From ArticleWorld

Semiconductor junctions which are composed of layers of different materials are heterojunctions. Each material involved in a heterojunction has a different energy band gap, thereby giving a certain desired behaviour to the heterojunction. In such a structure, diode characteristics that are implementable can closely approach those of a diode which is idealised. Several of a diode model’s specifications that decide that voltage response for a certain diode current value can be tuned according to requirements by adjusting the thickness of semiconductor layers and taking into consideration their band gaps.

Manufacture of heterojunctions

The manufacture of heterojunctions is carried out by molecular beam epitaxy (MBE) or by the use of metalorganic chemical vapour deposition (MOVCD) techniques. Both involve a method of epitaxial growth of semiconductor materials. This is observed in the manufacture of gallium arsenide. The principle of operation in the first case is i.e. MBE is that gases are bound to cold surfaces by means of cryocondensation. Epitaxial growth starts with the heating of the compound required for the addition of a certain atomic species into the vacuum chamber. In an advancement of this method, room temperature can be used to carry out the appropriate reactions. Inside the ultra-high vacuum, the free atoms have a long mean-free path and collisions with other atoms are not very frequent. Everything is remotely controlled by a computer, thus facilitating the beams of atoms from the so-called “effusion cells” to attach to the substrate material and allow an epitaxial layer to form. The MOCVD is another widely used method of preparing epitaxial layers on a substrate by carefully controlled deposition of atoms. The substrate wafer to be layered is kept on a susceptor made of graphite, inside a reaction chamber which is heated by a RF induction heater. The temperature maintained may be between 500°C and 700°C depending on the compounds used. A medium pressure metal-organic gas is kept in the reaction chamber. The growth precursors like arsine (AsH3¬¬¬) for group V, trimethyl gallium Ga(CH3)3 or TMG for group III decompose on contact with the heated substrate, resulting in epitaxial layers being formed. Whereas molecular beam epitaxy requires a vacuum in order to facilitate the growth of crystals, metalorganic gases are used in the reactor at moderate pressures of 2 to 100kPa. Using technologies such as the ones mentioned above, the deposition thickness and alternating band gap values can be precisely controlled.

Applications and future scope

Semiconductor diodes using heterojunctions can be used in CD and DVD players and fibre-optic systems. When alternating layers of heterojunctions are used as base-emitter junctions of bipolar junction transistors (BJT), high forward gain and low reverse gain values are observed. This means extremely high frequency of operation in the order of several GHz. Low leakage currents obtained bring about an advantage. Gallium arsenide is an important semiconductor used in heterojunctions. Though it has not been able to gain more importance than silicon, future advances and research may see more of its widespread use.