Influences of AlGaAs Emitter Band Gaps on Current Gains in AlGaAs/GaAs HBTs

Abstract

The influences of an emitter Al content on current gains in AlGaAs/GaAs heterojunction bipolar transistors are investigated experimentally. In the HBTs examined, a linearly graded band-gap base with heavy doping of 1×10¹⁹ cm^-3 is utilized, in which the AlAs mole fraction at the emitter-base interface is 0.1. Current gain increases from 6 to 68 for emitter Al content from 0.1 to 0.3. Experimental results are compared with the calculation, including Fermi-Dirac carrier distribution, band-gap shrinkage, and diffusion and drift transport in the base. The Al content difference between emitter and base junction necessary for obtaining sufficient current gain value is found to be approximately Δ x= 0.1.