Thermal velocity limits to diffusive electron transport in thin-base np⁺n GaAs bipolar transistors

Abstract

We present experimental evidence that minority electron transport across a thin, quasineutral p+ GaAs region is limited by the thermal velocity of the electrons rather than by conventional diffusive transport. A set of GaAs homojunction np+n transistors with base widths of 4000, 2000, 1000, and 500 Å was fabricated and characterized. The diffusive model predicts that the dc collector current of the 500-Å base width transistors should be eight times larger than the collector current of transistors with a 4000-Å-wide base. The experimental results, however, show only a factor of ∼3.5 increase in collector current. The measured collector current versus base width characteristic agrees well with theoretical treatments of thin-base transport. These new results present evidence of quasiballistic electron transport in p+ GaAs and have important implications for GaAs transistor design.