Semiconducting Materials and their Applications | Electrical Engineering

In this article we will discuss about the commonly used semiconducting materials and their applications.

Commonly Used Semiconducting Materials:

1. Germanium:

Germanium, the material in which transistor action was first observed, is one of the most important semiconductors.

The following points relate to germanium:

i. It is a grey metallic looking material.

ii. It is brittle and glass like in its mechanical properties.

iii. It crystalline in the diamond cubic lattice.

iv. It has an intrinsic resistivity of 47 Ω cm but may be doped with antimony or arsenic or give n-type resistivity of 0.01 or less Ω cm and with boron, gallium or aluminium to give p-type resistivity of 0.001 Ω cm or less.

v. The valence band of germanium is of interest, since it degenerate, so that there are two types of positive-hole conducting current namely, “light” and “heavy” holes, the former having 0.044, the latter having 0.28, the mass of free electron. The third degenerate “spin-orbit split” band is of minor significance.

vi. Germanium is usually prepared in high purity single-crystal form for electronics use by pulling from the melt, either vertically or horizontally. To obtain intrinsic material, impurities of active elements must be in the range of 1 part/billion or less.

vii. Junctions of p-n type have been made in germanium; alloyed junctions involve greater amounts of impurity, are produced at lower temperatures and are made directly from metals, which form a eutectic with germanium. Such alloyed junctions tend to do abrupt in transition and usually involve recrystallization phenomena.

viii. The ordinarily active impurities such as gallium, indium, arsenic and antimony tend to diffuse rather slowly in germanium. The donors diffuse roughly an order of magnitude faster than the acceptors.

ix. Germanium is notable, not only for the number of impurity elements with which it may be used to create deep impurity energy levels, but also for sensitivity it has to other elements (copper, lithium, nickel and gold) which can move about in the crystal at high speeds at low temperature.

2. Silicon:

Silicon has become increasingly important as a transistor material, to such an extent that its properties are now being studied more than those of germanium. Its higher band gap offers a greater temperature range than that of germanium but silicon is handicapped by the lower carrier mobilities 1200, and 250 cm²/volt-sec electrons and holes respectively compared with values of 3600 and 1700 for germanium. The drawback limits applications of silicon in high frequency transistors.

The following points relate to silicon:

i. Silicon is somewhat more difficult than germanium to produce and purify because of its higher melting point (1420°C).

ii. The properties of silicon are notably sensitive to the presence of oxygen, which is usually present to a level of 10¹⁷ to 10¹⁸ atoms/cm³ unless special care is taken to exclude it.

iii. Oxygen in silicon tends to introduce instabilities when the material is subjected to heat treatments at high temperatures. The mechanism is not well understood, although it is likely that interestial oxygen acts as a donor. It may also be combined with other impurity atoms to produce neutral complexes.

iv. Silicon is one of the most sensitive elements to nuclear radiations, stemming from its low atomic weight and high resistivity of the material as generally used.

v. The bulk properties of silicon under radiation are particularly sensitive to the presence of oxygen, since the defects produced seem to combine readily with oxygen to produce active centres.

vi. Silicon behaves much like germanium in its sensitivity to a wide variety of impurities.

vii. Silicon like germanium is also characterised by the number of impurities which besides having deep levels, move rather rapidly through the lattice at moderate to high temperature. Among these are copper, iron, manganese, nickel and cobalt.

3. Compound Semiconductors:

Even before germanium and silicon became important, compound semiconductor were the objects of much interest. The foundations of semiconductor science were laid on the basis of studies with copper oxide, zinc sulphide, silicon carbide, and zinc oxide among others.

Some of the compound semiconductors are discussed below:

i. Gallium Arsenide:

The characteristics of gallium arsenide are very closely related to those of germanium. This is understandable, since gallium is the third column neighbour and arsenic the fifth-column neighbour of germanium in the periodic table.

The following properties of gallium arsenide make it inferior to silicon and germanium:

(a) The high melting point (1300°C) combined with the high vapour pressure of arsenic at 1200 to 1300°C, makes the production of gallium arsenide of electronic grade an extremely difficult one.

(b) Horizontal furnaces using zone refining techniques have proved the best, but the resulting material still is not competitive with silicon and germanium in purity or structural perfection.

(c) Resistivities of 1 to 10 Q cm are, generally obtained and mobilities of 5000 to 6000 cm²/volt-sec are common. However life time values are low.

Gallium arsenide has proved to be useful for a number of very important devices, including some varieties of switching and parametric diodes, tunnel diodes, semiconductors lasers and hot electron “Gun-effect” diodes.

ii. Indium Antimonide:

It is of interest because it has the highest room-temperature electron mobility (70,000 cm²/volt-sec) of any known material. Because of the low melting point. 525°C, Indium antimonide (InSb) is much easier to prepare in single crystal form than gallium arsenide (GaAs).

iii. Cadmium Sulphide:

Cadmium sulphide (CdS) melts only under at high pressure, it has been used commercially as a photo conductor for many years, as well as constituent of cathode-ray phosphorous.

It can be prepared in the resistivity range from 10 to about 10¹² Ω-cm, depending on the presence of defects and the impurities. It is extremely difficult to produce p-type CdS.

Applications of Semiconducting Materials:

1. Copper oxide and selenium, were the first materials to be used to serve as rectifiers.

2. Germanium and silicon rectifiers came into commercial use somewhat later after copper-oxide and selenium. Germanium rectifiers found application earlier than silicon ones. One of the reasons for this is that it is easier and simpler to produce germanium mono-crystals, although the process involves considerable technological difficulties. The Germanium and silicon semiconductors find wide use in both high frequency and commercial-frequency circuits particularly as non-controlled rectifiers (diodes) and controlled rectifiers (for example, silicon controlled rectifiers (SCR)).

3. Non-linear resistors. These are also called varistors. These are the semiconductors whose resistance is marked by dependence on the applied voltage, due to which the current rise non-linearly with rise in voltage. These are made mainly from silicon carbide obtained by electrically heating a mixture of quartz sand with carbon to temperature of about 2000°C. This is commonly known as synthetic (electrical) carborundum.

4. Temperature-sensitive resistors. These are also called Thermistors. They possess a negative temperature resistivity of high absolute value. They are made from oxides of certain metals such as copper, manganese cobalt, iron and zinc. Thermistors are produced in the form of disc, short rods, beads etc. by ceramic techniques.

5. Photo-conductive and photo-voltaic cells. These are prepared from the materials which possess high sensitivity to light. The materials are sulphide, selenides and tellurides. Sometimes germanium and silicon are also used for the purpose. These days considerable use is made of silicon photo-voltaic cells, more commonly called solar cells which serve to convert solar radiant energy into electric energy for spacecraft power supply, etc.

Photo-voltaic cells find wide applications in the following:

(i) Automatic control systems.

(ii) Television circuits.

(iii) Sound motion picture recording and reproducing equipment.