Gallium arsenide electronic properties

Gallium arsenide is epitaxially deposited on a silicon substrate and the resulting composite combines the mechanical and thermal properties of silicon with the photonic capabilities and fast electronics of gallium arsenide. 

Silicon is not as prominent a material in optoelectronics as it is in purely electronic applications, since its optical properties are limited. Yet it finds use as a photodetector with a response time in the nanosecond range and a spectral response band from 0.4 to 1.1 im, which matches the 0.905 im photoemission line of gallium arsenide. Silicon is transparent beyond 1.1 im and experiments have shown that a red light can be produced by shining an unfocused green laser beam on a specially prepared ultrathin crystal-silicon slice.CVD may prove useful in preparing such a material. 

Very broadly speaking, two situations have to be considered in explaining devices such as those we have mentioned. In the first, which is relevant to the ruby laser and to phosphors for fluorescent lights, the light is emitted by an impurity ion in a host lattice. We are concerned here with what is essentially an atomic spectrum modified by the lattice. In the second case, which applies to LEDs and the gallium arsenide laser, the optical properties of the delocalised electrons in the bulk solid are important. 

A major advantage of the gallium arsenide (GaAs) laser is that it has the electron distribution of a semiconductor The main difference between electrons in semiconductors and electrons in other laser media is that in semiconductors all of the electrons occupy and thus share the entire crystal volume. Although all semiconductors possess this property, not all of them can be used as lasers. See Fig. 4. 

Gallium arsenide is a compound semiconductor with a combination of physical properties that has made it an attractive candidate for many electronic applications. From a comparison of various physical and electronic properties of GaAs with those of Si (Table 4), the advantages of GaAs over Si can be readily ascertained. 

Gunn devices belong to a group called transferred electron oscillators and are the ones most often encountered in MMW spectrometry, as they offer the lowest noise figure. They rely on a bulk property of gallium arsenide and indium phosphide when a DC voltage is applied across the end contacts of the n-type material. As the voltage is increased, the current initially increases linearly and then starts to oscillate, with a period closely related to the transit time of the carriers between the contacts across the bulk material. The device is housed in a cavity coupled to a transmission line and is used as a source of MMW radiation, the frequency of which can be tuned mechanically and electronically. 

Arsenic is an element with the symbol As and the atomic number 33. It can occur as a pure element but is most often found in minerals containing sulfur and metals. Arsenic can exist in different structural forms (allotropes). However, gray arsenic is the most common. It is a metalloid that is brittle and a bit shiny. See Fig. 5 [25]. This form has metallic properties and has been used in industry to strengthen alloys of copper and lead. Arsenic is also a common n-type dopant in semiconductor electronic devices (example gallium arsenide is a semiconductor). Over the years arsenic and its compounds were used in the production of products like pesticides, insecticides, and treated wood items. However, because of its toxicity and harmful effects to humans, arsenic s applications have decreased. 

Owing to the fact that valence electrons determine bonds, the electrical properties of a material are related to the bond type. In conductors such as metals, alloys, and intermetallics, the atoms are bound to each other primarily by metallic bonds, and metals such as tungsten or aluminum are good conductors of electrons or heat. Covalent bonds occur in insulators such as diamond and silicon carbide and in semiconductors such as silicon or gallium arsenide. Complexes and salts have ions that are bound with electrostatic forces. Ionic conductors can be used as solid electrolytes for fuel cells because solids with ionic bonds may have mobile ions. Most polymers have covalent bonds in their chains but the mechanical... 

Elemental silicon is central to the vast industry of solid-state electronics. Appropriately doped with other elements, it forms a variety of semiconductors that constitute most transistors and integrated circuits. Other elements and compounds such as germanium or gallium arsenide have also found a niche as semicondnctors in electronics, but silicon occupies the prime position. How fortunate that it is the second most abundant element in the Earth s crust. Its compounds with the most abundant crustal element, oxygen, are equally central in many different aspects of chromatography. Silica, silica gel, glass, qnartz, fused silica, and silicones all have a remarkable variety of key roles to play in chromatography. Let us familiarize ourselves with some of their relevant properties. 

The promising electronic properties of beta-silicon carbide are compared to those of other semiconductor materials in Table 8.3 of Ch. 8. A major advantage of this material is its high-temperature potential (>1000"C) which far surpasses that of other semiconductors. Beta-SiC should also be more effective than silicon or gallium arsenide particularly in microwave and millimeter-wave devices and in high-voltage power devices. The development of SiC as a semiconductor is still in the laboratory state. 

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