Electrical and magnetic properties of solids - class-XI

$As$ is a member of group $V$ and has 5 electrons in its outermost shell. 

An n-type semiconductor is due to the presence of a free electron in the semiconductor. Since silicon is an element having $4$ valence electrons, in order to have a free electron, it must be doped with an element having one more valence electron than silicon. 

In semiconductor two types of the semiconductor are present. They are an intrinsic and extrinsic semiconductor. A pure silicon crystal is an example of an intrinsic semiconductor. 

To form a p-type semiconductor, one must add elements from boron family to the silicon atom.

Arsenic is a Group 15 element. Doping Germanium (Group 14) with a Group 15 element provides an extra electron, resulting in an n-type semiconductor.

To get n-type doped semiconductor, pentavalent impurity should be added to silicon. Example- As in Si.

Lithium (Li) is a metal, and metals are characterized by high electrical and thermal conductivity due to the presence of free electrons in their metallic lattice.

Silicon and germanium belong to group 14 of the periodic table and have four valence electrons each. In their crystals each atom forms four covalent bonds with its neighbours. When doped with a group 15 element like P or As, which contains five valence electrons, they occupy some of the lattice sites in silicon or germanium crystal. Four out of five electrons are used in the formation of four covalent bonds with the four neighbouring silicon atoms. The fifth electron is extra and becomes delocalised.These delocalised electrons increase the conductivity of doped silicon(or germanium). Here the increase in conductivity is due to the negatively charged electron, hence silicon doped with electron-rich impurity is called n-type semiconductor.

A semiconductor of Ge can be made p-type by adding trivalent impurity and for n-type SC pentavalent impurity is dopped.

Doping silicon with Group 13 elements creates holes because there are fewer valence electrons than in silicon. These holes act as positive charge carriers, making it a p-type semiconductor.

Group-15 doped crystals of silicon are called a n-type semiconductors because of the presence of a valence electron and electrons are responsible for the semiconducting properties.

In an insulator, the bandgap is large. More energy is required to promote electrons from valence band to conduction band. This energy is not available. The electrons remain in the valence band and cannot move freely. Hence, insulators are bad conductors of heat and electricity.

Frenkel defect is the defect which is created when an ion leaves its appropriate site in the lattice and occupies an interstitial site. A hole or vacancy is thus produced in the lattice.

As the temperature increase the electrons in the valence band get excited and jump into the conduction band and hence the conductance increases and conductance is inversely proportional to resistivity. Hence, resistivity decreases.

When a pure semiconductor is doped with trivalent elemental impurity $p$ type semiconductors are formed

Super conductors are the substance which provide the conductivity of electricity at different temperatures. Mercury becomes superconducting at 4 K.

Superconductivity may be defined as a phenomenon in which metals, alloys and chemical compounds become perfect conductors with zero resistivity at temperatures approaching absolute zero.

For most metals, the transition temperature lies between 2 and 5 K. The
temperature at which a substance starts behaving as super-conductor is
called transition temperature.

The following represents true statements. The electrical resistance of metal depends upon temperature. Electrical resistance decreases with decrease in temperature. The electrical resistance becomes almost zero near the absolute temperature.

The temperature at which a substance starts behaving as super-conductor
is called transition temperature. For most metals, the transition
temperature lies between 2 and 5 K.

Superconductors are diamagnetic.

The highest temperature at which superconductivity has been observed is 23 K for alloys of niobium $(Nb _3Ge)$.

The n-type semiconductor can be produced  by doping impurity atoms of valence 5 i.e, pentavalent atoms like phosphorous and so electrons are majority charge carriers and Holes are minority charge carriers.

$Tl _2Ca _2Ba _2Cu _3O _{10}$ possess superconductivity at 125 K.

Superconductors have applications in electronics, building magnets, aviation, transportation (trains which move in air without rails) and power transmission.

The n-type semiconductors are obtained when $Si$ or $Ge$ are doped with elements of group 15, eg, Arsenic $(As)$, while p-type semiconductors are obtained when $Si$ or $Ge$ are doped with traces of group 13 , ie Indium $(In)$ , Boron $(B)$.

An extrinsic semiconductor is one that has been doped; during manufacture of the semiconductor crystal a trace element or chemical called a doping agent has been incorporated chemically into the crystal, for the purpose of giving it different electrical properties than the pure semiconductor crystal.

$p-$type semiconductors have a larger hole concentration than electron concentration. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. $p-$type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities. A common $p-$type dopant for silicon is boron or gallium. There will be no charge on the semiconductor.

$TiO _{ 3 }$ behaves as conductor or insulator depending on temperature because of variation of energy gap between valence band and conduction band with the variation of temperature.

An $n-$type semiconductor is made by adding a small amount of a Group-V element such as phosphorous ($P$) or arsenic ($As$) to the intrinsic semiconductor. Group-V elements have five valence electrons per atom.

The conductance through electrons is called n-type conduction and if through positive semiholes, it is called p-type conduction.
Doping involves preparation of semi-conductors by the presence of impurities in the intrinsic semiconductor.With donor impurities, we get n-type semi-conductors and with acceptor impurities, we get p-type semi-conductors.

The addition of small amount of foreign impurity in the host crystal is called as doping. It results in an increase in the electrical conductivity of the crystal. Doping of group 14 elements (such as Si, Ge etc.) with elements of group 15 (such as As) produces an excess of electrons in the crystals and thus, gives n-type semiconductors. Doping of groups 14 elements with group 13 elements (such as Indium) produces holes (electron deficiency) in the crystals. Thus, p-type semiconductors are produced. The symbol 'p' indicates flow of positive charge. Also, the band gap in germanium is small as Ge is a semi-conductor.

The addition of a small amount of foreign impurity in the host crystal is called doping. 

The addition of small amount of foreign impurity in the host crystal is called as doping. It results in an increase in the electrical conductivity of the crystal. Doping of group 14 elements (such as Si, Ge etc.) with elements of group 15 (such as As) produces an excess of electrons in the crystals, thus, giving n-type semiconductors. Doping of groups 14 elements with group 13 elements (such as Indium) produces holes (electron deficiency) in the crystals. Thus, p-type semiconductors are produced. The symbol 'p' indicates flow of positive charge. Positive holes are responsible for the semiconducting properties like conduction.

A compound may have excess metal ion if an anion (negative ion) is absent from its appropriate lattice site creating a 'void' which is occupied by an electron. The ionic crystal which is likely to possess Schottky defect may also develop this type of metal excess defect. When alkali metal halides are heated in an atmosphere of vapours of the alkali metal, anion vacancies are created. The anions (halide ions) diffuse to the surface of the crystal from their appropriate lattice sites to combine with the newly generated metal cations. The electron lost by the metal atom diffuse through the crystal is known as F-centres. The main consequence of a metal excess defect in the development of colour in the crystal. For example, when NaCl crystal is heated in an atmosphere of Na vapours, it becomes yellow. Similarly, KCI crystal when heated in an atmosphere of potassium vapours, it appears violet.
The addition of a small amount of foreign impurity in the host crystal is called doping. It increases the electrical conductivity of the crystal. Doping of group 14 elements (such as Si, Ge etc.) with elements of group 15 (such as As) produces an excess of electrons in the crystals, thus, giving n-types semiconductors. Doping of groups 14 elements with group 13 elements (such as Indium) produces holes (electron deficiency) in the crystals. Thus, p-type semiconductors are produced. Then symbol 'p' indicates the flow of positive charge.
Paramagnetic (weakly magnetic): Such materials contain permanent magnetic dipoles due to the presence of atoms, ion or molecules with unpaired electrons.
Due to unpair electron of F-centers, solids containing F-centers (Farbe) are paramagnetic.

We know that resistance of a conductor is directly proportional to the temperature. So, when it is cooled below a certain temperature called critical temperature, the resistance tends to drop to zero and superconductivity can be observed.

Around $1993$, the highest temperature superconducting material was known. It has been a ceramic material consisting of $Hg, Ba, Ca, Cu$ and $O$ i.e, the compound was $HgBa _2Ca _2Cu _3O _2$

Intrinsic (pure) semiconductors are undoped semiconductors. The charge carriers i.e electrons and holes in this kind of semiconductor are equal in number. The electrical conductivity of these type of semiconductors is due to electron excitation.

Eg: $Si, \ Ge, \ etc…$