The fabrication and characterization of MBE-grown AlGaAs/GaAs heterojunction bipolar transistors(HBT's) are described for graded and abrupt heterojunction. Linear grading is accepted for 300Å undoped grading layer. To prevent Be diffusion to the wide band gap emitter, undoped GaAs space layer is introduced. Emitter edge thinning is used to reduce surface recombination current. The layer structure consists of a 500Å-thick collector layer doped $5\times10^{16}cm^{-3}$ with Si, a 2000Å-thick base layer doped $1\times10^{19}cm^{-3}$ with Be, a space layer, a grading layer, and a 3000Å $Al_{0.3}Ga_{0.7}$ As layer doped with Si to $5\times10^{18}cm^{-3}$. Emitter cap layer, and sub-collector layer is introduced to get better ohmic characteristics.
Current gain of abrupt HBT's β=18 is greater than graded HBT's β=12 at collector current density $J_C=2\times10^3$ A/㎠. But, the ideality factor of graded HBT's is better than abrupt HBT's. In graded HBT, severer negative differential output resistance(NDR) was obtained. Measured AC characteristics for abrupt HBT's of emitter size 3×20μ㎡ is $f_T=8GHz$, $f_{max}=3.5GHz$ with relatively large base width. The better AC characteristics is expected when base width is reduced to typical value 500∼1000Å.
