DRAMATIC ENHANCEMENT IN THE GAIN OF ALGAAS/GAAS HETEROSTRUCTURE BIPOLAR TRANSISTORS BY SURFACE PASSIVATION.

A promising chemical technique capable of reducing the surface recombination current in a three-terminal device is discussed. By spin coating a thin film of Na//2S//0//. //9H//2O after device fabrication it is possible to realize a sixtyfold increase in the current gain ( beta ) of an AlGaAs/GaAs heterostructure bipolar transistor (HBT), attributable to a reduction in the surface recombination velocity to a value of minus 7000 cm/s. HBTs grown by OMCVD (organometallic chemical vapor deposition) were fabricated with emitter sizes of 50 multiplied by 150 mu m**2, 40 multiplied by 100 mu m**2, 18 multiplied by 40 mu m**2, and 6 multiplied by 20 mu m**2. As processed transistors exhibited incremental current gain as high as 4000 (collector current density J//c approximately 500 angstrom/cm**2) for a base thickness and doping level of 0. 15 mu m and 1 multiplied by 10**1**8 cm** minus **3, respectively. The current gain of these devices was found to decrease markedly at low collector currents and to depend on the perimeter-to-area ratio. At a collector current density as low as 1 mA/cm**2 the passivated transistors exhibited beta 1500 as compared to beta approximately 25 for untreated devices.