Homojunction is a semiconductor device or interface that occurs between similar material layers with the same band gap but different doping concentrations. In most cases, it occurs at the interface between n-type (called donor doping) and p-type (called acceptor doping) semiconductors, such as silicon. For example, n-type to n-type junctions will also be considered as homojunction, even if the doping level is different. In other words, homojunction is a junction formed by the same semiconductor, including pn junction, pp junction and nn junction.
What is heterojunction?
Heterojunction is an interface between two layers or regions of different materials/crystal semiconductors or solid materials. A device in which multiple heterojunctions are concentrated is called a heterostructure.
Molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) technology is generally required to manufacture heterojunction. Some special applications of heterojunction are:
Solar cell: In 1983, the heterojunction (HIT) solar cell structure with inner thin layer was first developed and finally commercialized by Sanyo/Panasonic. The intrinsic thin layer (HIT) solar cell is one of the most effective single junction silicon cells at present, because its conversion efficiency is 26.7%.
Laser: – By combining small direct bandgap materials such as gallium arsenide (GaAs) and two large bandgap layers such as aluminum arsenide (AlAs), the carrier can be very small, so that the laser can be generated at low threshold current at room temperature. One of the main advantages of using semiconductor lasers is that heterostructures can be used as waveguides.
Bipolar transistor: – When heterojunction is used for bipolar crystal transistor, it will obtain extremely high forward gain and low reverse gain. This results in very good high-frequency operation (values in the tens to hundreds of GHz) and low leakage current. Such devices are called heterojunction bipolar transistors (HBTs).
Field effect transistor: – It is used for high electron mobility transistor (HEMT), which can work at significantly higher frequencies (more than 500 GHz).
In a word, heterojunction is a special PN junction, which is formed by two or more layers of different semiconductor material films deposited on the same base in turn. These materials have different energy band gaps. They can be compounds such as gallium arsenide or semiconductor alloys such as silicon-germanium.
Extended data
Schottky junction solar cell is a solar cell formed by using the Schottky barrier on the metal-semiconductor interface. It is called Schottky junction solar cell, or MS cell for short. At present, it has developed into metal-oxide-semiconductor (MOS), metal-insulator-semiconductor (MIS) solar cells, etc.
Composite junction solar cells, which are formed by two or more P-N junctions, are called composite junction solar cells. They are also divided into vertical multi-junction solar cells and horizontal multi-junction solar cells. For example, a high-efficiency MISNP composite junction silicon solar cell can be formed by the combination of a (MIS) solar cell and a P-N junction silicon cell, and its efficiency has reached 22%. Composite junction solar cells are often made into cascades, with wide band gap materials placed in the top region to absorb high-energy photons in the sun; The narrow band gap material is used to absorb low energy photons, which broadens the spectral response of the whole cell. At present, the efficiency of aluminum gallium arsenide-gallium arsenide-silicon solar cells has reached 31%.
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