21.1.4 Doped Semiconductors

Question 1

Doped Semiconductors

Answer 1

• Doped semiconductors have added impurities to make them act in a predictable and useful manner.
• N-type doping involves the addition of group 15(VA) elements into the matrix of a semiconductor to introduce valence electrons into the conduction band.
• P-type doping involves the addition of group 13(IIIA) elements into the matrix of a semiconductor to remove valence electrons from the valence band.

Question 2

note

Answer 2

• Semiconductors made from pure elements are
  intrinsic semiconductors.
• Impurities can be added into intrinsic semiconductors to change the properties of the semiconductor.
• Such semiconductors are called doped semiconductors.
• In n-type doping, group 15(VA) elements such as phosphorus are added to the semiconductor.
• Group 15(VA) elements have one more valence electron than group 14(IVA) elements such as silicon. The difference in energy between this electron and the conduction band is smaller than the band gap (E g). This electron is therefore likely to jump to the conduction band.
• This moving electron results in a negative charge moving through the silicon matrix. The n in n-type doping stands for negative. The negative charge is analogous to an extra marble being placed on the empty conduction band Chinese checkers board.
• In p-type doping, group 13(IIIA) elements such as boron are added to the semiconductor.
• Group 13(IIIA) elements have one less valence electron than group 14(IVA) elements such as silicon. An electron can therefore move from the valence band to the boron atom.
• This results in a positive charge moving through the silicon matrix. The p in p-type doping stands for positive. The positive charge is analogous to a marble being removed from the valence band Chinese checkers board.

Question 3

A silicon crystal is doped with a small amount of phosphorus. The unbonded electron from phosphorus is free to move about because it moves into an orbital where the charge is _____ .

Answer 3

delocalized

Question 4

Certain types of semiconductors have places in the crystal lattice called “holes.” What is a positive hole in a semiconductor?

Answer 4

The vacancy left by an electron

Question 5

The valence band contains ____ orbitals and the conduction band contains ____ orbitals

Answer 5

bonding, antibonding

Question 6

A certain semiconductor is made by adding a small amount of antimony to a crystal of pure germanium. Which of the following is not a characteristic of this semiconductor?

Answer 6

Extra electrons from antimony occupy energy levels in the conduction band

Question 7

Suppose silicon is doped with boron to make a p-type semiconductor. Why don’t the orbitals of boron blend together into a band the way the orbitals of the silicon atoms do?

Answer 7

The boron atoms are few and far apart

Question 8

As the temperature of a metal is increased, the conductivity decreases because molecular orbitals are perturbed by the vibrational energy of the atoms. Which of the following describes what happens to the conductivity of semiconductors with increasing temperature?

Answer 8

The conductivity increases but then decreases at even higher temperatures.

Question 9

Which of the following results in an n-type semiconductor?

Answer 9

Ge doped with As

Question 10

Which of the following describes an n-type semiconductor?

Answer 10

Conducts electricity by movement of electrons

Question 11

Which of the following results in a p-type semiconductor?

Answer 11

Si doped with Al

Question 12

Which of the following mixtures is least likely to have the characteristics of a semiconductor?

Answer 12

ZnO