In this article we will discuss about:- 1. Introduction to Semiconductor 2. Classification of Materials in Terms of Energy Bands 3. Types 4. Classification of Elemental Semiconductors 5. Position of Fermi Level 6. Conductivity 7. Hall Effect 8. Merits 9. Formulated Materials 10. Applications.
Introduction to Semiconductor:
A semiconductor is defined as a material whose conductivity falls intermediate between that of metals, 106 to 108 ohm-1m-1, and that of dielectrics (insulators); 10-20 to 10-8 ohm-1m-1.
Semiconductors are materials which have resistivity between 10-5 to about 107 ohm-m (at ordinary temperature).
The resistivity of semiconductors depends greatly on:
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i. The Temperature:
Resistivity decreases as the temperature increases. Hence, the temperature coefficient for the resistivity of a semiconductor is usually negative.
ii. The Illumination:
Resistivity decreases in brighter surroundings.
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iii. The Magnitude of Electric Field in Semiconductors:
Current in semiconductors does not obey Ohm’s law and will increase rapidly than voltage, i.e. semiconductors are non-linear resistors.
iv. External Impurities:
A minute amounts of certain other substances change resistivity of semiconductor considerably; certain impurities even change the mechanism of conduction in semiconductors.
Classification of Materials in Terms of Energy Bands:
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i. The band theory provides a clear-cut classification of the materials according to which there are three categories normally, metals, insulators and semiconductors.
ii. If the energy levels of the filled band and the conduction (empty) band overlap, a slight excitation will cause electrons to pass from a filled to a conduction band. A material consisting of such atoms will therefore exhibit high conductivity, and be a conductor.
iii. In cases, where the atoms of the material concerned have an energy gap lying between filled and conduction band, the material will not be able to act as a conductor. If the difference between the energy levels of filled and conduction band is small, the electrons will only require a small amount of energy (e.g. thermal energy) to become excited and pass into the conduction band. Materials of such a class are semiconductors. In semiconductors the forbidden region (empty band, Eg) has a width of 0.12 to 5.3 eV (normally 1 eV at room temperature).
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iv. The materials, in which the energy gap is still higher (Eg ≅ 15 eV), the electrons do not leave the filled band even by thermal excitation, such materials are dielectrics (insulators 6 eV).
Types of Semiconducting Materials:
A. Elemental Semiconductors (Group IV Element):
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(a) Intrinsic semiconductor
(b) Extrinsic semiconductor
B. Compound Semiconductors:
(a) II – VI compound semiconductors
(b) III – V compound semiconductors
(c) IV – IV compound semiconductors
(d) IV – VI compound semiconductors
As conductivity and band gap of elemental semiconductor materials are limited hence their usefulness is limited. So group III-V, II-VI, IV—IV, IV—VI, semiconductors is used to provide better properties.
A. Elemental Semiconductor Material:
Example:
Ge, Si, C, B, AI, Ga, P, As, Sb, Bi etc.
a. It is pentavalent semiconductor material
b. It is used as donor N-type semiconductor material
c. When it is alloyed with Gallium, then it is used in fabrication of LED. Se (Selenium): It is used in photo voltaic cell.
B. Compound Semiconductor Material:
(i) III-V Semiconductor Materials:
a. They provide wider range of band gap and extended temperature range.
b. Structure is diamond cubic.
Example: GaAs, AIP etc. GaAs
c. This is large band gap material.
d. It has large electron mobility which helps in high speed switching.
e. It is 10 times costlier than Si
f. It is 2.5 times faster than Si based devices
g. In GaAs crystal, Ga takes place corner and face atom whereas As takes place of four inside atoms.
Application:
i. LED
ii. Laser
iii. Satellite Amplifier.
(ii) Group II—VI Semiconductor Materials:
Example:
Cds, CdSe, CdTe, ZnS, ZnSe etc.
a. They are used in photo conductors.
b. Band gap is larger than group III-V semiconductor material.
(iii) Group IV – IV Semiconductor materials:
Example: SiC
a. Its band gap is 3eV.
b. α-SiC can be used for high temperature devices.
c. Its drawback is that it is expensive and not easy to manufacture.
(iv) Group IV – VI Semiconductor Materials:
Example:
PbS, PbSe, PbTe
In these semiconductors excess Pb give rise to N-type semiconductors and less Pb give rise to p-type semi-conductors.
Classification of Elemental Semiconductors:
A. Intrinsic Semiconductor:
The semiconducting material in its pure (no impurity added) form is generally known as intrinsic semiconductor.
In such materials there are no charge carriers at 0°k as the valence band is completely full of electrons and the conduction band is empty.
At higher temperatures, electron – hole pairs are generated as the electron of valence band can cross the energy gap by incident thermal energy.
B. Extrinsic Semiconductor:
The real power of semiconductor materials is realised when these are doped. Even a very little impurity addition (nearly 1: 1012 atom) leads to a drastic change in the electrical properties of these materials.
The drastic influence of an impurity on the conductivity of a semiconductor is a result of change in the energy band spectrum.
Extrinsic semiconductor can be obtained by doping the pure semiconductor by IIIrd or Vth group elements of the periodic table.
The most frequently used elements from the third column are boron, gallium, indium and aluminium, and from fifth: phosphorus, antimony, or arsenic.
(a) n-Type Semiconductor:
n- type semiconductors are those in which Ge or Si is doped with an element from the fifth column, for example, Ge or Si doped with phosphorus, antimony or arsenic. These impurities from the fifth column have five electrons in its outer shell available for interaction with other atoms.
The four nearest silicon atoms then form covalent bonds with four of the five valance electrons of the arsenic atom. In the energy band diagram, the energy level of this electron will be close to the conduction band as indicated in figure.
Impurity atoms like arsenic, that contribute free electrons, are called donors, because they donate electrons.
Introducing a group-V element in a pure semiconductor create a level close to the conduction band. The impurity levels created by group-V impurity atoms is known as donor level.
This addition of group-V element leads to increase in the number of electrons. The majority carriers in n-type semiconductor are “electrons”.
(b) p-Type Semiconductors:
p-type semiconductors are those semiconductors in which Ge or Si is doped with an element from the third column, for example, Ge or Si doped with boron, gallium, or aluminium.
Three of the neighbouring silicon atoms form covalent bonds with the three valance electrons of the aluminium atom (trivalent impurity).
Impurity atoms that contribute holes in this manner are termed acceptors, because they accept electrons from Si or Ge atoms.
Introducing a group-III element in pure semiconductor creates a level near to the valence band. The impurity levels created by group-III elements is known as acceptor level.
Thus addition of group-III impurity leads to increase in number of holes. Majority carriers in p-type semiconductor are “holes”.
Position of Fermi Level in Extrinsic Semiconductors:
A. n-Type Extrinsic Semiconductor:
In intrinsic semiconductor, fermi level lies very close to the middle of forbidden energy gap (Eg) indicating equal concentrations of free electrons and holes.
When donor type impurity is added to the crystal (if we assume that all the donor atoms are ionised). The donor electron will occupy the states near the bottom of the conduction band.
Hence it will be more difficult for the electrons from the valence band to cross energy gap by thermal agitations. Consequently, the number of holes in the valence band is decreased. Hence EF for n-type semiconductor must move closer to the conduction band.
ED → Energy levels corresponding to the Donor impurities
B. p-Type Extrinsic Semiconductor:
When a group III impurity is added, it creates new energy levels that are near to the top of the valence band.
The energies for holes are highest near the valence band and decrease vertically upward in energy level diagram.
When an intrinsic semiconductor is doped with acceptor type impurity. The concentration of holes in the valence band is more than the concentration of electrons in the conduction band; Fermi level shifts towards the valence band.
EA → Energy levels corresponding to the acceptor impurities
Conductivity of Semiconductor:
Since both types of charge carriers (electrons and holes) are available in a semiconductor and both are influenced by external applied electric field, therefore the net conductivity of a semiconductor will be a sum of conductivity of holes and electrons.
If n be the no. of electrons and p be the no. of holes.
The current density due to electrons Je = n.e.ve
The current density due to holes Jh = p.e.vh
The net current density of a semiconductor;
Where, me → mobility of electron
mh → mobility of hole
m = Vd/E = Average drift velocity/Applied electric field.
And conductivity due to drift motion of charge carriers
s = J/E = e [n.µe + p-µh]
For an intrinsic semiconductor;
n = p = ni
The conductivity will; therefore be-
σ= nie (µe + µh)
The reciprocal of conductivity is resistivity-
r = 1/σ = 1/σ = 1/e (n.µe + p.µh)
The intrinsic concentration (ni) is very sensitive to temperature because of the effect of temperature on the rate of production of electron-hole pairs. Theory shows that for any intrinsic semiconductor, the intrinsic concentration ni, and hence the conductivity, follows the relation;
Ni = AT3/2e –eV/2kT
Here V = The energy in volts required to break the covalent bond and is called ionization energy,
T = The absolute temperature,
k = Boltzman’s constant,
e = The charge of an electron,
and A = A constant that depends on the semiconducting material.
In pure germanium V = 0.75 volt and A = 9.64 × 1015, so that at room temperature of T = 300 K, ni = 2.5 × 1010 electrons per m3.
The mobilities of holes and free electrons in germanium are 0.17 and 0.36 m2 volt-1, sec-1, respectively.
Hall Effect in Semiconductors:
The Hall Effect occurs when a transverse magnetic field is applied to a specimen (metal or semiconductor) carrying current. Under these conditions, a transverse electric field is produced (perpendicular to both the direction of electric current and the applied magnetic field). The effect thus produces a measurable transverse voltage across the specimen.
The conductivity of a semiconductor does not determine the mobility and the density of charge carriers separately (determine the product of the two). However, in case there is only one type of charge carriers (either electron or holes); the density of charge carriers can be found from a measurement of the, so called Hall coefficient, RH of the material.
Let us discuss the Hall Effect for a p-type material, assuming the carriers are positive holes then by applying electric field they will drift with an average velocity vx in the x-direction.
When a magnetic field of flux density BZ (Wb/m2) is applied along the z-direction, the carriers will experience a Lorentz force perpendicular to vx & Bz, resulting in an excess of holes near the front face and deficiency of holes near the back face. These charges will in turn create an electric field along the negative y-axis i.e. Ey.
eEy = e.vx.Bz ……(1)
vx = Ey/Bz ………. (2)
also, the current density,
Jx = nhe.vx ………. (3)
where,
Jx – is the current density in the x-direction.
nh – is the density of holes, (only positive charges).
Then “Hall coefficient” RH is defined as-
If instead of holes the carrier were electrons, then the voltages measured across the Hall probes would be reversed. Hence, for electrons,
where, ne is the density of conduction electrons.
Note:
Measurement of the hall coefficient in a semiconductor gives us the information about the density of the carriers and their sign (whether electron or holes).
Due to the presence of the hall field Ey, the resultant field in the material no longer remains longitudinal but makes a angle q with the x axis (previous field direction). The angle q is called “Hall angle”.
Calculation of Hall Voltage:
Application of Hall Effect:
i. To determine the type of semiconductor,
ii. To determine the carrier concentration,
iii. To calculate mobility of carriers.
Merits of Semiconductor Materials:
i. Smaller in size
ii. Very light in weight
iii. They consume negligible power
iv. Their operating voltage in low
v. Do not show ageing effect
T.S. Applications of Semiconductor Materials:
i. Thermistors,
ii. Varistor,
iii. Photo Electric Device,
iv. Photo Cells,
v. Refrigerators.
Formulated (Compound and Alloyed) Semiconducting Materials:
i. Gallium Arsenide (GaAs):
Gallium (a third column element) and arsenic (a fifth column element) in periodic table made GaAs. It is made by zone refining technique.
Its melting point is about 1250°C. Production of GaAs is a difficult task due to generation of high vapour pressure in arsenic at about 1200°C. It has high mobility and large energy gap.
Uses:
a Switching and parametric diodes
b. Tunnel diodes
c. Semiconductor lasers
d. Hot electron diodes
ii. Indium Antimonide (InSb):
This is a compound of indium (In) an element of IIIrd column and antimony (Sb) an element of Vth column in periodic table.
Electron mobility of InSb at room temperature is highest (≈ 10 m2A/s) among all known semiconductors. It has a low melting point (525°C) and is easier to produce in single crystal form.
Its electrical resistivity at 20°C is ≈ 20000 ohm m.
Uses:
a. Infrared detectors
b. Hall Effect devices
c. Laser diodes
d. Tunnel diodes
e. Infrared filter material
f. Transistors
iii. Oxides, Sulphides, Halides, Telluride and Sellurides:
These are extrinsic semiconductors of compound form. Some of them are n-type and others are p-type. Zinc oxide (ZnO) is a n-type and Cuprous oxide (Cu20) is a p-type semiconductor.
Sulphides such as PbS (Gelina), CdS, Cu2S, ZnS; and halides such as CuBr, Cul, CuCI etc. have deviations from stoichiometric compositions.
Tellurides such as Bi2Te3, PbTe and sellurides such as PbSe, CdSe and other elements are also the semiconductor compounds.
Uses:
a. BaO- in oxide coated cathodes
b. CdS, CdSe, CdTe- in photoconductivity based automatic door opener, street light switching, burglar alarm
c. PbSO4, CdS, PbS- in photoconductive devices such as photocells of TV camera and cinematography
d. GaP, GaAs, GaSb- in semiconductor lasers
e. CsSb- in photomultipliers
iv. Cadmium Sulphide (CdS):
It used in manufacture of photoconductors of high dissipation capability and excellent sensitivity in visible spectrum, and to prepare cadmium sulphide cell by depositing a layer of CdS.
This layer generally contains Ag, Sb, In etc. as impurity. Its energy gap is 2.4 eV.
Uses:
a. As constituent of cathode-ray phosphorus
b. To measure a fixed amount of illumination as with light meter
c. To record modulating light intensity on sound track
d. As ON-OFF light relay in digital and control circuits.
v. Silicon Carbide (SiC):
Hard and refractory in nature.
Its energy gap is large, because of 3 eV. Its melting point is also very high (about 2400°C).
Application:
a. High temperature rectifiers
b. High-temperature transistors
Microelectronics:
In microelectronics single piece of semiconductor can connect more than 1000 transistors on an area of about 1 square centimetre.
Processes of microelectronics may be classified as:
a. Semiconductor microelectronics, and
b. Thin film microelectronics
Diodes, pnpn switches, transistors and resistors belong to semiconductor microelectronics group while the interconnections of various electronic circuits and capacitors are the products of thin film microelectronics.
Fabrication of Thin Film Microelectronic Circuit:
In fabricating of these circuits, glazed ceramics and glass substrates are used.
Substrate is a polished surface on which many hundred I.Cs may be located.
Deposition of I.Cs on thin films can be done by one or more of the following processes:
(i) Vaccum deposition
(ii) Silk screening
(iii) Vapour plating
(iv) Electron beam decomposition
(v) Plasma decomposition
(vi) Sputtering, and
(vii) Anodization
Thin film resistors employ metallic film while the conductors employ film of gold or aluminium; and capacitors use a dielectric film.
Ion implantation is a most modern method of doping microelectronic devices such as in MOSFET.
Applications of Semiconductor Devices:
i. Equilibrium Condition Junction Diodes e.g.:
a. Rectifying diodes as- (i) half-wave rectifier, (ii) full-wave rectifier
b. Zener diode- (i) for meter protection (ii) as peak clipper
ii. Transient Condition Diodes e.g.:
a. Switching diodes – Varactor diodes
b. Metal-semiconductor junction Schottky diode
iii. Bipolar junction transistors (BJT) e.g.:
a. Field effect transistor (FET)
b. Metal-semiconductor field-effect transistor (MESFET)
c. Metal-insulator-semiconductor field effect transistor (MISFET)
iv. Optoelectronic Devices e.g.:
i. Photodiodes
ii. Photocells
iii. Light emitting diodes (LEDs)
iv. Photodetectors
v. Solar cells
vi. Optical fibres
vii. Semiconductor lasers
v. Negative conductance microwave devices e.g.:
i. Tunnel diodes
ii. The Gunn diode
iii. Impatt diode
vi. Power Devices e.g.:
i. p-n-p-n diode
ii. Silicon controlled rectifiers (SCRs)
Semiconductor Devices:
i. Junction:
Boundary between p and n semiconductors is known as junction. The region of small thickness at the junction is called depletion layer. The rectifying action takes place in this region.
ii. Transistor:
Two junction (n-p and p-n) system shows amplifying action of electrical signals. Miniature, cheap, reliable, instant acting and less power consuming solid state transistors. Metal oxide semiconductor field-effect transistor (MOSFET) is a modern transistor used in street lights and as burglar alarm. CdS, CdSe and CdTe are commonly used photo-conducting semiconductors.
iii. Photodiodes:
A photodiode is a two terminal device, which responds to photon absorption.
iv. Photoconductors (or Photo Detectors):
Used to detect and measure the quanta of light such as in automatic door opener, in switching the street lights and as burglar alarm. CdS, CdSe and CdTe are commonly used photo-conducting semiconductors.
v. Photocells:
It convert the light energy into electrical energy. Made from CdS, Se and PbSO4. Photocells are used in cinematography, fire alarms and television cameras etc.
vi. Solar Cells
Solar cells from semiconducting materials are of immense utility in satellites and space-going vehicles. They are also used in calculators, solar power generation and solar auto vehicles.
vii. Light-Emitting Diode:
It is an incoherent light source that is used as a light source in fiber optic systems and other devices.
viii. Laser:
It is the source of a highly directional, monochromatic coherent light which is used as a light source for various optical and electronic devices.
ix. Optical Fiber:
It is a means of transmitting optical signals from a source to a detector. Thermistors are used to determine the temperature of systems in process industries, ovens and furnaces etc. They are manufactured from sintered manganese oxide that contains dissolved germanium or lithium.
x. Integrated Circuits:
These are made on a single chip that contains diodes, transistors, resistors and capacitors etc. These are generally monolithically constructed using either unipolar or bipolar techniques.
xi. Thermistors:
These work on temperature dependency effect of semiconductors.
xii. Varistors:
These work on voltage dependency effect of semiconductors.
xiii. Rectifiers:
These work on impurity dependency effect of semiconductors.
xiv. Strain Gauges:
These work on change in resistance effect of semiconductors.
xv. Zener Diodes:
These work on electrical field effect of semiconductors.
xvi. Transistors:
These work on amplification effects of semiconductors.
xvii. Photoconductive Cells:
These work on the light illumination effect of semiconductors.
xviii. Photovoltaic Cells:
These work on the optical characteristics of semiconductors.
xix. Hall Effect Generators:
These work on the carrier drift effect in semiconductors.
Semiconductor Lasers:
Laser (light amplification by stimulated emission of radiation) is a quantum electronic device that generates intense electromagnetic radiation.
Now a days solid ruby laser (chromium ions in alumina), liquid lasers (having rare- earth ions), and gaseous laser (such as carbon dioxide laser) are used in successful applications but semiconductor lasers are more efficient than these lasers.
GaP, GaAs and GaSb are the most significant compounds for this purpose.
In semiconductors lasers, light emission is obtained through a pn junction as a result of recombination of electrons and holes.
Materials for Semiconductor Lasers:
It must be efficient light emitter.
It ease in formation of p-n junctions.
It is heterojunction barriers.
II-VI compounds efficiency high for light emission, but formation of their junction is difficult. However using nitrogen (N) as acceptor, the junction can be grown easily in the following materials by crystal growth techniques like MBE (molecular beam epitaxy) and MOVPE (metal-organic vapour-phase epitaxy).
a. ZnS
b. ZnSe
c. ZnTe
Lasers made of these materials emit in the region of green and blue-green light spectrum.
Large Bandgap Semiconductors- Semiconductor of large band gaps (about 2 to 5 eV) offer good candidature as semiconductor lasers materials, e.g.
a. InN (Eg = 2 eV)
b. GaN (Eg = 3.4 eV)
c. AIN (Eg = 5 eV)
This covers a wide range of light emission from blue to UV spectrum.
Ternary alloy system- These are used in the lasers used in fibre-optic communication systems.
a. AIGaAs
b. PbSnTe
c. InGaSb
The PbSnTe is able to provide laser output in the wavelength range from 7µm to above 30µm at low temperatures. The InGaSb is suitable for use at intermediate wavelengths.
Quaternary alloy system- These are versatile in fabricating, by allowing flexibility in lattice matching, e.g. InGaAsP.
Short wavelength emitters- Blue/UV semiconductor lasers are very suitable for storage purposes such as compact discs (CDs), digital versatile discs (DVDs) etc.
By reducing the laser wavelength i.e. by using short wavelength emitters, the storage density of these devices can be increased many times, e.g. InGaN.
This laser has multi-quantum-well hetero structures.