THERMAL VELOCITY LIMITS TO DIFFUSIVE ELECTRON-TRANSPORT IN THIN-BASE NP(+)N GAAS BIPOLAR-TRANSISTORS

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 Angstrom was fabricated and characterized. The diffusive model predicts that the de collector current of the 500-Angstrom base width transistors should be eight times larger than the collector current of transistors with a 4000-Angstrom-wide base. The experimental results, however, show only a factor of similar to 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.