Heterojunction
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A heterojunction is a semiconductor junction which is composed of layers of dissimilar semiconductor material, these materials having non-equal band gaps. In such a structure, the implementable diode characteristics can closely approach those of an idealized diode. Furthermore, the diode model parameters that define the diode current vs. voltage 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 compound semiconductors to form lasing heterojunctions.
When a heterojunction is used as the base-emitter junction of a bipolar junction transistor, extremely high forward gain and low reverse gain result. This translates into very good high frequency operation (values in tens to hundreds of GHz) and low leakage currents.
Heterojunction manufacturing requires the use of molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD) technologies into order to precisely control the deposition thickness and alternating band gap value of the semiconductor. MBE tends to be very production uneconomical compared to traditional silicon device fabrication. Gallium arsenide (GaAs) is a typical semiconductor used in heterojunctions.
