Comparison of Current-Induced Migration of Be and C in GaAs/AlGaAs HBTs
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COMPARISON OF CURRENT-INDUCED MIGRATION OF Be AND C IN GaAs/AlGaAs HBTs F. REN, T. R. FULLOWAN, J. R. LOTHIAN, P. W. WISK, C. R. ABERNATHY, R. F. KOPF, A. B. EMERSON, S. W. DOWNEY AND S. J. PEARTON AT&T Bell Laboratories, Murray Hill, NJ ABSTRACT We contrast the stability under bias-aging conditions of GaAs/AlGaAs HBTs utilizing highly Be- or C-doped base layers. Devices with Be doping display a rapid degradation of dc current gain and junction ideality factor. At 200 0 C, a 2 x 10 gm 2 Be-doped device (4 x 10 19 cm- 3 base doping) operated at a current density of 2.5 x 104 A . cm- 2 shows a decrease in gain from 16 to 1.5 within 2h. Under the same conditions a C-doped device with even higher base-doping (7 x 1019 cm=3 ) is stable over periods of 36h, the longest time we tested our structures. The degradation of Be-doped devices is consistent with the mechanism of recombination-enhanced diffusion of interstitials into the adjoining layers. Similar results are obtained with Zn-doped devices. Since C occupies the As sub-lattice rather than the Ga sublattice as with Be and Zn, it is not susceptible to reaction with Ga interstitials injected during growth or bias-aging.
INTRODUCTION Even though GaAs/AlGaAs Heterojunction Biplar Transistors (HBTs) have been fabricated for over a decade with relatively sophisticated growth and processing techniques, it is only recently that two problems related to the use of high base doping levels have been reported. The first is the diffusion of Zn and Be during Metal Organic Chemical Vapor Deposition (MOCVD) or Molecular Beam Epitaxial (MBE) growth.(1-3) It has been demonstrated by a number of authors that not only does this diffusion occur at high doping levels because of the concentration-dependent nature of Zn and Be redistribution and also the fact that the solubility limit may be exceeded leading to high concentrations of interstitials, but also that the presence of n-type doping in the sub-collector and emitter layers may significantly enhance the diffusion. A second problem is the current-induced degradation of the dc characteritics of Zn- or Bedoped HBTs during operation of the devices.(4-6) To simulate this effect, Uematsu and Wada(7 ) used Be-doped GaAs tunnel diodes to show that the Be diffusion under forward bias conditions was enhanced by a factor of -1015 at 300 K, and that the enhancement mechanism was most likely recombination-enhanced diffusion of Be interstitials. In HBTs the current densities are typically in the range 104 - 105 A cm- 2 somewhat larger than those use in the tunnel diodes,' 7 ) and one might expect substantial problems with the stability of such devices. In this paper, we show that in contrast to the problems encountered with Be-doped HBTs, devices utilizing carbon as the base dopant are not subject to significant degradation during operation. We also examine the effect of implant damage isolation in promoting Be diffusion within the HBT structure. Tie HBT layer structures consisting of a 6000A GaAs subcollector (n = 3 x 1018 cm 4000 A GaAs collecto
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