 GHz and a maximum oscillation frequency (f/sub max/) of 30 GHz. Further analysis of f/sub t/ associated with collector current densities indicates the intrinsic time of the device, which is the sum of the base transit time (/spl tau//sub b/) and the collector depletion region transition time (/spl tau//sub SC/), is as low as 2.0 ps.<>