GaAsP/InGaP heterojunction bipolar transistors grown by MOCVD

Abstract

Heterojunction bipolar transistors with GaAs[subscript x]P[subscript 1−x] bases and collectors and In[subscript y]Ga[subscript 1−y]P emitters were grown on GaAs substrates via metalorganic chemical vapor deposition, fabricated using conventional techniques, and electrically tested. Four different GaAs[subscript x]P[subscript 1−x] compositions were used, ranging from x = 0.825 to x = 1 (GaAs), while the In[subscript y]Ga[subscript 1−y]P composition was adjusted to remain lattice-matched to the GaAsP. DC gain close to or exceeding 100 is measured for 60 μm diameter devices of all compositions. Physical mechanisms governing base current and therefore current gain are investigated. The collector current is determined not to be affected by the barrier caused by the conduction band offset between the InGaP emitter and GaAsP base. While the collector current for the GaAs/InGaP devices is well-predicted by diffusion of electrons across the quasi-neutral base, the collector current of the GaAsP/InGaP devices exceeds this estimate by an order of magnitude. This results in higher transconductance for GaAsP/InGaP than would be estimated from known material properties.