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physics h > Semiconductors > Flashcards

Semiconductors Flashcards

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1
Q

What are semiconductors?

A

materials with a resistance somewhere in between the resistance of a conductor and an insulator

A solid in which the gap between the valence band and the conduction band is small

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2
Q

Band gap

A

A range of energies that are NOT ALLOWED

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3
Q

Valence band

A

The highest band filled with electrons in a solid (not in conductors though)

Next highest energy band after conduction band

(There will be filled energy levels below the valence band but these are not important to us)

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4
Q

Conduction band

A

The first unfilled band above the valence band in a solid

Highest energy band

Electrons can move freely here as the band is not full

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5
Q

What are Insulators?

A

A solid in with the valence band is full. Large gap between valence and conduction bands

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6
Q

How do we decrease the resistance in the semiconductor?

A

Heating the semiconductor (giving the electrons enough energy to jump into the conduction band)

Doping the semiconductor with an impurity atom

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7
Q

What are Intrinsic semiconductors

A

(temp up, resistance down eg thermisors)

A ‘pure’ semiconductor material (eg germanium and silicon) with some free electrons in the conduction band at room temperature

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8
Q

What is Doping (extrinsic conductors)?

A

adding an impurity atom into the semiconductor (that has too many or too few (1) outer electrons)

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9
Q

How do intrinsic semiconductors work?

A

-most electrons in the valence band)
-band gap small (e- excited to conduction band when heated)
-charge can flow when e- are in the conduction band

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10
Q

How does Doping work?

A

Doping will either create an extra ‘free’ electron, or a ‘hole’ where an electron will normally be, increasing conductivity

group IV elements can be doped by adding elements from group I or VIII

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11
Q

n type semiconductors

A

‘free’ electron

extra electron free to move in conduction band

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12
Q

p type semiconductors

A

‘hole’ is introduced in valence band

thought of as +
creates a flow of e-
e- can move into hole

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13
Q

How is a p-n junction formed?

A

When a semiconductor contains 2 types of doping which are next to each other, a p-n junction is formed

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14
Q

How can a p-n junction be represented?

A

by considering the energy difference between the bands of the p-type and the n-type

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15
Q

draw a p-n junction diagram

A

(check jotter)

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16
Q

What exists inside the p-n junction

A

an electric field

17
Q

What is biasing

A

In order for the semiconductor to conduct, a voltage is required to allow the electrons and holes to overcome the electric field.
This is knows as biasing

18
Q

What is applied to bias the p-n junction?

A

an external voltage

19
Q

Forward-biased?

A

(jotter for diagram)
Cell connected-positive end to p-type

negative end to n-type

electric field is smaller

20
Q

reverse biased?

A

cell connected-negative end to p-type positive end to n-type

the electric field is bigger

This does not conduct

21
Q

Are LEDs forward or reverse biased?

A

forward

22
Q

How do LEDs work?

A

Voltage applied caused electrons to move away from the n-type conduction band towards the conduction band of p-type

electrons drop from conduction bad to valence brand EITHER side of the junction

Photon emitted when electron drops

(hole moves up)

23
Q

For LEDs what do different band gaps (different materials change)

A

band gap size—>energy lost —>different wavelength of photon —>different colour LED

24
Q

What formulas are used in colour of LEDs questions?

A

E=hf
and maybe v=fxlamda

25
Q

Draw a diagram of an LED

A

Jotter

26
Q

Draw a diagram of a photodiode

A

jotter

27
Q

draw symbol for photodiode

A

jotter

28
Q

What is the photovoltaic effect?

A

At the junction, photons provide energy to raise the electrons from the valence band of the semiconductor to the conduction band

This is known as the photovoltaic effect

29
Q

Draw a diagram of a solar cell

A

jotter

30
Q

What are solar cells

A

p-n junctions that are designed so that a potential difference is produced when photons are absorbed

31
Q

how do solar cells work?

A

The electric field in the p-n junction caused the electrons in the conduction band to move towards the n-type semiconductor and a potential difference is produced across the solar cell

32
Q

What are conductors?

A

A solid which the conduction and valence band overlap, or the valence band is only partially filled

physics h (5 decks)

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