What are Compound Semiconductors

Semiconductors Includes:
What is a semiconductor     Holes & electrons     Semiconductor materials     Compound semiconductors     Silicon carbide, SiC     Gallium nitride, GaN    

Silicon is by far the most widely used semiconductor material used, but in the past germanium was also used.

Nowadays there are many compound semiconductors that are formed from two or more elements that are being used in specific areas to give considerably enhanced performance in many electronic circuit designs.

New electronic components using compound semiconductors, with materials like silicon carbide, gallium arsenide, gallium nitride and the like are able to give enhanced performance in a number of areas and their use is increasing, although silicon still remains the dominant semiconductor material and will remain so for the foreseeable future.

Compound semiconductor basics

The basic form of semiconductor uses materials from single types of atom. Silicon, Si, is the most widely used, but other elementary semiconductors include germanium, Ge, tin, Sn, selenium, Se, and tellurium, Te.

Silicon, germanium and tin come from column IV in the periodic table of elements, and selenium, and tellurium are from column VI.

Although silicon is a great semiconductor to use and possesses many attractive properties that can be used within semiconductor devices, it is not perfect in every sense.

In some cases compound semiconductor materials are able to provide improvements in some areas of performance, and it is for this reason that they are used.

A compound semiconductor is basically a single-crystal semiconductor material that contains two or more elements.

By using two and sometimes more elements in a single crystal of semiconductor, soem properties are changed whereas others are introduced. For example, light emitting diodes use compound semiconductors, whereas silicon is not used because it does not exhibit this feature.

Common compound semiconductors

There are many different types of compound semiconductor material, but some have gained far more use than others.

It will be seen from the list that most compound semiconductors are from combinations of elements from Group III and Group V of the Periodic Table of the Elements. However other compound semiconductors are made from elements in Groups II and VI. It is also possible to use different elements from within the same group, to make compound semiconductors.

Some of the key materials have been listed below.

• Gallium arsenide, GaAs:   Gallium arsenide is a type III-V compound semiconductor as the elements come from columns III and V of the periodic table. GaAs is the second most widely used type of semiconductor after silicon.

  Gallium arsenide is widely used in high performance RF devices where its high electron mobility is the key to its high level of performance. It is also used as substrate for other III-V semiconductors, and especially those using three elements, e.g. InGaAs and GaInNAs.

  However it is a brittle material and has a lower hole mobility than silicon which makes applications such as P-type CMOS transistors not feasible.

  It is also relatively difficult to fabricate and this increases the costs of GaAs devices. It's use tends to be limited to applications where a cost premium is viable to obtain the higher level of performance.




• Silicon carbide, SiC:   Silicon carbide is a compound semiconductor material where both elements come from group IV of the periodic table of elements. This compound semiconductor finds uses in a number of applications. It is often used in power devices where its losses are significantly lower and operating temperatures can be higher than those of silicon based devices. Silicon carbide has a breakdown capability which is about ten times that of silicon itself. Forms of silicon carbide were types of semiconductor material that were used with some early forms of yellow and blue LEDs.

  Read more about . . . . Silicon Carbide SiC.
  
  

• Gallium Nitride, GaN:   This type of semiconductor material uses elements from columns III and V of the periodic table of elements.

  GaN is starting to be more widely in microwave transistors where high temperatures and powers are needed. It is also being used in some microwave ICs. GaN is difficult to dope to give p-type regions and it is also sensitive to ESD, but relatively insensitive to ionising radiation. It is also used in some blue LEDs.

• Gallium phosphide, GaP:   Tis is a groups III-V material and it finds its major uses within LED products and technology.

  It was used in many early low to medium brightness LEDs producing a variety of colours dependent upon the addition of other dopants. Pure Gallium phosphide produces a green light, nitrogen-doped, it emits yellow-green, ZnO-doped it emits red.

• Cadmium Sulphide, CdS:   This compound semiconductor uses elements from groups II and VI of the periodic table. Interestingly there is a large difference within the group numbers for this compound semiconductor.

  It is not as widely used as many other compound semiconductors, but it does find uses within photo-resistors and solar cells.

• Lead sulphide, PbS:   This compound semiconductor uses elements from groups IV and VI in the periodic table of elements

  It was widely used in early crystal radio sets where in its form as galena, it was used in many Cat's Whisker radio detectors. In this very early semiconductor diodes a point contact was made with the tin wire onto the galena crystal to provide the PN junction and the resulting rectification of the signals.

Cost of compound semiconductors

Silicon os the most widely used semiconductor and there are many reasons for this.

Not only is its performance more than adequate for most semiconductor devices, but it is also easy to process. The raw silicon needs only to be refined (very well refined) but there are no additional processes to make the raw silicon into a compound semiconductor etc.

As there are now additional chemical processes required to create the raw material, apart from refining it, the material is relatively low cost. Other compound semiconductors require additional stages to create the raw semiconductor material. Often the crystals of these compound semiconductors are more difficult to grow.

It is also found that silicon is a very easy material to process as a semiconductor. It's properties are ideal for the various semiconductor processes required.

As a result, the processes for silicon are well established and have now been finely honed so that they are reliable and cheap.

Conversely the processes for many compound semiconductor materials are not so finely honed as they are used in a limited number of products. Many of the processes are more complicated as a result of the nature of the materials used - for example gallium arsenide is more brittle than silicon and needs to be processed more carefully.

As a result of the lower usage of compound semiconductors and the greater difficulties of using them, costs are higher.

In recent years the cost of manufacturing compound semiconductors has fallen significantly. The costs are still much higher than those for processing silicon. However the special properties of these compound semiconductor crystals have become more important in many areas. Often the fundamental material properties mean that devices fabricated from the various compound semiconductors can do things that are not possible with silicon. As a result they are being used increasingly in many areas.

Although silicon will remain the main material used for semiconductor devices, there are many areas where devices fabricated from compound semiconductor materials are able to provide a far higher level of performance for a given parameter or parameters. As a result semiconductor electronic components using materials like silicon carbide, gallium arsenide, gallium nitride and many more are being seen more frequently in new electronic circuit designs.

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