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LC Physics > Semiconductors > Flashcards

Semiconductors Flashcards

1
Q

Insulators explanation

A
  • Have tightly bound electrons in outer shell, require large amount of energy to free them for conduction
  • If we apply a potential difference, force on each electron is not enough to free it from its orbit + insulator does not conduct
  • have high resistivity/resistance
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2
Q

Insulator definition

A

a substance or device which does not readily conduct electricity.

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3
Q

Conductors explanation

A
  • have loosely bound electrons in their outer shell
  • electrons require small amount of energy to free them for conduction
  • if we apply a p.d, force on each electron is enough to free it from its orbit + can jump from atom to atom and the conductor conducts
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4
Q

Conductors definition

A

A material through which electric current can pass

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5
Q

Semiconductors explanation

A
  • have high resistivity/resistance between that of conductors and insulators
  • electrons not free ot move but little energy will free them for conduction
  • two most common semiconductors are silicon and germanium
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6
Q

Semiconductors definition

A

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

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7
Q

Silicon atom (Si)

A

-valency of 4
-if heat applied,
more heat - more current - less resistance + acts as a thermistor, its resistance decreases with temperature

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8
Q

Intrinsic conduction

A
  • a pure semiconductor is called an intrinsic conductor
  • conduction in a pure semiconductor due to electrons moving from negative to positive and an equal no. of holes moving in opposite direction
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9
Q

The thermistor

A
  • a heat sensitive resistor
  • when cold, behaves as insulator ie. v high resistance
  • when heated, electron-hole pairs released + available for conduction, resistance reduced
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10
Q

Uses for thermistors

A
  • measure temp
  • turn devices on or off, as temp changes
  • used in fire-warning or frost-warning circuits
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11
Q

the Light dependent Resistor (LDR)

A

v similar to thermistor - but uses light energy instead of heat energy

  • when dark, resistance high
  • as light falls on it, energy releases electron-hole pairs. Thenfree for conduction, resistance reduced
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12
Q

uses for LDRs

A
  • used as light meters
  • control automatic lighting
  • used where light is needed to control a circuit eg. light operated burglar alarm
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13
Q

N-type silicon

A
  • majority of charge carriers are negative electrons
  • small no. of minority charge carriers - holes - will exist due to electrons-hole pairs being created in silicon atoms due to heat
  • silicon is still electrically neutral as the no. of protons is equal to no. of electrons
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14
Q

the boron atom

A
  • no.5 in periodic table

- 3 electrons in outer shell. 5 protons + 5 electrons

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15
Q

doping: making p-type silicon

A
  • remove silicon atom from crystal lattice
  • replace with boron am
  • we have a hole in a bond, this hole is free for conduction
  • remove another silicon atom + replace w/ another boron atom
  • as more holes are available for conduction, we have increased the conductivity of the material
  • Boron is the dopant in this case
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16
Q

extrinsic conduction: p-type silicon

A
  • apply a p.d. across the silicon

- a current will flow - this time carried by positive holes The positive holes move towards the negative terminal

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17
Q

p-type silicon

A
  • majority of charge carriers are positive holes
  • small no. of minority charge carriers - electrons - will exist due to electrons-hole pairs being created in silicon atoms due to heat
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18
Q

p-n junction

A
  • when initially joined electrons from n-type migrate into the p-type, less electron density there
  • when electrons fill a hole, electron + hole disappear as gap in bond is filled
  • leaves region w/ no free charge carriers - the depletion layer - this acts as an insulator
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19
Q

the phosphorous atom

A

number 15 on periodic table

15 protons, 15 electrons, 5 electrons in outermost shell

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20
Q

doping: making n-type silicon

A
  • Relying on heat or light for conduction does not make for reliable electronics
  • suppose we remove a silicon atom from crystal lattice + replace w/ a phosphorous atom
  • we not have an electron that is not bonded - thus, free for conduction
  • Remove another silicon atom, replace with phosphorous atom
  • More electrons available for conduction, increased conductivity of material
  • Phosphorous is the dopant
  • Apply p.d. across silicon, a current will flow. Negative electrons move towards positive terminal
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21
Q

Intrinsic conduction definition

A
  • conduction through a pure sample of a semiconductor

- equal no. of electrons and positive holes

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22
Q

extrinsic conduction definition

A
  • conduction through a semiconducting material that contains impurities and is either n-type or p-type in nature
  • unequal number of electrons and holes
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23
Q

how a depletion layer is formed

A
  • Free electrons in n-type material wander into p-type, as mobile electron concentration higher in n-type.
  • Here, they meet holes. When electron meets hole, it fills hole. Both are no longer available for conduction
  • Similarly, holes in p-type wander in to the n-type.
  • A region forms near junction containing virtually no free majority charge carriers, called depletion layer.
  • acts as an insulator.
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24
Q

depletion layer definition

A

a depletion later is the region at both sides of a p-n junction that contains no free majority charge carriers. It behaves as an insulator.

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25
Q

how to prevent damage to diode

A

there should be a protective resistor in series to prevent damage by a large current

26
Q

what a rectifier does

A

changes alternating current to direct current

27
Q

why is a diode useful as a rectifier

A

Diode only carries current in one direction

  • makes it useful as a rectifier
  • in forward bias, a current flows but in reverse bias, current is blocked
28
Q

n-type silicon

A
  • type of silicon
  • majority of charge carriers are negative electrons
  • small no. of minority charge carriers - holes - will exist due to electrons-hole pairs being created in silicon atoms due to heat
  • silicon still electrically neutral as no. of protons equal to no. of electrons
29
Q

doping

A

addition of small controlled amounts of impurities

30
Q

types of extrinsic conduction

A

can be p-type (extra positive holes) or n-type (extra negative electrons) conductors

31
Q

making n-type silicon simplified

A

remove silicon, add phosphorous

phosphorous is the dopant

32
Q

p-n junction

A
  • when initially joined electrons from n-type migrate into p-type - less electron density there
  • when an electron fills a hole, both disappear as gap in bond is filled
  • leaves region with no free charge carriers - the depletion later - this layer acts as an insulator
33
Q

the p-n junction

A
  • take a p-n junction
  • apply a voltage across it with p-type negative, n0type positive
  • close switch
  • voltage sets up electric field throughout junction
  • junction said to be reverse-biased/forward-biased
34
Q

reverse-biased p-n junction

A
  • negative electrons in n-type feel attractive force pulling them away from depletion layer
  • positive holes in p-type also experience this
  • thus, depletion layer is widened + no current flows thru p-n junction
35
Q

forward-biased p-n junction + why

A
  • negative electrons in n-type feel repulsive force which pushes them into depletion layer
  • positive holes in p-type experience this force
  • depletion layer is eliminated and current flows through p-n junction
  • at junction, electrons fill holes
  • both disappear as they are no longer free for conduction
  • replenished by the external cell and current flows
  • continues as long as external voltage greater than junction voltage
36
Q

what a p-n junction is called

A

diode

37
Q

applying a higher voltage to p-n junction

A
  • electrons feel a greater force, and move faster

- current will be greater

38
Q

symbol of diode

A

in hardback

39
Q

conducting currents - forward biased and reverse biased p-n junction

A

forward-biased junction conducts current

reverse-biased junction does not conduct current

40
Q

the semiconductor diode

A
  • a p-n junction
  • reverse biased: does not conduct
  • forward-biased: conducts as long as external voltage is greater than junction voltage
  • should always have a protective resistor in series, can be damaged by a large current
41
Q

how should semiconductor diode be connected

A
  • silver line drawn on one side represents the line in its symbol
  • thus should be connected to negative terminal for diode to be forward biased
42
Q

uses of diodes

A
  • change a.c. to d.c.

- prevent damage in a circuit by connecting a battery or power supply the wrong way around

43
Q

light emitting diode (led)

A
  • some diodes emit light as they conduct, called lEds, come in various colours
  • have one leg longer than the other
  • longer leg should be connected to positive terminal for LED to be forward biased
44
Q

uses of LEDs

A

-often used as power indicators on radios, TVs, other electronic devices

45
Q

characteristic curve of a diode

A
  • diodes do not obey Ohm’s law
  • graph of Current vs Voltage will not be a straight line through origin
  • increases dramatically once voltage reaches 0.6V approx (ie the junction voltage [for silicon]?)
  • curve is known as the Characteristic Curve of a Diode
46
Q

how to form a depletion layer in p-n junction

A

free electrons in the n-type material

47
Q

explain how a pure semiconductor can be converted into a p-type semiconductor

A

-doped with a group III element / boron

48
Q

explain how a pure semiconductor can be converted into an n-type semiconductor

A

-doped with a group V element / phosphorous

49
Q

describe, with a labelled diagram, how a depletion layer is formed at the p-n junction

A
  • p-type material connected to n-type material
  • electrons move from n-type to p-type

diagram

50
Q

indicate on your p-n junction diagram the sections that are positively charged, negatively charged, and neutral

A

positively charged: in n-type side of layer

negatively charged: in p-type side of layer

neutral: on both sides of depletion layer

51
Q

explain how a diode may be protected from having too large a current flowing through it when connected across a battery

A

resistor in series

52
Q

what would be the effect on current flowing in diode if terminals of battery were reversed? + why

A

small/zero current

bc diodie in reverse bias + larger resistance/depletion layer

53
Q

function of a rectifier

A

coverts a.c. to d.c.

54
Q

what property of a diode makes it useful in a rectifier circuit?

A

allows current to flow in one direction only

55
Q

explain how the presence of phosphorous and boron makes silicon a better conductor

A
  • more electrons available as charge carriers when phosphorous is added
  • more positive holes as charge carriers when boron is added
56
Q

what happens at the boundary of two adjacent layers

A

-electrons move from n-type to p-type region and depletion layer is formed

57
Q

describe what happens at the boundary when the semiconductor diode is

(i) forward biased
(ii) reverse biased

A

(i) width of depletion reduced/diode conducts

(ii) width of depletion layer increased/no conduction

58
Q

use of a semiconductor diode

A

rectifier

59
Q

What is a p-n junction

A

where a p-type semiconductor and an n-type semiconductor meet

60
Q

why is silicon a semiconductor

A

-it has a resistivity/resistance/conductivity beween that of a conductor and an insulator

61
Q

learn circuit diagrams

A

eg. the side diode is in forward/reverse bias

LC Physics (25 decks)

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