Role of neutral base recombination in high gain AlGaAs/GaAs HBT's

Abstract

Neutral base recombination is a limiting factor controlling the maximum gain of AlGaAs/GaAs HBT's with base sheet resistances between 100 and 350 /spl Omega///spl square/. In this work, we investigate five series of AlGaAs/GaAs HBT growths in which the base thickness was varied between 500 and 1600 /spl Aring/ and the base doping level between 2.9/spl times/ and 4.7/spl times/10/sup 19/ cm/sup -3/. The dc current gain of large area devices (L=75 /spl mu/m/spl times/75 /spl mu/m) varies by as much as a factor of two at high injection levels for a fixed base sheet resistance, depending on the growth optimization. One of these series (Series TA) has the highest current gains ever reported in this base sheet resistance range, with dc current gains over 225 (@ 200 A/cm/sup 2/) at a base sheet resistance of 330 /spl Omega///spl square/. A high dc current gain of 220 (@ 10 kA/cm/sup 2/) was also confirmed in small area devices (L=8 /spl mu/m/spl times/8 /spl mu/m). High-frequency tests on a separate set of wafers grown under the same conditions indicate these high current gains can be achieved without compromising the RF characteristics: Both high and normal gain devices exhibit an f/sub t//spl sim/68 GHz and f/sub max//spl sim/100 GHz. By fitting the base current as a sum of two components, one due to recombination in the neutral base and the other in the space charge region, we conclude that an improvement in the minority carrier lifetime is responsible for the observed increase in dc current gain. Moreover, we observe a thickness-dependent variation in the effective minority carrier lifetime as the gains increase, along with a nonlinear dependence of current gain on base doping. Both phenomena are discussed in terms of an increase in Auger and radiative recombination relative to Hall-Shockley-Read recombination in optimized samples.