Specialized semiconductor materials

Early semiconductor devices were based on germanium, although there were others around such as selenium used in rectifiers. Germanium doesn’t need to be as pure as silicon for making devices but has higher leakage and is not so good at high temperatures so silicon has taken over and is the mainstay of current electronics. However, there are a number of other semiconductor compounds which are in common use for specific purposes, such as speed and for special devices such as LEDs, lasers and photodetectors.

Gallium arsenide (GaAs) has been around for a long time and has been the choice for high speed for devices such as microwave transistors. It is also used as the base for LEDs or combined with other elements to make LEDs. However, newer compounds offer advantages for high frequency devices such as silicon germanium (SiGe). SiGe appeared a while ago and there are now a significant number of high speed integrated circuits based on SiGe. Texas Instruments have a large number of SiGe opamps, some of which are devices they acquired when buying National Semiconductor. The LMH6629 for example is a low noise SiGe opamp with 0.69nV/rtHz and 900MHz bandwidth.

Some of the Texas Instruments devices own devices such as the THS4511 are SiGe but they don’t often seem to publicize the technology in their datasheets very much, although they may make reference to the BiCom III process which is their SiGe complementary bipolar process.

Infineon have a silicon germanium carbon (SiGe:C) process which gives some very impressive discrete devices such as the BFP640ESD transistor. With low noise and wide bandwidth (around 46GHz) they are worth looking at as an alternative to GaAs transistors. At around $0.40 each they are not expensive.

Another new development is silicon carbide (SiC) which is being used and developed for high voltage, high current transistors. Cree make a range of SiC MOSFETs, Schottky diodes and power modules promising improved efficiency, higher switching speeds and reduced size. These could find their way into switch mode power supplies and high power, high voltage LED drivers. Anvil Semiconductors are working on various SiC developments as well.

While not strictly a different semiconductor, silicon on insulator (SOI) and silicon on sapphire (SOS) are worth mentioning. They are essentially the same but sapphire is probably a more expensive insulating material on which to base the process whereas silicon dioxide can be used as the base. This has the advantage of requiring little special processing compared to a conventional silicon IC process. The benefits of SOI are the low capacitance between devices and to substrate, low leakage and high isolation between devices allowing high voltage operation. This also allows the use of devices at higher temperatures as the leakage isn’t such a problem. Cissoid of Belgium are one company specializing in high temperature products which can operate from -55°C to +225°C and up to -200°C to +300°C with reduced performance. In order to utilize the high temperature benefits other parts of the process also need to be able to handle the higher temperatures such as the metalization.

SOI also offers radiation hardening i.e. the ability to withstand higher levels of ionizing radiation than conventional silicon devices so has uses in military ICs and the nuclear industry.

Various other materials such as GaAsP and InGaAlP are commonplace in conventional LEDs, and materials such as InGaN or GaN for UV LEDs and lasers, and other materials such as InGaAs for photodiodes for long wavelengths (1300nm to 1500nm) for fiber optic communications. However, the number of materials used for transistors, diodes and integrated circuits is a lot more limited.

www.compoundsemiconductor.net is one place to look for developments in the various compounds used for semiconductors.