Substantial improvement by substrate misorientation in dc performance of Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As double-heterojunction NpN bipolar transistors grown by molecular beam epitaxy

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Abstract
Al0.5Ga0.5As/GaAs/Al0.5Ga0.5As NpN double‐heterojunction bipolar transistors have been grown simultaneously by molecular beam epitaxy on (100) and 3° off (100) towards 〈111〉A GaAs substrates. On the tilted substrate, the current gain is significantly higher, comparable to the maximum expected value, with a marked reduction of its dependence on current and device geometry. For 10 μm×40 μm emitter devices, maximum common emitter current gains (β) of 1630 and 725 were measured at a current density of ∼6.3 kA/cm2 on the tilted and flat substrates, respectively. On the tilted substrate, both the emitter injection efficiency and base transport factor are increased. We have used compositionally graded emitter‐base (e‐b) and abrupt base‐collector (b‐c) junctions. We find that the abrupt b‐c junction does not result in an offset voltage but certainly reduces the electron collection efficiency, and hence the gain, in the region where it is forward biased. The device characteristics and the current gain on both substrates were essentially independent of temperature between 25 and 100 °C, except for a slight decrease of gain with increasing temperature.