Abstract
The effect of hot-electron injection energy (E/sub i/) into the base on the high-frequency characteristics of In/sub 0/52/(Ga/sub 1-x/Al/sub x/)/sub 0.48/As/InGaAs abrupt heterojunction bipolar transistors (HBTs) is investigated by changing the composition of the emitter. There exists an optimum E/sub i/ at which a maximum current gain cutoff frequency (ft) is obtained. Analysis of hot-electron transport in the base and collector by Monte Carlo simulation is carried out to understand the above phenomenon. The collector transit time ( tau /sub c/) increases with E/sub i/, because electrons with higher energy transfer from the Gamma valley into the upper L and X valleys. At first, the base transit time ( tau /sub b/) decreases with E/sub i/ at the low E/sub i/ region. However, tau /sub b/ does not decrease monotically with E/sub i/, because of the nonparabolicity in the energy-band structure of InGaAs. Consequently, there exists a minimum in the sum of tau /sub b/ and tau /sub c/, in other words a maximum f/sub t/, at an intermediate value of E/sub i/. >
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