HETEROJUNCTION BIPOLAR TRANSISTOR (HBT) FOR HIGH POWER APPLICATIONS

Abstract

In this report, a non-self aligned process, entirely based on wet-etching, for the fabrication of Al0.3Ga0.7As/GaAs based HBT with a very thin base (ranging from 400A to 800A) and a minimum emitter area of 6x 10µm2 was presented. An etchant, which consists of 1 part of (27.8g)KI/(16.25g)I2/(25ml)H2O with three days of aging (to attain stable etching characteristics) and 1 part of diluted H2SO4 with a pH value of 0.9, was employed to selectively etch Al0.3Ga0.7As emitter against GaAs base. This simplifies the fabrication and enhances the controllability by avoiding the overetching of the thin GaAs base. The best selectivity of 22 was obtained at 0°C with a GaAs etch rate of 80A/min±10%. It has also been found that the potassium ions do not have deleterious effect on the device performance. In addition, it has been found that the etching of emitter and base mesas needs to be carried out immediately after photolithography steps to avoid excessive lateral etching, which could render the subsequent metalization infeasible. Also, the thickness of polyimide used for passivation has to be restricted to 4000A to increase the yield of functional HBTs by avoiding either insufficient etching of smaller area contact windows or over-etching of bigger area contact windows. From the DC characterizations, DHBT with 800A base region has been observed to have an excessive base current density at medium bias (Vbe = 0.8V to 1.1V) and a very low current gain (?<10). The former could be due to the high base-emitter heterointerface states whereas the latter could be caused by the energy spike presents at the base-collector heterojunction, which is not fully eliminated by the 200A grading layer at the base-collector heterojunction. For the AC characteristics, DHBT with 800A base region was found to have the highest measured fT of 5GHz at a collector current density of 1800A/cm2 and the highest fmax of 3.5GHz was achieved by SHBT with an abrupt base-emitter junction. The measured values are about 5 times smaller than the analytical predictions. This could be due to the lower applied collector current density of approximately 2000A/cm2, limited by heat dissipation of the devices, which is about one order of magnitude smaller than the theoretical value of 1x104 A/cm2. Also, the non-self aligned process employed has resulted in a high extrinsic base resistance and a large base-collector junction capacitance, which reduced fT and fmax.