ALUMINUM-GRADED-BASE PNP ALGAAS GAAS HETEROJUNCTION TRANSISTOR WITH 37 GHZ CUTOFF FREQUENCY/

The base transit time of an Aluminum-graded-base PNp AlGaAs/GaAs heter ojunction bipolar transistor (HBT) was studied in order to clarify the effect of aluminum grading in the base. Theoretical analysis using a classical drift diffusion model with velocity saturation at the base-c ollector junction and a high base quasi electric field (58 keV/cm) cre ated by 20%-aluminum linear grading in a 400 Angstrom base, leads to a base transit time (tau(b)) of 0.9 ps. The base transit time is reduce d by four times, compared to the base transit time of 3.6 ps without a luminum grading in the base. In order to demonstrate this advantage, w e fabricated aluminum-graded-base PNp AlGaAs/GaAs heterojunction trans istor which employs a 20%-aluminum linear graded 400 Angstrom-wide bas e. The device with a 2 mu m x 10 mu m emitter showed high RF performan ce with a cut-off frequency (f(t)) of 37 GHz and a maximum oscillation frequency (f(max)) of 30 GHz at a collector current density of 3.4 x 10(4) A/cm(2). Further analysis using direct parameter extraction of a small signal circuit model under the collector current density of 1.1 -9.9 x 10(4) A/cm(2) indicated the intrinsic transit time, which is th e sum of the base transit time and the collector depletion layer trans it time (tau(sc)), was as low as 2.3 ps under low injection level. Sub tracting the collector depletion-layer transit time from the intrinsic time leads to a base transit time of 1.1 ps, which is close to the th eoretical base transit time and is the shortest value ever reported. T he structure is very attractive for pnp-type AlGaAs HBTs combined with Npn HBTs for complementary applications.