High-Current-Gain Direct-Growth GaN/InGaN Double Heterojunction Bipolar Transistors

We report high-current-gain n-p-n GaN/InGaN double-heterojunction bipolar transistors (DHBTs) using a direct-growth fabrication processing approach. The impact of the indium composition in the base layer was studied, and a burn-in effect using a constant-base-current stressing method was observed. We found that DHBTs with higher indium composition in the InGaN base layer may help reduce the base resistance and lower the surface recombination current but may result in higher bulk recombination current. A device burn-in effect was also studied. The postprocessing current stressing step helps increase free-hole concentration in the base, reduce the bulk recombination current, and enhance the current gain. As a result, a direct-growth
$\hbox{GaN/In}_{0.03}\hbox{Ga}_{0.97}\hbox{N}$
DHBT with a peak current gain of 105 and a collector current density
$ > \hbox{6.5}\ \hbox{kA/cm}^{2}$
was demonstrated on a sapphire substrate.