Effect of reduced temperature on the fT ofAlGaAs/GaAs heterojunction bipolar transistors

The high-frequency and DC performances of single-heterojunction Al _0.25Ga_0.75As/GaAs heterojunction bipolar transistors (HBTs) have been measured at temperatures between 300 and 110 K. It is found that the maximum unity-current-gain cutoff frequency increases from 26 GHz at 300 K to 34 GHz at 110 K. It is shown that electron diffusion as determined from the majority-carrier mobility does not accurately estimate the base transit time, at least until corrections for degeneracy and minority-carrier mobility enhancement are included. Reasonable agreement is obtained assuming that base transport is limited by the thermal velocity of electrons at reduced temperatures