semiconductors

Question 1

Electronic Switches

Answer 1

Electronic Switches
 Needed to control the flow of electricity
 Manual switches: Slow ,very limited use
 Electric Relays: slow ,cumbersome only 2 states
 Electric Valves: Fragile, Large & expensive
 All these solutions where bulky, fragile and slow
 The first working transistor was made in 1947

eg - manual switches , electrical relays

Question 2

The Atom

Answer 2

The Atom
 The atom is made of Protons, Neutrons and Electrons.
 Electricity, as the name implies, is the flow of electrons.
 Because electrons move very fast, electricity can be used to
transmit energy and information at very high speeds.
 The flow of electricity is caused by applying a voltage across a
conductor.

Question 3

Atomic Energy Levels

Answer 3

Atomic Energy Levels
* The negatively charged electrons orbit the
positively charged nucleus.
* These are arranged in “bands” or energy levels
of varying distance from the nucleus.
* The greater the distance from the nucleus, the
higher the energy level.
* There is a distance from the nucleus known as
the “conduction band” whereby electrons can
escape from the conduction of the nucleus and
move freely.

Question 4

Conductors

Answer 4

Conductors
 A conductor allows the flow of electrons within its structure.
 This is achieved by the presence of free electrons.
 The free electrons are part of the outer layer (or valance band) of
each atom.
 The outer electrons are free to move because they are more loosely
attached to the rest of the atom.
 A conductor will contain less than 4 electrons in its valence band.
 The valence band overlaps with the conduction band.

Question 5

Insulators

Answer 5

Insulators
 An insulator is simply a material with no free electrons.
 I.e. the electrons in the outer shell are too strongly bonded to the atom so they can not move
freely.
 The energy level of the valence band is not close to the
conduction ban

Question 6

SEMICONDUCTORS

Answer 6

SEMICONDUCTORS
* Pure semiconductor materials have electrical
properties half way between conductors and
insulators.
* Certain pure elements that have 4 valance
electrons in their outer shell can act as
semiconductors. E.g. Silicon, Germanium.
* Also some Compounds can act as
semiconductors, such as Gallium Arsenide which
contains 3 valence electrons.
* The valence shell is close to the conduction band,
but does not overlap as with conductors.

Question 7

Semiconductor Materials

Question 8

DOPING:

Answer 8

DOPING:
*Adding very small amounts of a different a
element with either more, or less electrons in
its outer shell

Question 9

Doping

Answer 9

 An impurity is introduced to the semiconductor
material in order to change its electrical conductivity.
 This process is called doping.
 The semiconductor is modified (or doped) so that
sometime it acts like a conductor, and other times like
an insulator.

Question 10

Doping: N Type Semi-Conductor

Answer 10

Doping: N Type Semi-Conductor
 Adding a small number of atoms of e.g.
Arsenic (As) with five electrons in its
outer shell to pure Silicon with four. We
end up with a spare (donor) electron in
the conduction layer.
 Semiconductors doped with such donor
impurities are referred to as N-Type,
because the spare electron is negatively
charged.

Question 11

N Type

Question 12

Doping: P Type Semi-Conductor

Answer 12

Doping: P Type Semi-Conductor
 If we add a small number of atoms of e.g
Gallium (Ga) with three electrons in its outer
shell to pure Silicon with four. We end up with a
spare (Acceptor) electron hole in the
conduction layer.
 Semiconductors doped with acceptor impurities
are referred to as P-Type, because the spare
electron hole is positively charged.

Question 13

P Type

Question 14

DOPING

Question 15

The Depletion Layer

Answer 15

The Depletion Layer
 So what happens if we attach a piece
of P-Type to a piece of N-Type
semiconductor.
 At the join between the two types of
semi-conductors we get what is
referred to a the depletion layer,
because it is depleted of free charge
carriers.

Question 16

DIODE

Answer 16

DIODE
* N-Type joined to a P-Type doped
semiconductors
* The fundamental property of a
diode is its tendency to conduct
electric current in only one
direction

Question 17

How a Diode Works

Question 18

Diodes and the Depletion Layer

Answer 18

Diodes and the Depletion Layer
 The depletion layer within a diode will
only allow current to pass in one direction
 We have now created something useful
with semi-conductors: A one way
electronic gate
 But this is not a switch!

Question 19

Diodes and the Depletion Layer

Question 20

transistor

Answer 20

• two diodes using three layers of semiconductors
• applying two different power supply with enough voltage a transistor will function as an elector switch
• electron switches extremely fast and small - about 22nm / 50 atoms

Question 21

Transistors

Answer 21

Transistors
 To use semiconductors to create a
switch we now add three layers of
semiconductors together
 E.g. With a Field Effect Transistor (FET)
a voltage applied to the Gate creates a
electrical field within the depletion
layer, allowing electrons to flow from
the Source to the Drain

Question 22

Transistors

Question 23

Integrated Circuits
and CPUs

Answer 23

Integrated Circuits
and CPUs
 Transistors can be made very small
 Transistors can also be created using advanced printing
techniques
 The world record in 2013 was 2.3 Trillion on one 8-Core
Xeon chip.
 Yes that’s 2 300 000 000 on one piece of silicon the size
of your finger nail

Question 24

PHOTODIODE

Answer 24

PHOTODIODE
 A photodiode is a semiconductor device that
converts light into an electrical current. The
current is generated when photons are
absorbed in the photodiode
 The energy of a photon creates an
electron/hole combination (or Charged Couple
)within the depletion layer which allows current
to flow across the terminals of the Photodiode

Question 25

PHOTODIODE

Answer 25

 Very fast switch activated by light

Question 26

Things can get very complicated !
Important concepts you need to know are :

Answer 26

CCD (Charged Couple Device) and CMOS
Sensors
* CCDs are made up of a two dimensional
array of photo diodes that change light
into a grid of different amounts of
electrical charge.
* Each photodiode represent a pixel in the
image, the charge created is proportional
to the amount and of light falling on it
Things can get very complicated !
Important concepts you need to know are :

Question 27

Charged Couple Device

Answer 28

CMOS (complementary metal oxide
semiconductor)
Sensors

 Both CCD and CMOS are semiconductor devices that serve as “electronic
eyes.”
 While they both use photodiodes, they differ in terms of manufacturing
process and signal readout method.
 Although the CCD technology was dominant at first due to superior sensitivity
and picture quality, various improvements in CMOS sensors led them to
surpass CCD sensors from 2004 onwards in shipment volume.
 CMOS are FASTER

Question 29

Shift Registers

Answer 29

each photo sensor represents one pixel a number of pixels is a byte. the register moves the image out one boyte at a time vertically and then horizontally.

because the image is moved out of the ccd in a systematic way by the register, it can be reconstructed byte by byte back into the original image.

• To become a useful image, the charge in each Photo-Diode within the CCD must be moved
  into the main memory of a computer, byte by byte using a shift register.

Question 30

Summary

Answer 30

Summary
 Electrical conduction involves free electrons and electron holes
 We can control the flow of charge carriers by using doped
semiconductors
 Integrated semiconductor devices can be made very small
 Photo diodes are semiconductor switches that react to light
 A grid of photo diodes is called a CCD which can be used to
capture digital images.
 CCD are used in Radiography to capture images