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

Neutral base recombination is a limiting factor controlling the maximum gai n of AlGaAs/GaAs HBT's with base sheet resistances between 100 and 350 Omeg a/square. In this work, we investigate five series of AlGaAs/GaAs HBT growt hs in which the base thickness was varied between 500 and 1600 Angstrom and the base doping level between 2.9 x and 4.7 x 10(19) cm(-3). The de curren t gain of large area devices (L = 75 mu m x 75 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 t he highest current gains ever reported in this base sheet resistance range, with de current gains over 225 (@ 200 A/cm(2)) at a base sheet resistance of 330 Omega/square. A high de current gain of 220 (@ 10 kA/cm(2)) was also confirmed in small area devices (L = 8 mu m x 8 mu m). High-frequency test s on a separate set of wafers grown under the same conditions indicate thes e high current gains can be achieved without compromising the RF characteri stics: Both high and normal gain devices exhibit an f(t) similar to 68 GHz and f(max) similar to 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 t he space charge region, we conclude that an improvement in the minority car rier lifetime is responsible for the observed increase in de current gain. Moreover, we observe a thickness-dependent variation in the effective minor ity carrier lifetime as the gains increase, along with a nonlinear dependen ce 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.