21.1.3 Intrinsic Semiconductors

Question 1

Intrinsic Semiconductors

Answer 1

• Intrinsic semiconductors are constructed from group 14(IVA) elements.
• The electrical conductivity of the group 14(IVA) elements can be explained by band theory.

Question 2

note

Answer 2

• Group 14(IVA) elements crystallize in the diamond lattice, a face-centered cubic lattice with half of the tetrahedral holes filled.
• Each group 14(IVA) element has four valence electrons. These electrons completely fill the sp 3-bonding orbitals. This is called the valence band.
• The unfilled antibonding orbitals ( *) are called the
  conduction band.
• These two bands are analogous to two Chinese checkers boards—one completely full and one completely empty. There is not space for electrons to move on the bottom board, and there are not any electrons on the top board. This is why diamond is a good electrical insulator.
• The energy gap between the valence band and the conduction band is the band gap (E g ). This gap gets smaller going down the column on the periodic table.
• If E g is small enough, an electron can become excited and jump from the valence band to the conduction band.
• This relocation creates a hole in the valence band, allowing for the movement of electrons. The electron now in the conduction band is also free to move. This freedom to move gives rise to electrical conductivity.
• Elements capable of this transition from insulators to
  conductors are called semiconductors.
• This excitation can be caused by thermal energy or photon absorption.
• This manner of excitation explains why semiconductors conduct electricity better at higher temperatures. As more thermal energy is available, more electrons are able to jump to the conduction band.
• The free electron model cannot explain this change in electrical conductivity.

Question 3

As one moves down the column of Group 14 (4A) elements, the size of the energy gap between the valence band and the conducting band gets

Answer 3

smaller

Question 4

Which of the following does not describe the Group 14 elements?

Answer 4

All of the elements in Group 14 are conductors.

Question 5

What happens to the conductivity of a semiconductor as the temperature increases?

Answer 5

It increases

Question 6

What is a positive hole in a semiconductor?

Answer 6

The vacancy left by an electron

Question 7

Which of the following is TRUE concerning semiconductors at room temperature?

Answer 7

The conduction band is partially filled

Question 8

How do insulators and semiconductors compare?

Answer 8

Semiconductors have partially filled conduction bands, while insulators have completely empty conduction bands

Question 9

Germanium is used in nuclear physics as a radiation detector. The energy gap in germanium is 0.72 eV. Suppose a 600-keV photon passes through a germanium crystal, gradually losing all of its energy by exciting electrons to the conduction band. About how many electrons will be moved to the conduction band?

Answer 9

8.3 × 10^5 electrons

Question 10

What do the bands represent in band theory that describes the behavior of metals and semiconductors?

Answer 10

Energy levels

Question 11

Which one of the following has the largest energy gap between the valence band and the conduction band?

Answer 11

An insulator

Question 12

What type of solid is diamond?

Answer 12

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