# [MCQ] Conductor, Semiconductor and Insulator rrb je electronics

This post contains all the theoretical explanations of conductor, semiconductor and insulator, followed by a Mock Test with Multiple Choice Questions .

## Difference between Conductor, Semiconductor and insulator

 parameter Conductor Semiconductor Insulator Resistance Low medium High Conductance High medium Low Temperature +ve temp coeff. -ve temp coeff. -ve temp coeff. Current High medium Low Free electrons Very High medium Low Bond metalic bond Covalent bond Ionic bond Band gap Zero Si = 1.14, Ge =0.67(less than 2 ev) greater than 6 eV

Materials are classified into conductors, insulators and semiconductors. This classification is based on the conductivity of material. Conductors have high conductance (Iron, Copper, Silver etc.,) hence it got that name. insulators have poor conductance (Wood, paper etc..). Conductance of semiconductor is in between conductor & insulator and it is used in manufacturing of electronic components (silicon, germanium). Conductance is denoted by G.

In terms of Resistance, which is opposite of conductance. in other words resistance is inversely proportional to conductance. That is

Resistance $$\alpha$$ $$\frac{1}{conductance}$$ .

So Resistance is high in insulator, low in conductor and medium in semiconductor. Resistance is denoted by R.

As Temperature of a Conductor increases its resistance also increases. thus we can say Conductors have positive temperature coefficient. Insulator and Semiconductor have negative temperature coefficient, because here temperature and resistance are inversely proportional. That is when temperature increases resistance decreases.

## Conduction Band and Valance Band

There are two bands in materials. which are conduction and valance band. conductivity depends on number of electrons present in conduction band. these electrons are called free electrons. Electron in the valance band jumps to conduction band, then conduction take place.

Gap between conduction and valance band is called Forbidden gap. imaginary line drawn between these bands are called Fermi level. In metal or conductor two bands overlap each other. In semi conductor band gap is very small. Large band gap is present in insulators, so it is difficult for an electron to jump from valance band to conduction band.

## Semiconductor Classification

Semi conductors are classified in three ways.

• Direct and Indirect Band gap semiconductor
• Elemental and compound semiconductor
• intrinsic and Extrinsic semiconductor

### Direct and Indirect band gap semiconductor

electrons in valance band acquire energy and jump to conduction band. This electron will conduct but it is unstable, thus it have a tendency to release energy and returns to valance bond.

If electron falls from conduction band to an intermediate band between conduction and valance band and from their to valance band, then such semiconductors are called Indirect band gap semiconductor. Here heat is emitted during electron movement from conduction to valance band

If electron falls directly from conduction band to valance band, such semiconductors are called direct band gap conductors. Here light is emitted during electron movement from conduction to valance band.

### Elemental and compound semiconductor

Semiconductors formed by elements itself is called elemental semiconductor. example : silicon, Germanium. Both the examples are tetravalent elements (elements with four valance electrons).

Semiconductors which are compounds are called compound semiconductor. Here one element of the compound is trivalent element and another one is pentavalent element. Example : Gallium Arsenide, Indium Phosphide etc..

### Intrinsic and Extrinsic semiconductor

Intrinsic semiconductor are pure semiconductors. because which does not have impurities. In other words it is not doped with other elements. number of electrons and holes are same. At 0 kelvin it will act as insulator.

Extrinsic semiconductor are impure semiconductors. Because it is doped with an impurity. Here trivalent element or pentavalent element is doped with an intrinsic semiconductor. If trivalent (Boron, Aluminum etc..) is used as impurity then a P type semiconductor will form, which have more number of positive charged holes than negative charged electrons. If pentavalent(phosphorous) is used as impurity then an N type semiconductor will form, which have more number of negative charged electrons than positive charged holes . Pentavalent element donates an electron so it is called donar atom. Trivalent element accepts an electron

In P type semiconductor majority charge carriers are holes and minority charge carriers are electrons. In N type semiconductor majority charge carriers are electrons and minority charge carriers are holes. N type semiconductor is a donor, because it have more number of electrons so it can donate electron. Due to the large number of holes P type semiconductor is an acceptor.

#### Rank Point

By doping only majority charge carriers will increase. minority charge carriers will be constant

## Difference between Silicon and Germanium

 silicon Germanium (comparatively) leakage current lower leakage current little higher high power handling low power handling high temperature handling low temperature handling Available less available cheapest not so cheap

## Multiple Choice Questions

2. Atom of any element is

3. Conduction of electricity through conductor takes place by

4. Intrinsic semiconductor at room – temperature will have ………..

5. Intrinsic semiconductor at 0° K will have …….. available for conduction

6. At room temperature N – type material will have ……

7. A room temperature P – type material will have …….

8. Which of the following statements is incorrect regarding conduction band ?

9. The forbidden band in germanium at 0°K is

10. Donor type semi – conductor is formed by adding impurity of………… valency

11. Fermi level represents the energy level with probability of its occupation of ………. %

12. Consider the energy level diagram of an intrinsic semiconductor. The Fermi level lies in the

13. A semiconductor has ……. Temperature co – efficient of resistance

14. A semiconductor in its purest form is called …….

15. An electron in the conduction band

16. …….. has the highest mobility.

17. The donor impurity must have only …….. valence electrons

18. Acceptor – type impurities ……..

19. Which of the following cannot exist outside a semiconductor ?

20. In a N – type semiconductor, the position of the Fermi level………

21. Fermi energy is the amount of energy which

22. An intrinsic semiconductor has the following properties :
1. Its electron concentration equals its hole concentration.
2. Its carrier density increases with temperature.
3. Its conductivity decrease with temperature.
Which of these statements are correct ?

23. Medium doping in Silicon and Germanium corresponds to impurity of the order of

24. A ‘hole’ in a semiconductor has

1. Positive charge equal to the electron charge.
2. Positive mass equal to the mass of the electron.
3. An ‘effective mass’ greater than the effective mass of electron.
4. Negative mass and positive charge equal to the charge in nucleus.

Which of these statements are correct ?

25. The Fermi level in a p – type semiconductor lies close to

26. Covalent bond energy in germanium is about

27. The energy gap in the energy band structure of a material is 9 eV at room temperature. The material is

28. By doping germanium with Gallium, the types of semiconductors formed are :
1. N type
2. P type
3. Intrinsic
4. Extrinsic

Which of the above are correct : –

29. An n – type can be formed by adding impurity of
1. Phosphorous
2. Arsenic
3. Boron
4. Aluminum
Which of the above are correct ?

We prepared this list from the suggestions of  Junior Engineers who got selection in 2014.

RRB JE PREMIUM STUDY KIT from Bookhive

RRB JE Recruitment exam  from GK Publications

COMBO PACK OF RRB JE Study Guide 2019 from ARIHANT

Shortcut in Reasoning from Disha Publications