The Electron

By a Gentleman

Insulators and Conductors

Conduction

All conduction is due to the movement of free electrons.

+ -

I’m free

In a Semiconductor the electrons are fixed until they receive a little energy

The Silicon, Si, Atom

Silicon has a valency of 4 i.e. 4 electrons in its outer shell

Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms

These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms

This picture shows the shared electrons

Intrinsic Semiconductors Conduction half way between a

conductor and an insulator Crystals of Silica

I’m free

A photon releases an electron that now can carry current

Intrinsic Semiconductors

A photon releases an electron that now can carry current

Heating Silicon

We have seen that, in silicon, heat releases electrons from their bonds…

This creates electron-hole pairs which are then available for conduction

Intrinsic Conduction

If more heat is applies the process continues…

Slide 8

More heat…

More current…

Less resistance…

The silicon is acting as a thermistor

Its resistance decreases with temperature

The Thermistor Thermistors are used to

measure temperature

They are used to turn devices on, or off, as temperature changes

They are also used in fire-warning or frost-warning circuits

Thermistor Symbol

Light Dependent Resistor (LDR) The LDR is very similar to

the thermistor – but uses light energy instead of heat energy

When dark its resistance is high

As light falls on it, the energy releases electron-hole pairs

They are then free for conduction

Thus, its resistance is reduced

LDR Symbol

Two semiconductor devices

2) Thermistor – resistance DECREASES when temperature INCREASES

1) Light dependant resistor – resistance DECREASES when light intensity INCREASES

Resistance

Amount of light

Resistance

Temperature

THE VARIATION OF THE RESISTANCE OF A THERMISTOR WITH TEMPERATURE

Thermistor

Digitalthermometer

10°C

Water

Heat source

Ω

Glycerol

Method1. Set up the apparatus as shown.2. Use the thermometer to note the

temperature of the glycerol and thermistor.3. Record the resistance of the

thermistor using the ohmmeter.4. Heat the beaker.5. For each 10 C rise in temperature,

record the resistance and the temperature using the ohmmeter and the thermometer.

6. Plot a graph of resistance against temperature and join the points in a smooth, continuous curve.

Precautions

Heat the water slowly so temperature does not rise at end of experiment

Wait until glycerol is the same temperature as water before taking a reading.

Extrinsic Semiconductors Doping is adding an element of different

valency to increase conductivity of semiconductor

Extrinsic Semiconductors P-type have more holes (Add

Group3)

The Boron Atom

Boron is number 5 in the periodic table

It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell

Extrinsic Semiconductors N-type have more electrons (Add

Group5)

The Phosphorus Atom

Phosphorus is number 15 in the periodic table

It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell

Extrinsic Conduction – p-type silicon

A current will flow – this time carried by positive holes

Note:

The positive holes move towards the negative terminal

Junction Diode

Two types grown on the same crystal

P-type N-type

Junction Diode

Near the junction some electrons from the ‘N’ fill the holes in the ‘P’ crystal.

N-typeP-type

Junction Diode

This creates area in the middle where there are no carriers so no conduction

P-type N-type

This barrier is called the DEPLETION LAYER

Junction Diode

When the diode is in FORWARD BIAS the depletion layer disappears. The diode conducts.

P-type N-type+ -

Junction Diode

When the diode is in REVERSE BIAS the depletion layer increases. The diode acts as a barrier or insulator.

P-type

N-type

- +

Homework

2004 HL Q12(d)

The p-n Junction – no potential

As the p-type has gained electrons – it is left with an overall negative charge…

As the n-type has lost electrons – it is left with an overall positive charge…

Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V

0.6 V

The Reverse Biased P-N Junction

Take a p-n junction

Apply a voltage across it with the

p-type negative

n-type positive

Close the switch

The voltage sets up an electric field throughout the junction The junction is said to be reverse – biased

The Reverse Biased P-N Junction

Negative electrons in the n-type feel an attractive force which pulls them away from the depletion layer

Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer

Thus, the depletion layer ( INSULATOR ) is widened and no current flows through thep-n junction

The Forward Biased P-N Junction

Take a p-n junction

Apply a voltage across it with the

p-type postitive

n-type negative

Close the switch

The voltage sets up an electric field throughout the junction

The junction is said to be forward – biased

The Forward Biased P-N Junction

Negative electrons in the n-type feel a repulsive force which pushes them into the depletion layer

Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer

Therefore, the depletion layer is eliminated and a current flows through the p-n junction

The Forward Biased P-N Junction

At the junction electrons fill holes

They are replenished by the external cell and current flows

Both disappear as they are no longer free for conduction

This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V

The Forward Biased P-N Junction

If we apply a higher voltage…

The electrons feel a greater force and move faster

The current will be greater and will look like

The p-n junction is called a DIODE and is represented by the symbol…

The arrow shows the direction in which it conducts current

this….

Diode as Valve

Only allows current in one direction

Forward Bias Reverse Bias

LED An LED (Light Emitting Diode) works in the

same way. We use it for pin lights.

Forward Bias Reverse Bias

Characteristic Curve - Diode

V/v

I/A

Junction Emf (0.6V)Must be Overcome

before Conduction starts

In reverse

BiasNo

conduction

VARIATION OF CURRENT (I) WITH P.D. (V)

mA

V

+6 V

-

Diode in forward bias

VARIATION OF CURRENT (I) WITH P.D. (V)

+6 V

-

Diode in Reverse bias V

A

Rectifier

Uses this to turn AC to DC

This is called half wave rectification

Mains

Resistor

Rectifier

We use a capacitor to smooth the signal to get something more like DC

Amplification

On 16 December 1947 William Shockley, John Bardeen and Walter Brattain built the first practical transistor at Bell Labs

Despite hardly talking to each other.

Transistors Small changes in the input signal

greatly changes the size of the depletion layer

10mA

3A1A

30mA

The current increases if the D.P. is small

Signal Amplification So small changes in input signal

create large charges in output.

Thermionic Emission

Electrons leaving the surface of a hot metal

Hot Metal

e-e-

e- e- e-

Cathode Rays (Really Electrons) First we heat the cathode to make the

electrons jump off by Thermionic Emission

CATHODE

e-e-

We can use a high voltage to accelerate the electrons to form a stream

ANODE

High Voltage

Electron Energy Units We calculate the energy of each

electron first in electron volts. The energy gained when an electron crosses a potential difference of 1Volt.

CATHODE

e-e-

Energy Gained = 1 eV

ANODE

1v

Electron Energy We calculate the energy of each

electron first in electron volts

CATHODE

e-e-

Energy Gained = 2000eV

ANODE

2000v

Electron Energy Then we convert this to joules ( Charge on the

electron = e = 1.6x10-19 C)

CATHODE

e-e-

Energy Gained = e.V = 1.6x10-19 . 2000

= 3.2x10-16 JoulesANODE

2000v

Electron Velocity All the energy on an electron must be kinetic

energy.

CATHODE

e-e-

Energy Gained = 3.2x10-16 = 0.5mv2

electron mass = 9.1 × 10-31 kg

ANODE

2000v

Electron Velocity

CATHODE

e-e-

Energy Gained = 3.2x10-16 = 0.5mv2

electron mass = 9.1 × 10-31 kg

3.2x10-16 = 0.5 (9.1 × 10-31) v2

V2=7x1015

V= 2.6x107 m/s

ANODE

2000v

CRT and Demo

H/W

2005 OL Q10

X-Rays

Electrons jump from the surface of a hot metal –

Thermionic Emission

Accelerated by high voltage they smash into tungsten

Most of the electron energy is lost as heat.-about 90%

X-rays very penetrating, fog film, not effected by fields.

High Tension Voltage

Photons Bohr first suggested a model for

the atom based on many orbits at different energy levels

E1E2

Photons

If the electron in E1 is excited it can only jump to E2.

E1E2

Photons Then the electron falls back.

The gap is fixed so the energy it gives out is always the same

E1

E2

A small amount of energy in the form of an e-m wave is produced

Photons So Max Planck said all energy must

come in these packets called photons. He came up with a formula for the

frequency

E1E2

E2 –E1 = h.f

Where f=frequency

h= Planck’s constant

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Now show them the spectra of different lights using linear disperser

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Demo Light Emission

Albert Einstein Uncle Albert was already a

published scientist but the relativity stuff had not set the world alight.

He set his career in real motion when he solved a problem and started the science of Quantum Mechanics that the old world Jew in him could never come to terms with.

The Problem If you shine light on the surface

of metals electrons jump off

Polished Sodium Metal

e

e

e

e

e

• Electrons emitted• This is The PHOTOELECTRIC

EFFECT

A charged Zinc plate is attached to an Electroscope

When a U.V. lamp is shone on the plate the leaf collapses as all the electrons leave the surface of the zinc

We can also prove this with the experiment below

The Photoelectric EffectThe more intensity you gave it the

more electrical current was produced

Current

(# of electrons)

Light Intensity

(# of photons)

The Photoelectric EffectHowever something strange

happened when you looked at frequency

Frequency of light

Electron Energy

Newtonian Physics could not explain this

So we define the Photoelectric effect as:-

Electrons being ejected from the surface of a metal by incident light of a suitable frequency.

Albert used Planck’s theory that as energy came in packets each packet gives energy to 1 electron only

A small packet would not give the electron enough energy to leave

Low frequency light had too small a parcel of energy to get the electron free.

Energy of each photon = h.f

Einstein’s Law

Frequency of light

Electron Energy

f0=Threshold Frequency

Energy of incident photon =

h.f = h. f0+ KE of electron

Work Function,Energy to release Electron

Energy left over

turnedinto

velocity

Einstein's Explanation

Waves come in packets called photons Energy of a photon only depends on it’s

frequency One photon gives all it’s energy to one

electron If the energy is greater than the work

function the electron escapes Incident Frequency must be above a

threshold

H/W

2003 HL Q 9 2005 HL 12(d)