9. Semiconductors

The ability to conduct electricity is directly related to

the availability of and mobility of charge carriers (= electrons)

A large numbers of electrons in metals can easily become excited from the occupied states to the unoccupied states where they are...

free to move between atoms and provide electrical conduction.

The valance band in semiconductors is completely full and there is an energy gap between the valence and conduction band. Electrons can be excited to the conduction band by...

thermal energy (from the surrounding) leading to conductivity.

The energy gap in insulators is too large for a significant number of charge carriers to be excited to the conduction band, leading to...

very low electrical conductivity

Band gap

is the difference in energy between the valence band maximum (EV) and conduction band minimum (EC)

Good electronic conductors:

Cu, Ag, Au, Al, Fe, Pt

Good semiconductors:

Germanium (Ge), Silicon (Si), Copper(I)oxide (Cu2O)

Good insulators: Large band gaps

Diamond

Semiconductor types:

Undoped or intrinsic semiconductors.
Extrinsic semiconductors:
- n-type Donor impurity= pentravalent impurity
- p-type Acceptor impurity =trivalent impurity

Intrinsic semiconductors

- Charges move in both bands (Conduction Band: electrons; Valence Band: holes)
- Number of free electrons in CB = holes in VB
- Holes moves like positive charges
- Electrons have higher mobility ⇒ responds easier to a potential/voltage gradient; do not have to break any chemical bonds

The Fermi level is defined as

that hypothetical energy level where the probability is ½ that a level is occupied by an electron.

What is recombination of holes and electrons?

When the electron in the conduction band surpasses the energy gap and reaches the valence band, it becomes stable and occupies the position of a hole in the valence band. With this action, the electron and hole disappear

The electrical conductivity of intrinsic semiconductors can be changed by

doping resulting in extrinsic semiconductors

n-type extrinsically doped Si:

- Arsenic is an electron donor and has one additional valence electron compared to silicon.
- When arsenic (As) displaces a silicon atom in the crystal matrix, there is an extra electron that is easily excited or donated to the conduction band.

What does the extra electrons in n-type extrinsically doped Si serve as?

The extra electrons serve as charge carriers and lead to extrinsic conductivity = conductivity caused by doping.

p-type extrinsically doped Si:

- Boron is an electron acceptor and contains one less electron compared to silicon.
- When boron (B) displaces a silicon atom in the crystal matrix, it accepts an electron from the valence band, thus creating a hole in the VB.

What does p-type extrinsically doped Si result in?

Results also in extrinsic conductivity = conductivity caused by doping.

n-type doping:

The thermal energy at room temperature is sufficient to excite essentially all electrons from the dopant atoms to the conduction band.

p-type doping:

The thermal energy at room temperature is sufficient to permit all of the dopant atoms to capture an electron from the valence band.

n-type carriers

-Electrons majority carriers
-Holes minority carriers

p-type carriers

- Holes majority carriers
- Electrons minority carriers

After charge transfer from the n-type semiconductor to the electrolyte

Electrons move from the semiconductor to the redox couple until their Fermi levels are equal and a region depleted from electrons are formed at the semiconductor interface.

Band bending at the semiconductor:

The excess positive charge is not located at the
surface, as in metals. Instead is distributed in a space-charge region.
This charge distribution is analogous to that found
in the diffuse double layer that forms in solution