Condensed Matter - Semiconductor

In an n-type semiconductor, which of the following is true of impurity atoms?

1. They accept electrons from the filled valence band into empty energy levels just above the valence band.
2. They accept electrons from the filled valence band into empty energy levels just below the valence band.
3. They accept electrons from the conduction band into empty energy levels just below the conduction band.
4. They donate electrons to the filled valence band from donor levels just above the valence band.
5. They donate electrons to the conduction band from filled donor levels just below the conduction band.

(GR9677 #78)

Solution:

Semiconductor:

n-type:

• n for negative-charge
• atoms have extra electron
• donors to the conduction band

p-type:

• p for positive-charge (holes)
• atoms have missing electrons
• acceptors in the valence band

Source: physics.udel.edu

Answer: E 

Condensed Matter - Conductor and Semiconductor

Which of the following statements concerning the electrical conductivities at a room temperature of a pure copper sample and a pure silicon sample is NOT true? 

1. The conductivity of the copper sample is many orders of magnitude greater than that of silicon sample.
2. If the temperature of the copper sample is increased, its conductivity will decrease.
3. If the temperature of the silicon sample is increased, its conductivity will increase.
4. The addition of an impurity in the copper sample always decreases its conductivity.
5. The addition of an impurity in the silicon sample always decreases its conductivity.

(GR8677 #23)

Solution:

A. TRUE.
Copper → good conductor
Silicon → semi conductor

B. TRUE.
Conductivity is the inverse of resistivity, 
Copper → good conductor → resistivity, → , decreases.

C. TRUE.
Silicon → semi conductor → resistivity, → , exponentially increases

D. TRUE.
For conductor, doping will decrease conductivity.

E. FALSE.
For semiconductor, doping increases the charge carrier density (electron or electron holes) in the conduction band → increasing conductivity

Answer: E

Condensed Matter - Semiconductor

Which of the following best represents the temperature dependence of the resistivity of an undoped semiconductor? 

(GR0177 #53)

Solution:

ρ ~ e^(1/T)

As temperature increases, resistance decreases, in contrast to metals.

Answer: B