Semiconductors are those substances whose electrical conductivity lies in between conductors and insulators. In terms of energy band, the valence band is almost filled and conduction band is almost empty. Further, the energy gap between valence band and conduction band is very small. The semiconductor has filled valence band, empty conduction band, and small energy gap or Forbidden gap between valence and conduction band. Semiconductors virtually behave as an insulator at low temperature. However, even at room temperature, some electron cross over to the conduction band, imparting little conductivity.
Types of Semiconductor (SC)
A semiconductor (SC) in an extremely pure form is known as an intrinsic semiconductor.
In this case, the holes in the valence band are vacancies created by electrons that have been thermally excited to the conduction band and electron-hole pairs are created. When an electric field is applied across an intrinsic SC, the current conduction takes place by two processes namely; by free electrons and holes. The free electrons are produced due to the breaking up of some covalent bonds by thermal energy. At the same time, holes are created in the covalent bond. Under the influence of electric field, conduction through the SC is by both free electrons and holes. Therefore, the total current inside the SC is the sum of currents due to free electrons and holes. This creates new holes near the positive terminal which again drift towards the negative terminal.
An extrinsic semiconductor is a SC doped by addition of small amount of impurity which is able to change its electrical properties such as conduction, making it suitable for electronic applications like diodes, transistor, etc. This is achieved by adding a small amount of suitable impurity (having 3 or 5 valence electrons) to a SC (having four valence electrons). it is then called impurity or extrinsic semiconductor.
The process of adding impurities to an intrinsic semiconductor is known as doping. The purpose of adding impurity is to increase either the number of free electrons or holes in the SC crystal.
If a pentavalent impurity (having 5 valence electrons) is added to a SC, a large number of electrons are produced in the SC.
If trivalent impurity (having 3 valence electrons) added to the SC, a large number of holes are produced in the SC crystal.
Depending upon the type of impurity added, extrinsic semiconductors are classified into two types i.e.
- P-type semiconductor
- N-type semiconductor
When a small amount of pentavalent impurity is added to a pure SC, it is known as an n-type semiconductor. The addition of pentavalent impurity provides a large number of free electrons in the SC crystal. Typical examples of pentavalent impurities are arsenic, antimony, bismuth, phosphorus, etc. Such impurities which produce n-type semiconductor are known as Donor impurities because they donate or provide free electrons to the SC crystal.
Electrons = majority carriers
Holes = minority carriers
When a small quantity of trivalent impurities added to a pure SC it is called P-type semiconductor. The addition of trivalent impurity provides a large number of holes in the SC crystal. Typical examples of trivalent impurities are gallium, indium, Boron, etc. Such impurities which produce P-type semiconductors are known as acceptor impurities because holes created can accept the electrons.
Electrons = minority carriers
Holes = majority carriers
Semiconductor (SC) Materials Types
- Elemental SC materials
- Compound SC materials
- Amorphous SC materials
1. Elemental SC materials
Example: Ge, Si, C, B, Al, Ga, P, As, Sb, Bi, etc.
a). Arsenic (As)
- It is pentavalent SC material.
- It is used as donor N-type semiconductor material.
- When it is alloyed with gallium, then it is used in the fabrication of LED.
b). Selenium (Se)
- It is used in a photovoltaic cell.
2. Compound SC materials
As conductivity and band gap are limited for Elemental SC materials hence their usefulness is limited. So, group III-V, II-VI, IV-IV, IV-VI semiconductors are used to provide better properties.
a). III-V SC material
- They provide a wider range of band gap and extended ampere range of a device.
- The structure is zinc blende and diamond cubic.
- Example: GaAs, AlP, etc.
- large band gap material.
- large electron mobility which helps in high-speed switching.
- direct band gap material
- It is 10 times costlier than Silicon.
- It is 2.5 times faster than a silicon-based device.
- In GaAs crystals, Ga substitute corner and face atoms whereas takes place of 4 inside atoms.
- Satellite amplifier
b). Group II-VI SC material
- The band gap is larger than group III-V SC.
- Examples: CdS, CdSe, CdTe, ZnS, ZnSe, etc.
CdS, CdSe, CdTe can be used as photo conduphotoconductors
IV-IV SC material
- Example: SiC
- The band gap of SiC is 3eV.
- X-SiC can be used for high-temperature devices.
- One drawback is that it is expensive and not easy to manufacture.
d). Group IV-VI SC material
- Examples: PbS, PbSe, PbTe
- In this semiconductor, excess Pb gives rise to N-type semiconductor and less Pb gives rise to a P-type semiconductor.
3. Amorphous SC Material
- The structure is similar to super cooled liquid.
- Atoms up to first nearest neighbours are arranged periodically but the atoms which are away from the first nearest neighbour are found to be arranged randomly.
There are 3 types of Amorphous SC Material
a). Elemental Amorphous SC
Examples: Ge, Si, Se, Te, etc.
b). Covalent Amorphous SC
Examples: Ge, Te, etc.
c). Ionic Amorphous SC
Examples: Al2O3, V2O5, etc.
Q. What is forbidden gap?
Answer. The band separating the valence band from from the conduction band is called forbidden gap.
Q. What is the order of energy for forbidden band in (a) diamond (b) silicon (c) germanium, and (d) aluminium?
Answer. (a) 9 eV
(b) 1.2 eV
(c) 0.74 eV
Q. Mention some properties of semiconductors.
Answer. The properties of semiconductors are as follows.
- It has covalent bonding.
- It is crystalline.
- It has a negative temperature coefficient of resistance.
- Its conductivity increases with the addition of impurities.
Q. What is hole?
Answer. Hole is a seat of positive charge which is produced when an electron breaks away from a covalent bond in a semiconductor.
Q. What are the characteristics of a hole?
Answer. The characteristics of a hole are as follows.
- Hole carries a unit positive charge.
- It has the same charge as that of electron.
- Energy of a hole is high as compared to that of electron.
- The mobility of hole is smaller
Q. What is the difference between hole current and electron flow?
Answer. In a p-type semiconductor holes (vacancies of electrons) are majority carriers which are responsible for conduction. When such a material conducts, the electron from the neighbouring covalent bond jumps into a vacancy of electron in order to fill it. This will result in shifting of hole to the covalent bond from which electron has jumped. The movement of the holes constitutes the hole current.
In an n-type semiconductor, electrons are majority carriers which are responsible for conduction. The movement of electrons constitute the electron current.
Q. Distinguish between a metal, an insulator and semiconductor.
Ans: Conductors (metals), semiconductors and insulators are distinguished from one another on the basis of the forbidden band. In metals, the forbidden band does not exist, as the conduction and valence bands touch each other. In insulators, the forbidden band is about 6 eV while in semiconductors, it is about 1 eV.
Q. Explain that energy of a hole farther from the top of a valence band is high.
Answer. To understand it, consider an electron is removed from the filled valence band to the bottom of the empty conduction band. This creates a hole in the valence band. Since more energy is required to remove an electron which is farther from the top of the valence band, therefore, a hole in the valence state farther from the top of the valence band has higher energy, just as a conduction electron farther from the bottom of the conduction band has higher energy.
Q. What happens to the resistance and conductance of a semiconductor on heating? How does the conductivity of a semiconductor change with rise in temperature?
Answer. The resistance of a semiconductor decreases and conductivity increases on heating.
Q. How does the conductivity of semiconductors change with rise of temperature?
Answer. At absolute zero, all semiconductors are insulators. The valence band at absolute zero is completely filled and there are no free electrons in conduction band. At room.temperature, the electrons jump to the conduction band due to the thermal energy. When the temperature increases, a large number of electrons cross over the forbidden gap and jump from valence band to conduction band. Hence, conductivity of semiconductor increases with temperature.
Q. Why a semiconductor is virtually an insulator at room temperature?
Answer. A semiconductor is virtually an insulator at room temperature because almost all the valence electrons are involved in the formation of covalent bonds and there are practically very few free electrons.
Q. Doping of silicon with indium leads to which type of semiconductor?
Answer. Since indium belong to III group elements having 3 valence electrons. Hence forms p-type semiconductor.
Q. Name three acceptor impurities.
Answer. The three acceptor impurities are Aluminium, Indium, and gallium.
Q. Name three donor impurities.
Answer. The three donor impurities are Phosphorus, Antimony, and Arsenic.
Q. Name the minority carriers of a p-type Ge semiconductor.
Answer. Free electrons are the minority carriers of a p-type Ge semiconductor.
Q. Name the minority carriers of a n-type Ge semiconductor.
Answer. Free holes are the minority carriers of a p-type Ge semiconductor.
Q. What is the value of conductivity of a semiconductor at absolute zero?
Answer. The value of conductivity of a semiconductor at absolute zero is zero.
Q. What type of charge carriers are there in a n-type semiconductor?
Answer. Both electrons and holes exist as charge carriers in n-type semiconductor. However, electrons are majority charge carriers while holes are minority charge carriers.
Q. Why do conductors not form holes?
Answer. A hole can be formed only when an electron leaves an electron-pair bond. Conductors do not possess electron-pair bonds.
Q. The hole current is due to movement of valence electrons from one covalent bond to another. Why is then the name hole current?
Answer. The basic reason for current flow is the presence of holes in the covalent bonds. Therefore, it is more appropriate to consider the current due to the movement of hole.
Q. Why are hole carriers present in n-type semiconductor?
Answer. Due to the addition of pentavalent impurities, n-type material has a large number of free electrons. However, even at room temperature, some of the covalent bonds break, thus releasing an equal number of free electrons and holes. Therefore, an n type has a large number of free electrons and a small number of holes also.
Q. Why is semiconductor damaged by a strong current?
Answer. Strong current flowing through a semiconductor heats it up excessively. Its covalent bonds are broken and very large number of electrons are set free. Therefore, semiconductor starts behaving as a conductor.
Q. A n-type semi-conductor has a large number of electrons but still it is electrically neutral. Why?
Answer. In n-type semi-conductor pentavalent impurity is doped in pure germanium or silicon. Electrons of impurity atoms take part in forming covalent bond while the fifth electron of the impurity atom jumps to the conduction band. Atom as a whole remains neutral as all covalent bonds are complete.
Q. Explain, Why is the conductivity of n-type semiconductor greater than that of p-type semiconductor, for the same degree of doping?
Answer. The majority charge carriers are the free electrons in n-type semiconductor whereas there are holes in p-type semiconductor. As free electrons have more mobility than holes, n-type semiconductor has greater conductivity than that of p-type semiconductor. Actually, the conductivity of n-type semiconductor is approximately two times that of p-type semiconductor.
Q. Why is silicon preferred to germanium in the manufacture of semiconductor devices?
Answer. Silicon is preferred to germanium in the manufacture of semiconductor devices due to following reasons.
- The leakage current in silicon is very small as compared to that of germanium.
- In case of silicon, the leakage current is less effected with temperature as compared to germanium.
- The working temperature of silicon is more than that of germanium. The structure of germanium gets destroyed at a temperature of about 100 °C but silicon can be operated upto about 200°C.
Q. Why is the amount of impurity added to a pure semiconductor minutely controlled?
Answer. Addition of impurity to a pure semiconductor have a remarkable effect on the electrical conductivity of the semiconducting material. The addition of one impurity atom in one hundred million will increase the conductivity by 11 times at room temperature. Therefore, the amount of impurity added has to be minutely controlled during the preparation of extrinsic semiconductor.