| Research Abstract |
To realize ultra-high speed heterojunction bipolar transistors (HBTs), a "metallic" base layer with extremely low base resistance is necessary. In this work, carbon doped metallic p-type GaAs layers were grown by metalorganic molecular beam epitaxy (MOMBE) using trimethylgallium (TMG) and solid arsenic. A hole concentration of 1.5x10^<21> cm^<-3> with a resistivity of 1.9x10^<-4> OMEGAcm has been obtained, which is the highest hole concentration for GaAs reported so far. Furthermore, carbon was found to be much less diffusive than other pーtype dopants, such as beryllium and zin c, even at extremely high doping levels. Therefore, carbon is a promising dopant for the HBT applications. However, at high doping levels, the lattice constant decreases with increasing carrier concentration, which creates a lattice mismatch between heavily carbon doped pーGaAs and moderately doped GaAs. To compensate this lattice mismatch, a small amount of indium was added to grow InGaAs. A hole concentration of 2.6x10^<20> cm^<-3> was obtained for InGaAs (In-5%) lattice matched to GaAs substrate. It was also found that the band-gap of carbon doped GaAs decreases with carrier concentration. Since this shrinkage is as large as 200 meV for a hole concentration of 5x10^<20> cm^<-3>, a pseudo-HBT has been proposed, which uses GaAs instead of AlGaAS for the emitter layer. In pseudo-HBTs, because of the band-gap shrinkage of heavily carbon doped GaAs in the base, a high emitter injection efficiency, and excellent high frequency performance can be expected, as in conventional HBTs. A pseudo-HBT with 100 nm thick base (p=1x10^<20> cm^<-3>) was fabricated and a DC current gain of 1.7 was obtained.
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