Chapter 18. Semiconductors – Material Science and Metallurgy

18

Semiconductors

18.1 INTRODUCTION

Semiconductor devices, the electronic components made of semiconductor materials, are essential in modern-day electrical equipments, from computer to cellular phone to digital audio players. A semiconductor is a solid, whose electrical properties can be controlled over a wide range. Silicon is the most commercially exploited semiconductor.

The intrinsic electrical properties of a semiconductor can be permanently modified by introducing controlled quantities of impurity atoms. This process is called ‘doping’.

In many semiconductors, when electrons fall from the conductor band to the valence band, they often emit light, i.e. photoemission e.g. a light-emitting diode (LED).

This chapter studies the behaviour of intrinsic and extrinsic semiconductors, the effect of temperature on the conduction of intrinsic semiconductors; p- and n-type extrinsic semiconductors and Hall effect experiment to determine the electron mobility. Various semiconductor devices like p–n junctions, forward and reverse biasing voltage, infrared detectors and photo conductors are used in thermoelectricity, transistor and MOSFET and microelectronic circuitry used in computers, calculators, watches and cell phones. In the end, there is a discussion on semiconductor applications in light-emission microwave frequency-integrated circuits, solar cells, photovoltaic cell and laser diodes.

18.2 SEMICONDUCTION

The electrical conductivity of a semiconductor is much less than that of a pure metal. However, semiconducting materials are extremely sensitive to impurities; even a minute concentration of impurity atoms changes the electrical properties of these materials. There are two types of semiconductors: (1) intrinsic, where the electrical behaviour is based on the electronic configuration of the pure material and (2) extrinsic, where the electrical properties are changed by impurity atoms.

18.2.1 Intrinsic Semiconductors

These semiconductors are characterized by the electron band structure as shown in Figure 18.1, i.e. a valence bond separated from an empty conduction band by a narrow forbidden band gap of energy Eg, generally less than 2 eV.

Figure 18.1 Electron Board Structure of Semiconductor

 

There are two elemental semiconductors: (1) silicon and (2) germanium having band gap energies of 1.1 and 0.67 eV, respectively. Both Si and Ge are covalently bonded. (In both Si and Ge, the valence bands correspond to sp3 hybrid energy levels for the isolated atom. These hybridized valence bands are completely filled at 0 K.)

In intrinsic semiconductor, for every electron excited to conduction band, there is a missing electron in one of the covalent bonds, a vacant electron site or a hole in valence band as shown in Figure 18.2.

Figure 18.2 Electron Excited to Conduction Band, Creating a Hole in Valence Band

A hole is considered to have a charge of same magnitude as of an electron, but positive + 1.6 × 10−19 C.In the presence of electric field, electrons and holes move in opposite directions. Both holes and electrons are scattered by lattice imperfections. So there are two types of charge carriers in an intrinsic semiconductor.

Electrical conduction          σ = neeµe + nheµh,

where ne is the number of free electrons, nh is the number of holes, e is the absolute magnitude of electrical charge on an electron or on a hole, i.e. 1.0 × 10−19 C, µe is the electron mobility and µh is the hole mobility.

Table 18.1 provides band gap energies, electron and hole mobilities and electrical conductivities for some semiconducting materials.

 

Table 18.1 Band Gap Energies, Electron and Hole Mobilities and Electrical Conductivities of Some Semiconducting Materials

Example 18.1    Calculate the electrical conductivity of intrinsic silicon at 300 K. For silicon, ne = nh = 1.5 × 1016 per m3, e = 1.60 × 10−19 C, μe = 0.135 m2/(Vs) and µh = 0.048 m2/(Vs).

Solution:

Electrical conductivity,

 

σ

=

ne∣ (μe + μh)

ne

=

nh = n = 1.5 × 1016

e

=

1.60 × 10−19 C

μe + μh

=

0.135 + 0.048 = 0.183 m2/(Vs)

Resistivity,

18.3 TEMPERATURE EFFECT ON INTRINSIC SEMICONDUCTOR

As the temperature increases, the Fermi distribution changes, and there is some probability that conduction band is occupied (an equal probability that a level in the valence band is unoccupied) or that a hole is present. Number of electrons in conduction band or holes in valence band is

 

n = ne = nh = n0exp(−E/2KT)

where n0 is a constant.

Higher temperatures permit more electrons to cross the forbidden zone, so the conductivity

 

σ = n0(μe + μh)exp(−E/2KT)

The behaviour of semiconductors is opposite to the behaviour of metals. As the temperature increases, the conductivity of a semiconductor increases because more charge carriers are present, whereas the conductivity of a metal decreases due to lower mobility of charge carriers.

Figure 18.3 shows the variation of electrical conductivity of germanium semiconductor versus temperature as compared to conductivity of a metal.

18.4 EXTRINSIC SEMICONDUCTORS

By intentionally adding a few impurity atoms to a material, an extrinsic semiconductor is produced, and the conductivity of an extrinsic semiconductor depends upon the number of impurity atoms or dopant and in a certain range; the conductivity is independent of temperature. But the conductivity of an intrinsic semiconductor changes with temperature as shown in Figure 18.3. Therefore, the conductivity of an extrinsic semiconductor is controllable and stable.

Figure 18.3 Conductivity Versus Temperature

18.4.1 n-Type Semiconductors

If an impurity like antimony having five valence electrons is added to silicon or germanium, four of the electrons of antimony atom participate in the covalent bonding process, and the remaining one electron enters an energy level in a donor state, just below the conduction band as shown in Figure 18.4. The extra electron is not tightly bound to the atom, and only a small amount of energy Ed < Eg is required to enter the conduction band. No corresponding holes are created when donor electrons enter the conduction band.

Some intrinsic semiconductors still remain with a few electrons jumping the Eg gap.

Number of charge carriers

 

ntotal = ne (dopant) + ne (intrinsic) + nh (intrinsic)

At low temperatures, few intrinsic electrons and holes are produced. As the temperature increases, more of donor electrons jump the Eg gap, eventually, all the donor electrons enter the conduction bond. At this point, donor exhaustion is reached. The conductivity virtually becomes constant as no more donor electrons are available, yet the temperature is too low to produce many intrinsic electrons and holes. So,

Figure 18.4 n-Type Semiconductors

conductivity,                             σ = ndeμe

where nd is the maximum number of donor electrons and

 

μe

=

electron mobility

 

=

absolute magnitude of charge on electron or hole

 

=

1.6 × 10−19 C

The number of donor electrons, nd is determined by the number of impurity atoms that are added. A material of this type is said to be an n-type extrinsic semiconductor. For n-type, the Fermi level is shifted upward in the band gap, within the vicinity of the donor state.

Example 18.2    Phosphorus is added to a high-purity silicon to give a concentration of 1024/m3 of charge carriers at room temperature. Calculate the room temperature conductivity of this material, assuming that electron and hole mobilities are same for intrinsic material.

Solution:

Phosphorus acts as a donor in silicon. The 1024/m3 charge carriers will be virtually all electrons. The material is extrinsically n-type.

 

σ

=

neμe


 


=

× 1.6 × 10−9 × (0.14 m2/Vs)

 

=

22,400(Ωm)−1

18.4.2 p-Type Extrinsic Semiconductors

Gallium has a valence of 3, and if this is added to a semiconductor there are enough electrons to complete the covalent bonding process. An electron hole is produced in the valence band, which can be filled by an electron from other location as shown in Figure 18.5. The holes act as acceptors of electrons. These hole sites have a somewhat higher than normal energy and create an acceptor level of possible electron energies just above the valence band, Ea. The electrons must gain energy equal to Ea in order to create a hole in the valence band. The hole moves and carries the charge. This is a p-type semiconductor.

Figure 18.5 p-Type Semiconductors

If some intrinsic semiconduction occurs, total number of charge carriers is

 

nt = na (acceptors) + ne (intrinsic) + nh (intrinsic)

At low temperatures, the acceptor levels predominate. Eventually, the temperature is high enough to cause saturation of acceptors and the conductivity is

 

σ = naeµh,

where na is the maximum number of acceptors or holes, e is the absolute magnitude of charge on hole and µh is the mobility of the hole.

Extrinsic semiconductors (both n- and p-type) are produced from materials that are of extremely high purity, and total impurity content may be as low as 10−7 at %. (atom per cent) Controlled concentrations of specific donors or acceptors are added, using various techniques. This process is known as doping.

In extrinsic semiconductors, a large number of charge carriers (electrons or holes) are created at room temperature by the available thermal energy, and as a result, relatively high electrical conductivities at room temperatures are obtained. Semiconductor materials are designed for use in electronic devices to be operated at ambient conditions.

18.5 HALL EFFECT

By using simple electrical conductivity measurements, the type, concentration and mobility of charge carriers in a semiconductor cannot be determined. For the measurement of these quantities, a ‘Hall effect’ experiment is performed. Hall effect is the creation of a voltage, the Hall voltage VH, when a magnetic field is applied in a direction perpendicular to the motion of charged particles, which exerts a force on the particle perpendicular to both the magnetic field and the direction of particle motion.

Consider a parallelopiped specimen, base b.a, with origin at O of Cartesian co-ordinates. In response to externally applied electric field, the electrons or holes move in z-direction and a current Iz is produced. Now, a magnetic field is applied in the positive x-direction as shown by Bx in Figure 18.6. The resulting force brought to bear on the charge carriers (moving in z-direction) will cause them to be deflected in y-directions, i.e. holes (positively charged carriers) towards the right face of the specimen and electrons (negatively charged carriers) towards the left vertical face as shown in Figure 18.6. A voltage VH will be established in y-direction. The magnitude of VH will depend upon Iz, Bx and the thickness of specimen a

Figure 18.6 Hall Effect

where RH is the Hall coefficient.

For metals, conduction is by electrons, and RH is negative.

where n is the concentration that can be determined as RH is measured by Eq. (18.1).

e∣ is the absolute magnitude of charge on electron or hole. Moreover, electron mobility, μe is equal to

or

 

μe = ∣ RHμ        (18.3)

RH∣ is the absolute value of Hall coefficient from Eq. (18.3), and electron mobility μe can be measured.

Example 18.3    The electrical conductivity and electron mobility for aluminium are 3.7 × 107 Ωm−1 and 0.0012 m2/Vs. Calculate the Hall voltage for an aluminium specimen of 20-mm thickness for a current of 30 A and a magnetic field of 0.5 tesla (imposed in a direction perpendicular to current)

Solution:

Hall voltage may be determined by

Let us first determine Hall coefficient,

18.6 VARIATION OF CONDUCTIVITY WITH TEMPERATURE AND CARRIER CONCENTRATION

In the case of intrinsic semiconductors, electrical conductivity increases with rising temperature, because due to more thermal energy on account of increase in temperature, both electrons and holes increase. Both values of ne and nh increase in conductivity expression. The magnitude of electron mobility and hole mobility (μe and μh) decrease slightly with temperature as a result of more effective scattering of electrons and holes by thermal vibrations. However, reduction in μe and μh is very much smaller than increase in ne and nh, with the net result that conductivity is increased.

Mathematically,

where σ is the intrinsic conductivity, Eg is the gap energy, k is the Boltzmann's constant, T is the absolute temperature and C is a temperature-independent constant.

Since the increase in ne and nh is so large with increase in temperature, and there is only slight decrease in μe and μh, the dependence of carrier concentration on temperature for intrinsic behaviour is the same, i.e.

 

ln ne = ln nh =               (18.5)

where C′ is a constant, independent of temperature, but different from C.

18.6.1 Extrinsic Conductivity

At temperatures below 800 K (523°C), the boron-doped materials are extrinsically p-type. Virtually, all the carrier holes result from extrinsic excitations—electron transitions from valence band into the boron-acceptor band, which leaves behind valence band holes. The available thermal energies at these temperatures are sufficient to promote significant number of these excitations to acceptor level. So the extrinsic conductivity far exceeds that of intrinsic material. For example, at 127°C (400 K), σ for extrinsic silicon is 10−2.

Conductivity, σ, for extrinsic silicon and 0.0013 at% boron-doped silicon is 600 (Ωm)−1. This comparison indicates the sensitivity of conductivity to even extremely small concentration of the same impurity atom.

Another method of representing the temperature dependence of the electrical behaviour of semiconductors is from a graph between (1) natural log of electron and hole concentration and (2) reciprocal of absolute temperature.

 

ln ne = ln nh =               (18.6)

Slope of the line segment is −Eg/2k.

As shown in Figure 18.7, ln nh first increases linearly with decreasing (1/T) (on increasing temperature). A large number of extrinsic excitations are possible, even at relatively low temperatures, in as much as acceptor level lies just above the top of the valence band. With increase in temperature or decrease in 1/T, the hole concentration (nh) eventually becomes independent of temperature. At this point (in the case of silicon doped with boron), virtually all the boron atoms have accepted electrons from the valence band or said to be saturated—shown by saturation region (donor impurities become exhausted instead of saturated). The number of holes in this region is approximately equal to the number of dopant impurity (boron) atoms.

Figure 18.7 Extrinsic Conductivity

Example 18.4    At room temperature of 25°C, the electrical conductivity of intrinsic silicon is 4 × 10−4. Estimate its conductivity at 250°C.

Solution:

Constant,

where

 

σ

=

4 × 10−4

Eg

=

1.11 eV

k

=

8.62 × 10−5/ K

T

=

25 + 273 = 298 K

At 150°C (523 K)

 

 

=

8.353 − 12.3107

ln σ

=

− 3.958

Electrical conductivity,

 

σ = 0.0191 (Ωm)−1.
18.7 SEMICONDUCTOR DEVICES

Semiconductor devices as diodes and transistors have replaced the vacuum tubes used earlier. These are also known as solid state devices. Special features of semiconductor devices are small size, low power consumption, no warm-up time and extremely small circuits. Numerous electronic devices can be incorporated into a small silicon chip. In other words, the circuits are miniaturized. Some semiconductor devices are ‘diodes’, which act as rectifier and allow the current to flow only in one directions, and convert ac to dc signal. pn semiconductor (combined) junction is a semiconducting rectifier. Figure 18.8 shows a pn junction.

Figure 18.8 p–n Junction

A single piece of silicon is doped so that one side becomes p-side and other becomes n-side. Note that a piece of silicon or any semiconducting material is taken from a single crystal of the material, because the grain boundaries are deleterious to the operation of a semiconducting device.

18.7.1 Forward Bias

Before the application of any potential across the pn junction, holes on p-side and electrons on n-side are the dominant carriers of charge.

Now, a battery is connected with positive terminal to the p-side and negative terminal to the n-side. This type of connection is known as forward bias. Under the influence of applied potential, holes on p-side and electrons on n-side are attracted towards the junction. Flow of hole and electron is shown in Figure 18.9. Holes and electrons encounter each other at the junction, recombine and cancel each other producing energy

Figure 18.9 Forward Bias

For the forward bias, a large number of charge carriers flow across the junctions and an appreciable current is produced. The characteristics of current–voltage are shown in I quadrant of Figure 18.10.

18.7.2 Reverse Bias

As shown in Figure 18.11, both holes and electrons are rapidly drawn away from the junction and the junction relatively becomes free of charge carriers. Therefore, the junction is now highly insulating. The IV quadrant of Figure 18.11 shows the current–voltage characteristics of reverse bias on pn junction. Current IR is extremely small in comparison to IF.

Figure 18.10 Current Voltage Characteristics

Figure 18.12(a) shows sinusoidal variation of applied voltage ±V0. Maximum current for reverse bias is very small IR, as shown in comparison to IF, in forward bias. Sometimes, high reverse bias voltages are applied and a large number of electrons and holes are generated. This will increase the current abruptly, a phenomenon known as breakdown.

Figure 18.12(b) shows voltage versus time for a pn rectifying junction, while the current IR versus time showing rectification of voltage.

Figure 18.11 Reverse Bias

Figure 18.12 Sinusoidal Variation of Applied Voltage

18.8 INFRARED DETECTORS AND PHOTOCONDUCTORS

These are semiconductor devices. The energy required to break a chemical bond and to create a free electron hole pair is supplied by electromagnetic radiation of a certain wavelength. Minimum energy required to produce an electron–hole pair at 0°C absolute is Eg, gap energy between valence and conduction bands.

The electromagnetic radiation falling on a material is absorbed by the following:

  1. Fee electrons in conduction band.
  2. Bound electrons, which are energized to occupy higher unfilled orbits available.
  3. Ionizing the material with concomitant production of free electrons (photoemission).

In metals, there are many free electrons and adjacent orbits, and a quantum of very wide spectrum of radiation will be absorbed, including very small energies (of large wavelengths of visible light), so the metallic materials are opaque.

  1. Semiconducting materials have a very few electrons and large wavelengths to which the metals are opaque and are transmitted by semiconductors.
  2. As the wavelength decreases, the quantum of energy increases, and eventually the energy becomes large enough to exert the electron from valence to conduction band, producing electron–hole pair and raising electrical conductivity.
  3. As the wavelength becomes shorter and shorter, many electrons are produced that a semiconductor becomes opaque to these wavelengths. Shorter and shorter wavelengths beyond this stage produce photoemission.
  4. Electrons from any of the orbits within the valence band can be excited to any unfilled orbits within the conduction band or to newly vacated orbits within the valence band. However, both valence and conduction bands have finite widths, and a spectrum of energies is absorbed. Whether a given semiconductor is sensitive to visible light or infrared radiation depends upon Eg, gap between valence and conductor bands.

The materials sensitive to visible light are known as ‘photoconductors’ and the materials sensitive to infrared radiations are known as ‘infrared detectors’.

18.9 THERMOELECTRICS

Thermoelectric effects in semiconductors are 100 times greater than the effects in metals. For this reason, a semiconductor like Bi2Te3 (bismuth telluride) can be used to convert heat directly to electricity in a thermoelectric generator.

The principal thermoelectric effects are ‘Seebeck effect, Peltier effect and Thomson effect. Thomson effect states that heat is absorbed from or liberated to the surroundings by a conductor in which both temperature gradient and current exist.

If a temperature gradient exists in a semiconductor, the otherwise uniform distribution of electrons and excess holes tend to move from the hot end of a conductor towards the cold end. An equilibrium is reached when the charge established on the cold end becomes great enough to repel the additional carriers that tend to migrate there. The open circuit current is zero, though a potential remains. The hot ends of the metals and n-type semiconductors become positively charged, while their cold ends become negatively charged. In p-type semiconductors, reverse is true.

In addition to the tendency for charge carriers to move away from hot portion of a conductor, the charge carriers possess different energies in different materials. Therefore, movement of carriers across a junction of dissimilar materials results in either liberation or absorption of energy. Conversely, if the junction is heated or cooled, there will be a movement of charge carriers across the junction.

Consider the pn junction, a pn device and the energy level diagrams of p- and n-type semiconductors (Figures 18.13 and 18.14).

Figure 18.13 p–n Device

Figure 18.14 Energy Level Diagrams

If the applied voltage causes electrons to flow from n-material across the junctions to p-material, heating of the junction will occur. This is because the high-energy electrons from donor levels will move down the energy barrier and liberate energy as they occupy the acceptor levels in the valence band of p-material. In this case, the device can be used as a heat pump.

If the applied voltage is reversed so that the electrons move from acceptor levels near the valence band to donor levels near the conductor band, energy has to be supplied to the electrons. This energy is obtained in the form of heat from the surroundings, causing the junctions to become cold. This device is called a ‘refrigerator’.

Semiconductors have two advantages over metals when used in thermoelectrics:

  1. The voltages produced in semiconductors are 10 times greater than that for a given temperature difference in the metals.
  2. Most of the semiconductors are relatively poor conductors of heat, so it is easier to maintain a large temperature difference between hot and cold junctions under given conditions.
18.10 TRANSISTORS

Transistors are most important semiconducting devices in microelectronic circuitry and can perform two basic functions:

  1. They can amplify an electrical signal like a triode.
  2. In computers, they can serve as switching devices for processing and storage of data.

There are two types of transistors: (1) junction transistor and (2) MOSFET (metal oxide semiconductor field effect transistor) transistor.

18.10.1 Junction Transistor

In this transistor, there are two pn junctions arranged back to back as shown in Figure 18.15. A very thin n-type base region is sandwiched between p-type emitter on one side and p-type collector on the other side. Base–emitter junction is forward biased, while base–collector junction is reverse biased.

Figure 18.15 Junction Transistor

Operations

The p-type emitter and junction 1 is forward biased, and a large number of holes enter the base region. These are minority carriers in the n-type base and the same will combine with majority of electrons. However, if the base is very thin, most of these holes will be swept through the base without recombination and cross junction 2, go into the p-type collector. Holes have become a part of emitter–collector circuit. A small increase in input voltage Vi, produces a large increase in current across junction 2. This large increase in collector current produces large increase in output voltage (across the load resistor). Therefore, a voltage signal Vi that passes through junction transistor experiences amplification and becomes V0, as shown in Figure 18.16 (voltage–time plots).

Figure 18.16 Amplified Output Voltage

Similarly, the operations of npn transistor can be explained, but in this case electrons instead of holes are injected across the base into the collector.

18.10.2 MOSFET

In MOSFET, two small islands of p-type semiconductors are created within a substrate of n-type silicon semiconductor as shown in Figure 18.17. Two p-type islands are joined to a p-type channel. An insulating layer of silicon dioxide (SiO2) is formed on the surface of silicon.

Figure 18.17 MOSFET Transistor

Connection of source, gate and drain are made on p island, p channel and p island as shown in Figure 18.18. The conductivity of the channel is changed by the presence of electric field applied on the gate. A positive field applied on the gate will drive charge carriers, i.e. holes out of the channel, and the electrical conductivity is reduced. Therefore, a small change in the field at the gate will produce a large change in current between the source and the drain.

Basic difference between the operation of junction and MOSFET transistors is that the gate current is very small in comparison to the base current in a junction transistor. Therefore, MOSFET is employed where the signal sources to be amplified cannot sustain a significant current.

Figure 18.18 Connection of Source, Gate and Drain

In addition to their function in amplifying the input electrical signal, transistors and diodes are used as switching devices in computers.

18.11 MICROELECTRONIC CIRCUITRY

In microelectronic circuitry, thousands of electronic components and circuits are incorporated into a very small chip. Very small size of the chip and low power consumptions are the basic requirements in aerospace technology. Now, personal computers are easily affordable and integrated circuits are used in calculators, watches, communications, industrial production and control.

Microelectronics processes are grouped into (1) semiconductors as transistors, diodes, pnp switches and resistors and (2) thin film microelectronics used as resistors, capacitors and interconnectors of electronic circuits.

First of all, a single crystal is grown out of the melt doped with the proper element. At present, silicon and germanium are mainly used because their manufacturing technology is very well understood and established. Silicon is generally preferred over germanium, because of its greater energy gap and better thermal resistance. From a single crystal of silicon, thin wafers are cut with the help of diamond saws. Then from wafers, chips of rectangular shape are cut. A highly polished surface free of any surface damage is essential for the wafer. Each polished wafer constitutes a substrate on which typically 1000 integrated circuits can be located. Wafer is subjected to further processes as follows:

  1. Oxidation
  2. Photoengraving
  3. Diffusion
  4. Epitaxy
  5. Chemical processing
  6. Interconnection

Oxidation provides an insulating layer of silicon dioxide to isolate a number of pockets of a single-crystal semiconducting wafer. Photoengraving is used for cutting windows in the oxide layer. Diffusion forms p- and n-type areas. Epitaxy is the growth of a new layer having the same crystal orientations as that of the substrate. Within this layer, the components of integrated circuit are formed using diffusion, oxide isolations and again diffusion. Chemical processing involves the technique of etching and removing mask and oxide layers by chemical actions. Finally, interconnections are made by metalizing aluminium, which can be deposited readily by vacuum evaporation that forms an excellent band with silicon and silicon oxide surfaces. Aluminium has good conductivity and welds readily to gold attachment leads by thermocompression bonding.

18.12 APPLICATIONS OF SEMICONDUCTORS

Because of their applications in transistors and lasers, the search for new semiconducting materials and improvement in existing materials are important studies in material science.

Most commonly used semiconductors are crystalline organic materials. These materials are classified according to the periodic table groups from which their constituent atoms emerge.

Group III nitrides have high tolerance to ionizing radiations, making them suitable for radiation-hardened electronics. Group IV elemental semiconductors are diamond, silicon and germanium. Group IV compound semiconductors include SiC (silicon carbide) and SiGe (silicon germanide).

18.12.1 III–V Semiconductors

Following are few examples of III–V semiconductors:

  1. Aluminium anti-monide (AlSb) containing aluminium and antimony (lattice constant is 0.61 nm).

    Indirect band gap is 1.6 eV at 300 K and direct band gap is 2.22 eV.

  2. Aluminium arsenide (AlAs) (lattice constant is 0.61 nm): It has wider band gap than AlSb. It is hygroscopic.
  3. Aluminium nitride (AlN): It has extremely wide gap of 6.2 eV. It has potential applications in deep ultraviolet optoelectronics.
  4. Aluminium phosphide (AlP) along with other elements, used in devices such as light-emitting diodes (e.g. aluminium gallium indium phosphide).
  5. Boron nitride (BN): Recently discovered boron nitride nanotubes have homogeneous electronic behaviour.
  6. Boron arsenide (BAs): Solar cells are fabricated from this semiconducting material.
  7. Gallium anti-monide (GaSb): It is used in devices such as microwave frequency-integrated circuits, infrared light-emitting diodes, laser diodes and solar cells.
  8. Gallium phosphide (GaP): This is used in the manufacture of low- and standard-brightness red, orange and green light-emitting diodes. GaP has been used as an LED material since 1960.
  9. Indium anti-monide (InSb): This narrow gap semiconducting material is used in infrared detectors, thermal imaging cameras, infrared astronomy and infrared homing missile guidance systems. InSb is sensitive to 1–5 µm wavelengths.
  10. Indium nitride: It is a small band gap semiconductor material having potential application in solar cells and high-speed electronics.
  11. Indium phosphide: This is used in high-power, high-frequency electronics because of its superior electron velocity with respect to more common semiconductors as silicon and gallium arsenide.

18.12.2 II–VI Semiconductors

A few examples of II–VI semiconductors are as follows:

  1. Cadmium selenide (CdSe): Band gap is 1.74 eV at 300 K, and is used in optoelectronic devices, laser diodes, nanosensing and biomedical imaging.
  2. Cadmium sulphide (CdS): Band gap is 2.42 eV. It has useful properties for optoelectronics, and is used in both photosensitive and photovoltaic devices and in photoresistors—electrical resistance changes with incident light levels.
  3. Cadmium telluride (CdTe): This is a strong solar cell material. It is usually sandwiched with cadmium sulphide to form a pn junction photovoltaic solar cell.
  4. Zinc oxide: It has a direct band gap of 3.37 eV, and is commonly used in gas sensors, blue LESs, transparent TFTs and in transparent conducting oxide (TCO).
  5. Zinc selenide (ZnSe): It is used in light-emitting diodes and diode lasers; it emits blue light. It is susceptible to n-type doping, by halogen element. p-type doping is more difficult, but can be achieved by introducing nitrogen.
  6. Zinc telluride: It can be easily doped, and is used in optoelectronics.

18.12.3 Miscellaneous Oxides

Following are few examples of miscellaneous oxides:

  1. Cu2O(cupric oxide): Initially rectifier diodes were made from this material. Now silicon is used.
  2. CuO (cuprous oxide): Application is in p-type semiconductor, and has a narrow band gap of 1.2 eV. It is an abrasive to polish optical instruments. Cupric oxide is used to produce dry cell batteries.
  3. Uranium dioxide (UO2): Band gap is 1.3 eV, and is used in very efficient solar cells. It can absorb five different wavelengths, including infrared. It is also useful for red-hard devices for special military and aerospace applications.

18.12.4 Organic Semiconductors

Both short-chain oligomers and long-chain polymers are known organic conductors. For example, oligomers are pentacene, anthracene and rubrene. Polymers are poly (3-hexylthiophene), poly (p-phanylene vinylene) and polyacetylene.

18.12.5 Magnetic Semiconductors

Magnetic semiconductors are materials that exhibit ferromagnetism (and a similar response) and have useful semiconductor properties. They are used in spin transistors.

Manganese-doped indium arsenide and gallium arsenide are examples of ferromagnetic semiconductor (at Curie temperature). Manganese-doped indium anti-monide becomes ferromagnetic even at room temperature, even with less than 1 per cent Mn.

KEY POINTS TO REMEMBER
  • In intrinsic semiconductors, electrical behaviour is based on electronic configuration.
  • In extrinsic semiconductors, electrical behaviour is based on impurity atoms.
  • Silicon with Eg = 1.1 eV and germanium with Eg = 0.67 eV are most commonly used semiconductors.
  • Electrical conduction,

     

    σ = ∣e∣{ne µe + nhµh}

    where ne is the number of free electrons, nh is the number of holes, µe is the electron mobility and µh is the hole mobility.

  • Temperature effect on intrinsic semiconductors

     

    n = ne = nh = n0exp(−E/2kT)

    where n0 is a constant, k is the Boltzmann's constant and T is the absolute temperature.

  • In n-type semiconductors, electrons enter an energy level in a donor state Ed < Eg.
  • In p-type semiconductors, holes enter an energy level in an acceptor state Ea > Eg.
  • Total charge carriers in p-extrinsic

     

    nt = na (acceptor) + nh (intrinsic) + ne (intrinsic)

     

  • Hall effect: When a magnetic field is applied in a direction perpendicular to the motion of charged particles, which exerts a force on the particles perpendicular to both magnetic field and direction of particle motion, Hall voltage is created.
  • Hall coefficient,
  • Intrinsic semiconductor

    Variation of conductivity σ with temperature

    where C is a constant, Eg is the gap energy, k is the Boltzmann's constant and T is the absolute temperature.

  • pn junctions, when reverse biasing applied IR < IF (i.e. the current when forward biasing is applied), effect of rectifier is obtained.
  • Group III nitrides have high temperature to ionizing radiation. These are suitable for radiation-hardened electronics.
  • Gallium anti-monide (GaSb) is used in devices such as microwave frequency-integrated circuits, infrared light-emitting diodes, laser diodes and solar cells.
  • Indium anti-monide (InSb) is used in infrared detectors, thermal imaging cameras, infrared astronomy and missile guidance systems.
  • Cadmium sulphide (CdS) is used in photosensitive and photovoltaic devices, and in photo resistors.
  • Zinc selenide (ZnSe) is used in blue light-emitting diodes.
  • Photoemission is produced by shorter and shorter wavelengths.
  • Photoconductors are sensitive to visible light.
  • If a temperature gradient exists in a semiconductor (otherwise having uniform distribution of charge carriers), free electrons and excess holes tend to move from hot end of a conductor towards a cold end.
  • Transistors can amplify an electrical signal and in computers they serve as switching devices.
  • MOSFET (metal oxide semiconductor field effect transistor): A very small change in the field at gate produces a large change in current between the source and the drain.
  • Microelectronic circuits: Thousands of electronic components and circuits are incorporated in a very small chip. Small size and low power consumption are special properties of these chips.
MULTIPLE CHOICE QUESTIONS
  1. Which is a correct statement?
    1. Extrinsic semiconductor properties are dependent on electronic configuration only.
    2. Behaviour of intrinsic semiconductor is based on electronic configuration of pure material.
    3. Band gap energy of Si is 0.67 eV.
      1. A and B
      2. B and C
      3. B only
      4. C only
  2. Arrange the following semiconductors in increasing order of band gap energy.

    Silicon, germanium, gallium, arsenide

    1. GaS, Ge, Si
    2. Ge, Si, GaS
    3. Si, GaS, Ge
    4. GaS, Si, Ge
  3. Arrange the following semiconductors in decreasing order of electron mobility

    Si, GaP, CdS

    1. Si, GaP, CdS
    2. GaP, Si, CdS
    3. CdS, Si, GaP
    4. CdS, GaP, Si
  4. Resistivity of a semiconductor is 2.5 × 103 Ωm. What is its electrical conductivity in (Ωm)−1?
    1. 2.5 × 10−3
    2. 2.5 × 10−4
    3. 4 × 10−3
    4. 4 × 10−4
  5. What is the band gap energy in indium anti-monide (InSb) semiconductor?
    1. 2.27 eV
    2. 1.42 eV
    3. 0.17 eV
    4. None of these
  6. What is the expression of Hall coefficient, RH?
    1. None of these
  7. Under the influence of applied voltage, holes on p-side are attracted towards the junction and electrons on n-side are also attracted towards the junction. This statement is correct for which biasing in pn junction
    1. Forward biasing
    2. Reverse biasing
    3. Neither (a) nor (b)
    4. Both (a) and (b)
  8. If current in reverse biasing is IR and in forward biasing is IF, which is a correct statement?
    1. IR >> IF
    2. IR = IF
    3. IR << IF
    4. None of these
  9. Which of the following semiconductors has 6.2 eV as gap energy?

    Aluminium anitmonide, Aluminium Arsenide, Aluminium Nitride

    1. AlN
    2. AlSb
    3. AlAs
    4. None of these
  10. In a semiconductor, thermoelectric effect is produced by
    1. Seebeck effect
    2. Peltier effect
    3. Thompson effect
    4. None of these

Answers

1. (c)

2. (b)

3. (a)

4. (d)

5. (c)

6. (a)

7. (a)

8. (c)

9. (a)

10. (c)

REVIEW QUESTIONS
  1. What are intrinsic and extrinsic semiconductors. Explain. What are forbidden band gap, electrical conductivity, electron mobility and hole mobility?
  2. Explain the temperature effect on behaviour of intrinsic semiconductors.
  3. Describe n-type and p-type extrinsic semiconductors, explaining donor energy, Ed, acceptor energy, Ea and gap energy, Eg.
  4. What is Hall effect? How it is determined? How electron mobility is measured?
  5. Explain the variation of electrical conductivity with temperature in the case of extrinsic semiconductors.
  6. What is pn junction? What are forward biasing and reverse biasing voltages? How rectifying effect is obtained?
  7. Name a few semiconductors for application in
    1. Integrated circuits
    2. Infrared light-emitting diodes
    3. Solar cells
    4. Nanosensing
    5. Photovoltaic devices
    6. Dry cell batteries
  8. What are infrared detector and photoconductor? What is the difference in their principle of operation?
  9. What semiconductor is used in thermoelectricity? Explain Thomson effect in a semiconductor-type thermocouple.
  10. Explain how voltage is amplified by junction transistor.
  11. Describe the preparation of a MOSFET.
  12. Mention the achievements in microelectronic circuitry.
PRACTICE PROBLEMS
  1. For intrinsic silicon, electrical conductivity at room temperature is 4 × 10−4 Ωm−1. The electron and hole mobilities are 0.1 and 0.05 m2/Vs, respectively. Complete the electron and hole concentrations at room temperature.

     

    Ans. [1.316 × 1016/m3]

  2. The electrical conductivity and electron mobility for aluminium are 3 × 107 Ωm−1 and 0.0012 m2/Vs. Calculate the Hall voltage for an aluminium specimen that is 15-mm thick for a current of 20 A and a magnetic field of 0.6 tesla (imposed in a direction perpendicular to current).

     

    Ans. [−2.53 × 10−8 V]

  3. Phosphorus is added to high-purity silicon to give a concentration of 1023.5/m3 of charge carriers at room temperature. If electron mobility of silicon is 0.139 m2/Vs, calculate the room temperature conductivity of the materials assuming that ne = nh for intrinsic material.

     

    Ans. [7.33 (Ωm)−1]

  4. At room temperature of 25°C, the electrical conductivity of intrinsic germanium is 2.2 Ωm−1. Estimate its conductivity at 200°C (473 K).

     

    Ans. [274 (Ωm)−1]