Microwave properties of self-aligned GaAs/AlGaAs heterojunction bipolar transistors on silicon substrates

(Al,Ga)As/GaAs heterojunction transistors (HBT's) grown on Si substrates have been characterized at microwave frequencies and have been found to perform extremely well. For emitter dimensions of 4 × 20 µm
²
, current gain cutoff frequencies
f_{T} = 30
GHz and maximum oscillation frequencies
f_{\max}
of 11.5 GHz have been obtained in a mesa-type structure. These values compare very well with the
f_{T} = 40
GHz and
f_{\max} = 26
GHz which are the highest reported for HBT's on GaAs substrates in a nonmesa structure with an emitter width of ∼ 1.5 µm. These results clearly demonstrate the potential of HBT's in general at microwave frequencies, as well as the viability of GaAs on Si technology.