Semiconductors

Question 1

What is a semiconductor?

Answer 1

A material whose resistivity is between that of a conductor and insulator.

Question 2

As temperature increases…

Answer 2

…resistivity decreases.

Question 3

What are common types of semiconductor materials?

Answer 3

Silicon and germanium

Question 4

What is valence?

Answer 4

The number of electrons required to fill the outermost shell of an atom.

Question 5

When a valence electron gains energy…

Answer 5

…it becomes a conduction electron and leaves a positive hole behind.

Question 6

What is intrinsic conduction?

Answer 6

Conduction due to electrons and holes in a pure semiconductor material.

Question 7

What is extrinsic conduction?

Answer 7

The inrease in conductivity due to the controlled addition of impurities.

Question 8

How do you increase the conductivity of a semiconductor?

Answer 8

• Increase temperature (eg thermistor)
• Increase the amount of light (eg LDR)
• Add impurities (doping)

Question 9

What is doping?

Answer 9

The addition of controlled amounts of impurity to increase extrinsic conduction.

Question 10

What are the two types of semiconductor achieved from doping?

Answer 10

• n-type semiconductors
• p-type semiconductors

Question 11

Explain how n-type semiconductors work.

Answer 11

In n-type semiconductors, there are more free electrons than in a pure silicon sample. Adding phosphorous (5 valence electrons) results in a spare electron for conduction before any extra energy is required. The majority of charge carriers are electrons.

Question 12

Explain how p-type semiconductors work.

Answer 12

In p-type semiconductors, each bond has an extra positive hole. Boron (3 valence electrons) can be added which leaves a spare positive hole which increases conductivity before extra energy is required. The majority of charge carriers are positive holes.

Question 13

What are p-n diodes?

Answer 13

In a p-n diode, p-type and n-type materials are joined together to form a single semiconductor that allows current to flow in one way only.

Question 14

How do p-n diodes work?

Answer 14

• A p-type and n-type material are joined together
• Free electrons in the n-type move towards the p-type
• Electrons fill the positive holes they reach
• A depletion layer forms, which acts as an insulator
• Since some electrons leave the n-type, a small positive charge forms
• The opposite happens in the p-type
• The accumulation of charge on each side of the depletion layer causes a junction voltage
• The junction voltage for silicon is 0.7V and 0.3V for germanium
• In order for a p-n diode to work, you have to apply a higher voltage than the junction voltage

Question 15

What happens when a p-n diode is connected to a circuit in forward bias?

Answer 15

The positive terminal forces holes into the depletion layer and the negative forces electrons into the depletion layer. This eventually eliminates the layer and current flows.

Question 16

What happens when a p-n diode is connected to a circuit in reverse bias?

Answer 16

The diode will not conduct and the depletion layer will widen. Leakage current exists and eventually it will break.

Question 17

For forward bias:

Answer 17

• Current is measured in milliamps
• No major current flows until the junction voltage is reached
• Once the junction voltage is exceeded, current rises sharply

Question 18

For reverse bias:

Answer 18

• Current is measured in microamps
• There is a small ‘leakage current’ but this is almost negligible
• If the negative voltage is increased too much, ‘breakdown’ occurs
• Once ‘breakdown voltage’ is reached, the diode may conduct too much or (as in most cases) not at all and the diode is destroyed.

Question 19

What can semiconductors be used for?

Answer 19

• Rectifying alternating current
• Integrated circuits
• LEDs

Question 20

How can semiconductors be used to rectify alternating current?

Answer 20

If you put alternating current into the diode, it will eliminate the reverse-biased part and allow only the forward-biased part to flow.

Question 21

How can semiconductors be used in integrated circuits?

Answer 21

Integrated circuits contain transistors, diodes, resistors and capacitors on single pieces of silicon. A single IC can hold as many as 6 million components. They are used for diodes, LEDs, photocells etc.

Question 22

How can semiconductors be used in LEDs?

Answer 22

They are used in lighting displays, decorations, notifications, notification lights and new designs of torches.