Minority electron mobility and lifetime in the p⁺GaAs base of AlGaAs/GaAs heterojunction bipolar transistors

AlGaAs/GaAs heterojunction bipolar transistors with different base doping concentrations grown by gas source molecular beam epitaxy were fabricated and characterized at dc and high frequency. Three different base doping concentrations; 5×10¹⁸, 1×10¹⁹, and 5×10¹⁹ cm^-3 doped with Be were used for the characterization with the same structural and process parameters, including 0.2 μm base width. Minority electron mobilities in heavily doped p type were measured as 1.1×10³, 1.3×10³, and 3×10³ cm² V/s for 5×10¹⁸, 1×10¹⁹, and 5×10¹⁹ cm^-3, respectively, by using the current gain cutoff frequency (f_T) which agrees well with theoretical predictions in heavily doped p-type GaAs. Combining dc and high frequency measurements, electron lifetimes of 1.2×10 ^-9, 5.5×10^-10, and 2×10^-11 s have been obtained for these three dopings, respectively.