Low turn-on voltage, non-electron blocking double HBT structure
View Patent ↗A double heterojunction bipolar transistor structure having desirable properties of a low base-emitter turn-on voltage and no electron blocking discontinuities in the base-collector junction. These properties are achieved by selecting base, emitter and collector materials to provide a bandgap profile that exhibits abrupt transitions at the heterojunctions, such that both abrupt transitions are due to transitions in the valence band edge of the bandgap, but not in the conductive band edge of the bandgap.
1. A heterojunction bipolar transistor structure, comprising:
a substrate;
a semiconductor collector layer formed over the substrate;
a semiconductor base layer formed over the collector layer; and
a semiconductor emitter layer formed over the base layer;
wherein a base-emitter heterojunction is formed between the base layer and the emitter layer, and a base-collector heterojunction is formed between the base layer and the collector layer;
wherein the structure has a bandgap profile defined by a conduction band edge and a valence band edge;
wherein the bandgap profile when the base layer, emitter layer and collector layer are considered isolated makes an abrupt transition at the base-emitter heterojunction and at the base-collector heterojunction, and the abrupt transition is in each case defined by an abrupt transition in the valence band edge but not in the conduction band edge; and
wherein the bandgap profile when the structure layers are considered to be joined, provides for a reduced base-emitter turn-on voltage and no electron trapping at the base-collector heterojunction.
2. A heterojunction bipolar transistor structure as defined in claim 1 , wherein:
the emitter layer and the collector layer are of n-type indium phosphide (InP); and
the base layer is of p-type indium aluminum gallium arsenide (InAlGaAs).
3. A heterojunction bipolar transistor structure as defined in claim 2 , wherein the base layer is formulated as In 0.53 (Al 0.42 Ga 0.58 ) 0.47 As to provide the desired bandgap profile.