Chapter#3: Semiconductors

Team Quanta gladly presents all possible short questions of BS Physics book Solid State Physics – II’s Chapter#3: Semiconductors.

Q.3.1:- What are intrinsic and extrinsic semiconductors?

Intrinsic semiconductor: The semiconductor in which no impurity atoms are added is called intrinsic semiconductor.

Example: pure Ge or  Si are un doped semiconductors.

Extrinsic semiconductor: The semiconductor in which suitable impurity atoms are added to the crystal is called extrinsic semiconductor.

Example: The pentavalent impurity, such as phosphorus are doped to pure semiconductor.

Q.3.2:- What type of carriers are present in semiconductor.

There are two types of carriers.

• Minority carriers
• Majority carriers

Majority carriers:

• In n- type semiconductors majority carriers are electrons.
• In p- type semiconductor, the majority carriers are holes.

Minority carriers:

• In n- type semiconductor the minority carriers are holes.
• In p- type semiconductor semiconductor the minority carriers are electrons.

Q.3.3:-Define Mobility of charge carrier.

The drift velocity acquired by a carrier per unit electric field strength.

 is called mobility of charge carrier.

Q.3.4:- What is doping?

The process of mixing pentavalent or trivalent impurity to pure semiconductor Ge or Si is called doping.

Example: phosphorus mixing the Ge

Q.3.5:- Draw the energy level diagram for n- type and p- type semiconductor and Lable it.

n- type semiconductor:-

At 0K, all the donors are in the unionized state, all the donors levels are occupied with electrons. As the temperature increases slightly, some of the donors get ionized and contribute electrons to the conduction band. Also, some of the valance band electrons may jump to the conduction band leaving behind holes in the valance band. Therefore, the Fermi level must lie somewhere near the middle of the donor level and the bottom of the conduction band.

p- type semiconductor:-

 The acceptor impurity atoms occupy the acceptor levels Ea which lies above the valance band. For  T > 0K, apart of these acceptors are ionized by acquiring electrons from valence band, thus creating holes in the valence band which cause p- type conduction.

Q.3.6:- Define law of mass action. For what type of semiconductors does it hold?

The product of electron and hole concentration is a constant at a given temperature and is equal to the square of the intrinsic carrier concentration. This is called law of mass action. Law of mass action for both intrinsic and extrinsic semiconductor.If impurity atoms are added to a semiconductor to increase n, there will be corresponding decrease in p, such that the product np remains constant.

Q.3.7:- Can a semiconductor containing both n-type and p-type impurities behave as intrinsic semiconductor. Give reason.

The intrinsic carrier concentration  (mass action law), the product of carriers is independent of the added impurities. Hence Fermi level of intrinsic semiconductor normally lies very close to the middle of the band gap. . Completely compensated semiconductor behaves as intrinsic.

Q.3.8:- What are donors and accepters? Give two examples of each.

Donors: The pentavalent impurity donates one free electrons to the semiconductor. Such impurities are known as donor or n-type impurities and the semiconductor for containing such as impurities atoms.

Example: phosphorus, arsenic, antimony.

Acceptors: The trivalent impurity acceptor one free electron to the semiconductor such as Germanium or silicon. Such impurities are known acceptor or p-type impurities.

Q.3.9:- Explain the concept of drift current and diffusion current. How are they different.

Drift current:-

 Drift of carriers, under the effect of an applied field, the resulting current is called drift current.

Diffusion current:-

 Diffusion of carriers, under the effect of concentration gradient of dopants present inside the semiconductor, the corresponding current is called diffusion current.

Q.3.10:- What are degenerate semiconductor?

Degenerate semiconductor is a semiconductor with such high level of doping that a material start to act more like a metal than as semiconductor.

Example: Pentavalent has more doping.