Heterojunction bipolar transistor structure with a bandgap graded hole barrier layer

A heterojunction bipolar transistor (HBT) structure with a bandgap graded hole barrier layer, comprising:

• a sub-collector layer, formed of an N-type group III-V semiconductor and stacked on a substrate;
• a collector layer, stacked on the sub-collector layer and formed of a group III-V semiconductor;
• a hole barrier layer, stacked on the collector layer and formed of at least one of AlGaAs, AlGaAsN, AlGaAsP, AlGaAsSb, and InAlGaAs, wherein aluminum composition is less than 22%, and In, N, P, and Sb compositions are respectively less than or equal to 10%;
• a base layer, stacked on the hole barrier layer and formed of a P-type group III-V semiconductor;
• an emitter layer, stacked on the base layer and formed of an N-type group III-V semiconductor having bandgap larger than that of the base layer;
• an emitter cap layer, stacked on the emitter layer and formed of an N-type group III-V semiconductor;
• and
• an ohmic contact layer, stacked on the emitter cap layer and formed of an N-type group III-V semiconductor;
• wherein bandgaps of the hole barrier layer at least comprise a gradually increasing bandgap from the base layer towards the collector layer and a largest bandgap of the hole barrier layer is greater than bandgaps of the base layer and the collector layer respectively.