3.3(b) - Semiconductors

Question 0

What is ‘doping’?

Answer 0

The addition of impurity atoms, such as phosphorous or boron, to a pure semiconductor to decrease its resistance.

Question 1

Give an example of a semiconductor

Answer 1

Silicon doped with phosphorous

Question 2

What is an ‘N-type’ semiconductor? (3)

Answer 2

• This is when a pure semiconductor is doped with an impurity atom that has one more electron in its outer energy level.
• This means that there is a spare electron that is left as a free charge carrier
• The majority of the free charge carriers are now negative, and this is now known as an n-type material.

Question 3

What is a ‘P-type’ semiconductor? (3)

Answer 3

• This is when a pure semiconductor is doped with an impurity atom that has one less electron in its outer energy level.
• This means that there is a missing electron-a hole- and effectively carries a positive charge
• The majority of the free charge carriers are now positive, and this is now known as an p-type material.

Question 4

What is a diode? (2)

Answer 4

• It is a component consisting of a piece of p-type semiconductor in contact with an n-type semiconductor
• Electrons drift from n-type to p-type to fill adjacent holes, creating a depletion layer at the junction with no free charge carriers.

Question 5

Describe the movement of charges in a forward biased p-n junction diode

Answer 5

• When the negative end of the power supply is connected to the n-type, and vice verse, the diode conducts
• Electrons flow into the depletion layer from the n-type into the p-type
• Holes flow into the depletion layer from the p-type into the n-type
• The depletion layer is reduced, and only a very small voltage is required to overcome the voltage barrier.

Question 6

Describe the movement of charges in a reverse biased p-n junction diode

Answer 6

• When the negative end of the power supply is connected to the p-type, and vice verse, the diode does not conduct
• Electrons in the n-type are pulled by the positive terminal
• Holes in the p-type are pulled by the negative terminal
• The depletion layer is widened, and a ‘breakdown’ voltage is required to break the voltage barrier

Question 7

What may happen in the junction region of a forward biases p-n junction diode?

Answer 7

Positive and negative charge carriers may recombine to give quanta of radiation

Question 8

What is a photodiode?

Answer 8

It is a solid state device in which positive and negative charges are produced by the action of light on a p-n junction

Question 9

What is ‘photovoltaic mode’?

Answer 9

This is when a photodiode may be used to supply power to a load

Question 10

What is ‘photoconductive mode’?

Answer 10

This is when a photodiode may be used as a light sensor (requires a power supply)

Question 11

What is the relationship between the e.m.f/ current and irradiance?

Answer 11

The e.m.f is directly proportional to the irradiance (photovoltaic)
The current is directly proportional to the irradiance (photoconductive)

Question 12

What are the 5 parts of a MOSFET?

Answer 12

Substrate = P-type silicon semiconductor
Implants = Grown by diffusion
Source = N-type semiconductor that connects to substrate
Gate = Insulated from substrate by oxide layer
Drain = N-type semiconductor

Question 13

Explain the ‘OFF’ state of an n-channel MOSFET (4)

Answer 13

• No voltage applied to the gate (Less than 2V)
• The junction between the drain and the substrate is reverse-biased
• The reverse bias and the low voltage of the substrate prevent the flow of charge from the source to the drain(OFF)

Question 14

Explain the ‘ON’ state of an n-channel MOSFET (3)

Answer 14

• Positive voltage applied to the gate (Greater than 2V)
• The junction between the drain and the substrate is forward-biased
• The forward bias and the high voltage of the substrate increases the flow of charge, switching MOSFET ON

Question 15

What are two uses of a MOSFET (transistor)?

Answer 15

• An electronic switch

* An amplifier