Test: Electrical Properties of Solids (Old NCERT) - NEET MCQ

Conductors 10⁴ to 10⁷ohm⁻¹m⁻¹
Insulators : 10⁻²⁰ to 10⁻¹⁰ohm⁻¹m⁻¹
Semiconductors : 10⁻⁶ to 10⁴ohm⁻¹m⁻¹

Pure Si and Ge are insulators because of the absence of any hole or free electron. They show semiconductor behaviour only after doping.

Conductors, semi-conductors and insulators can be defined on the basis of conduction band and valence band.In conductors, the conduction band and valence band overlap each other.
In semi-conductor, the energy gap between the conduction band and valence band ∼1eV.
In insulators, the energy gap between the conduction band and valence band >9eV.

To increase the conductivity of intrinsic semiconductor, some impurities are added. This process is called doping (P). The impurity can be of two type - electron (Q) rich or deficient. Due to doping, electronic (R) defects are introduced in the matter. If the impurities are electron rich then it will lead to the formation of n-type (S) semiconductors and if the impurities are electron deficit then it will lead to the formation of p-type semiconductor (T).

For germanium to be a N- type semiconductor, it should be doped with group 15 elements. This will lead to excess of electrons.

In the given options, only element which belongs to group 15 is Arsenic.

For Si or Ge to be a p−type semiconductor, they have to be doped with trivalent elements. This will lead to electron scarcity, which is called as hole. For this reason, the elements should be of Group 13.

Impurity of lower group creates a positively charged hole. On applying potential difference , p-holes conduct electric current.

Intrinsic semiconductors are insulators at room temperature and become semiconductors when temperature is raised.

In n-type semiconductor, Si and Ge are doped with group 15 elements. Since group 15 elements have five electrons in valence shell which is one more than Si or Ge i.e. four electrons. So, the material becomes electron rich and it becomes n- type.
In p-type semiconductor, Si and Ge are doped with group 13 elements. Since group 13 elements have three electrons in valence shell which is one less than 
Si or Ge i.e. four electrons. So, the material becomes electron deficit and it becomes p- type.  

Fe is a metallic solid. It has free electrons. So, it conducts electricity is solid state. It is called as electronic conductor.
In molten state, NaCl has free ions i.e. Na⁺ & Cl⁻. These ions helps in flow of electric current. Molten NaCl is called as electrolytic conductor. In CO₂, C and O are bonded by covalent bonds and there are no free electron in CO₂. So, it is Non-Polar insulator.
Si is group 14 element having four electrons in valence shell and Al is 13 element having three electrons in valence shell. On doping Si with Al, it will form a electron deficit material. It is called as p-type semiconductor.