Heterojunction
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A heterojunction is a semiconductor diode junction which is composed of alternating layers of semicondcutor material. Each alternating layer has a alternating band-gap. In such a structure, the implementable diode characterstics can closely approach those of an ideal diode. Further the diode model parameters that define the diode's current-vs-voltage (IV) response can be tuned by adjusting the thicknesses and band-gaps of the layers.
Semiconductor diode lasers used in CD & DVD players and fiber optic transceivers are manufactured using alternating layers of various III-V and II-VI semiconductor to form lasing heterojunctions.
When a heterojunction is used as the base-emitter junction of a bipolar transistor, extremely high forward gain and low reverse gain result. This translates into very good high frequency response (Ft values in high 10s to 100s of GHz) and low leakage currents.
Heterojunction manufacturing requires the use of molecular beam epitaxy (MBE) or MOCVD technologies into order to precisely control the deposition thickness and alternating band-gap value of the semiconductor. Especially MBE tends to be very production uneconomical compared to traditional silicon semiconductor device fabrication. This is why Gallium arsenide has been the "next big technology" to wipe out Silicon for over 30 years.
Recent advances in SiGe manufacturing technologies will likely fulfill many of the promises made in the past by Gallium Arsenide proponents.