Fabrication and Analysis of Current Transport in N P N and P N P Gallium-Arsenide - Arsenide Heterojunction Bipolar Transistors.

An experimental study of heterojunction bipolar transistors (HBT's) fabricated in the GaAs/Al_ {x}Ga_{1-x}As material system is reported. Both molecular beam epitaxy (MBE) and organometallic vapor phase epitaxy (OMVPE) have been used to grow Npn and Pnp HBT device structures. The problem of base dopant redistribution into the emitter layer is examined with respect to variations in epitaxial growth temperature, undoped base spacer layer thickness, V/III molar ratio during growth, and alternative emitter structures utilizing binary GaAs/AlAs superlattices (SL's). Secondary Ion Mass Spectrometry (SIMS), measurements of transistor current-voltage (I-V) characteristics, and collection of electroluminescent (EL) spectra are combined to consistently indicate the presence of excess conduction band (Npn) or valence band (Pnp) energy barriers associated with the incorporation of base dopant in the wide bandgap emitter. The HBT current gain is primarily limited by surface recombination at low base-emitter bias (<1.0 V) and by lateral carrier spreading in the quasi-neutral base at higher values of bias. Linear grading of the AlAs mole fraction, either by continuous alloy compositional grading or by a chirped period binary GaAs/AlAs SL, causes the magnitude of the base current to increase and the ideality factor of the base current to decrease. Surface passivation with wide bandgap, semi-insulating Al_{x}Ga_ {1-x}As is shown to reduce the surface recombination current by over one order of magnitude in Be-implanted GaAs p-n junctions prepared with rapid thermal annealing. Surface passivation techniques are also examined for surface passivation of the GaAs/Al_ {x}Ga_{1-x}As HBT. The Npn HBT's exhibit useful current gain up to 14 GHz and maximum power gain to 10 GHz, while the Pnp HBT's have current gain to 1.5 GHz and power gain to 2 GHz. Calculations indicate that the Npn HBT's are limited by the transit time of electrons across the 1500 A base layer and by a large value of base series resistance due to unalloyed base ohmic contacts. The Php HBT's are limited by the transit time of holes across the base layer and by large values of collector and emitter series resistances associated with the ohmic contacts. Ring oscillator circuits utilizing Npn HBT's are operated with gate delay times in the 100-200 ps range. (Abstract shortened with permission of author.).