  We demonstrate graded AlGaN/GaN heterojunction bipolar transistors (HBTs)
with selective injection of minority carriers across a p-GaN base and patterned
regrown base contacts. The selective injection design regulates minority
carrier transport under emitter-base forward bias through a thin base region,
while thick and highly doped p$^+$ GaN regrown layers patterned alongside the
thin base regions are utilized to lower the base contact resistance. With
SiO$_2$ employed as a spacer between the emitter and the p$^+$ regrown layers,
the device with an interdigitated emitter/base-contact stripe design displayed
a maximum collector current density (I$_C$) of 101 kA/cm$^2$, a maximum current
gain ($\beta$) of 70 at I$_C$ $\sim$ 1 kA/cm$^2$ and $\sim$ 11 for I$_C$ $>$ 50
kA/cm$^2$. The reported results demonstrate the potential of the selective
injection approach to break the long-existing HBT design tradeoff between base
resistance and current gain for next-generation radio frequency and mm-Wave
applications.
