atoms - city university londondanny/6_introsc.pdf · • such atoms have three valence electrons,...
TRANSCRIPT
![Page 1: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/1.jpg)
6. Introduction to Semiconductor Devices
1
ATOMS
• All matter is made of atoms, which consist of electrons,
protons and neutrons
• An atom is the smallest particle of an element that
retains characteristics of that element
• There are 109 elements, each of which has a different
atomic structure
• In the classic Bohr model, an atom is visualised as
having a planetary type structure that consists of a
central nucleus, surrounded by orbiting electrons
• Nucleus consists of positively charged particles called
protons, and equal number of uncharged particles
called neutrons
• Electrons have a negative charge
• Each type of atom has a certain number of electrons
and protons that distinguishes it from other atoms of
other elements
• The simplest atom is that
of hydrogen, with one proton
and one orbiting electron
• Helium has two protons,
two neutrons and two electrons
Electron Proton Neutron
![Page 2: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/2.jpg)
6. Introduction to Semiconductor Devices
2
ATOMIC STRUCTURE
• Elements are arranged in the periodic table according
to their atomic number, which is the number of protons
in the nucleus
• Hydrogen has an atomic number of 1, and helium 2
• In their neutral state, all atoms of an element have an
equal number of electrons and protons
• Thus the positive charges cancel out the negative
charges, so an atom is electrically balanced
• Electrons orbit the nucleus of an atom at certain
distances from the centre
• Electrons near the nucleus have less energy than those
in more distant orbits
• Only separate and distinct (discrete) values of electron
energies exist within atomic structures
• Thus electrons orbit at discrete distances from nucleus
• Each discrete orbit corresponds to an energy level
called a shell
• Each shell has a fixed maximum number of electrons at
permissible energy levels (orbits)
• Shells are designated 1, 2, 3 and so on, with 1 nearest
the nucleus
• Maximum number of electrons permitted in each shell
follows 2N2, where N is number of shell
• So first shell has 2 electrons, 2nd has 8, 3rd 18, etc..
![Page 3: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/3.jpg)
6. Introduction to Semiconductor Devices
3
VALENCE ELECTRONS
• Electrons in orbits farthest away from the nucleus have
higher energy, and are less tightly bound to the nucleus
than those closer to it
• This is due to force of attraction between positively
charged nucleus and negatively charged electrons,
which decreases with distance
• Electrons with the highest energy exist in the outermost
shell of an atom called the valence shell
• Electrons in valence shell are called valence electrons
and determine a material’s electrical properties
• If an electron absorbs a photon (particle of
electromagnetic radiation) with sufficient energy, it can
escape the atom and becomes a free electron
• When an atom is left with a net charge (i.e. when there
are an unequal number of electrons and protons), it is
called an ion
• When an electron escapes from a parent atom, the
atom gains a net positive charge as there are now more
protons than electrons than protons – the atoms
becomes a positive ion
• When an atom acquires an electron, it becomes a
negative ion – more electrons than protons
![Page 4: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/4.jpg)
6. Introduction to Semiconductor Devices
4
THE COPPER ATOM
• Copper is the most commonly used metal in electrical
applications
• It has 29 electrons in orbit around nucleus in 4 shells
• In the valence shell, there is only 1 valence electron
• When this valence electron gains sufficient thermal
energy, it can break away from the parent atom and
thus becomes a free electron
• In a piece of copper at room temperature, several of
these free electrons are present, and are free to move
in the copper material
• These free electrons make copper an excellent
conductor and make electrical current possible
![Page 5: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/5.jpg)
6. Introduction to Semiconductor Devices
5
CATEGORIES OF MATERIALS
• Conductors are materials that readily allow current
• They have a large number of free electrons, and are
characterised by one to three valence electrons in their
atomic structure
• Most metals are good conductors
• Silver is the best conductor, followed by copper
• Copper is more widely used as it is less expensive than
silver
• Semiconductors are classed below conductors in their
ability to carry current as they have fewer free electrons
• Semiconductors have four valence electrons, yet
because of their unique characteristics, semiconductor
materials are the basis of electronic devices such as
diodes and transistors
• Silicon and germanium are common semiconductors
• Insulators are poor conductors of electrical current
• They are used to prevent current flow where it is not
wanted
• Insulators have very few free electrons and are
characterised by more than four valence electrons
![Page 6: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/6.jpg)
6. Introduction to Semiconductor Devices
6
ELECTRICAL CHARGE
• The charge on an electron and proton are equal in
magnitude
• Electrical charge exists because of an excess or
deficiency of electrons (Q)
• Static electricity is the presence of a net positive or
negative charge in a material
• Materials with charges of opposite polarity are attracted
to each other, those of the same polarity are repelled
• A force acts between the charges (attraction or
repulsion), and is called an electric field
• Unit of charge is the coulomb, where 1 coulomb is the
total charge possessed by 6.25×10-19 electrons
![Page 7: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/7.jpg)
6. Introduction to Semiconductor Devices
7
POSITIVE AND NEGATIVE CHARGE
• A neutral atom has the same number of electrons and
protons – it has no net charge
• When a valence electron gains enough energy to pull
away from the atom, the atom is left with a net positive
charge (more protons than electrons)
• It thus becomes a positive ion
• If the atom acquires an extra electron, it becomes a
negative ion as there are now more electrons than
protons, and so it has a net negative charge
• The amount of energy required to free a valence
electron is related to the number of electrons in the
outer shell
• The more complete an outer shell, the more stable the
atom and thus the more energy is required to release
an electron
![Page 8: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/8.jpg)
6. Introduction to Semiconductor Devices
8
SILICON AND GERMANIUM ATOMS
• Both silicon and germanium have 4 valence electrons
• Silicon has 14 protons, whereas germanium has 32
• Valence electrons in germanium are in the 4th shell, and
those of silicon are in the 3rd shell
• Germanium valence electrons are at higher energy
levels than those of silicon and thus require a smaller
amount of additional energy to escape from the atom
• This makes germanium more unstable than silicon at
high temperatures, and is the main reason silicon is the
most widely used semiconductive material
![Page 9: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/9.jpg)
6. Introduction to Semiconductor Devices
9
ATOMIC BONDING
• When certain atoms combine into molecules to form a
solid material, they arrange themselves into a fixed
pattern called a crystal
• Atoms within the crystal structure are held together by
covalent bonds
• These are created by the interaction of valence
electrons of each atom
• In a silicon crystal, each atom positions itself with four
adjacent atoms
• A silicon atom with its four valence electrons shares an
electron with each of its four neighbours
• This creates eight valence electrons for each silicon
atom, and hence improves chemical stability
• This sharing produces covalent bonds that hold the
atoms together
• Each shared electron is attracted equally by two
adjacent atoms
• An intrinsic crystal is one without impurities
![Page 10: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/10.jpg)
6. Introduction to Semiconductor Devices
10
CONDUCTION ELECTRONS AND
HOLES
• Here is an energy band diagram for a silicon crystal with only unexcited atoms (no external energy)
• An intrinsic (pure) silicon crystal at room temperature has enough heat energy for some valence electrons to jump the gap from the valence band into the conduction band, becoming free electrons
• When an electron jumps to the conduction band, a vacancy (a hole) is left in the valence band
• For every electron raised to the conduction band by external energy, there is one hole left in the valence band, creating an electron-hole pair
• Recombination occurs when a conduction band electron loses energy and falls back into a hole in the valence band
• Thus a piece of intrinsic silicon at room temperature has a number of free, drifting conduction band electrons, and an equal number of holes in the valence band
![Page 11: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/11.jpg)
6. Introduction to Semiconductor Devices
11
ELECTRON AND HOLE CURRENT
• When a voltage is applied across a piece of silicon, the
free electrons in the conduction band are attracted to
the positive terminal of the voltage source
• The corresponding movement of free electrons is one
type of current in semiconductive material called
electron current
• Another type of current occurs at valence level, where
the holes created by the free electrons exist
• Electrons that remain in the valence band are still
attached to their parent atoms and are not free to move
randomly in the crystal
• Yet a valence electron can move into a nearby hole,
thus leaving another hole where it came from
• The hole has effectively (not physically) moved from
one place to another
• This is called hole current
![Page 12: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/12.jpg)
6. Introduction to Semiconductor Devices
12
N TYPE AND P TYPE
SEMICONDUCTORS
• Semiconductors do not conduct current well, and are of
little use in their intrinsic state
• So intrinsic silicon (or germanium) must be modified by
increasing the free electrons and holes to increase its
conductivity
• This is done by adding impurities to form an extrinsic
semiconductive material
• There are two types of extrinsic semiconductors
• N-type and P-type
• Doping is the process where impurities are added to a
semiconductor to increase its conductivity
![Page 13: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/13.jpg)
6. Introduction to Semiconductor Devices
13
N-TYPE SEMICONDUCTOR
• Pentavalent impurity atoms
are added to intrinsic silicon
to increase the number of
conduction band electrons
• Such atoms have 5 valence
electrons, and are known as
donor atoms – arsenic (As),
Phosphorus (P), antimony (Sb)
• Donor atoms provide an extra electron to the
semiconductor’s crystal structure
• Each pentavalent atom forms covalent bonds with 4
adjacent silicon atoms
• Four of the pentavalent atom’s valence electrons are
used to form the covalent bonds with silicon atoms,
thus leaving one extra electron
• This extra electron becomes a conduction electron as it
is not attached to any atom
• In an n-type (n - negative electron charge) semi -
conductor, most of the current carriers are electrons
• Hence in this case the majority carriers are electrons
• There are a few holes, but in n-type material they are
minority carriers
![Page 14: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/14.jpg)
6. Introduction to Semiconductor Devices
14
P-TYPE SEMICONDUCTOR
• To increase the number of
holes in intrinsic silicon,
trivalent impurity atoms
are added
• Such atoms have three
valence electrons, such
as aluminium (Al), boron
(B), Gallium (Ga), and are
known as acceptor atoms
• Acceptor atoms leave a hole in the semiconductor’s
crystal structure
• Each trivalent atom forms covalent bonds with four
adjacent silicon atoms
• All three of the trivalent valence electrons are used in
the covalent bonds
• Since four electrons are required, a hole is thus formed
with each trivalent atom
• Here most of the current carriers are holes, which can
be thought of as positive charges
• Thus holes are the majority carriers in p-type material,
and electrons are the minority carriers
• Silicon doped with trivalent atoms is a p-type
semiconductor
![Page 15: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/15.jpg)
6. Introduction to Semiconductor Devices
15
DIODES
• If you take a block of silicon and dope one half of it with
a trivalent impurity and the other half with a pentavalent
impurity, the boundary between the two regions is
formed called the pn junction
• A diode consists of an n region and a p region
separated by a pn junction
• The n region has many conduction electrons, and the p
region has many holes
• There is no movement of electrons (current) through a
diode at equilibrium
• A diode has the ability to allow current flow in only one
direction, which is determined by the bias
• Bias refers to the use of a DC voltage to establish
certain operating conditions for a device
• For a diode, there are two bias conditions: forward and
reverse
• These conditions are created by application of a
sufficient external voltage of the proper polarity across
the pn junction
![Page 16: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/16.jpg)
6. Introduction to Semiconductor Devices
16
THE DEPLETION REGION
• With no external voltage, conduction electrons in the n region randomly drift in all directions
• At the instant of junction formation, some of the electrons nearthe junction drift into the p region and recombine with holes close to the junction
• For each electron that crosses the junction and recombines with a hole, a pentavalent atom is left with a net positive charge in the n region near the junction, making it a positive ion
• When an electron recombines with a hole in the p region, a trivalent atom acquires a net negative charge, making it a negative ion
• Due to this recombination process a large number of positive and negative ions build up at the pn junction
• Electrons in the n region must overcome attraction of the positive ions and repulsion of negative ions in order to migrateinto p region
• As the ion layers build up, both sides of the junction become depleted of any conduction electrons or holes, and forms the depletion region
• At equilibrium, the depletion region has widened to a point where no more electrons can cross the pn junction
• The barrier potential is the amount of voltage needed to move electrons through the depletion region (Silicon = 0.7V, germanium = 0.3V
![Page 17: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/17.jpg)
6. Introduction to Semiconductor Devices
17
DIODE: FORWARD BIAS
• Forward bias permits diode current flow
• Negative terminal of DC bias voltage connected to n region, positive terminal to p
• Negative terminal of bias voltage pushes conduction band electrons in the n region toward the pn junction
• Positive terminal pushes holes in the p region also toward the pn junction
• When the bias voltage is greater than the barrier potential, there is enough energy for the n region electrons to penetrate the depletion region, and move through the junction and recombine with p region holes
• As electrons leave the n region, more flow from the negative terminal of the dc voltage source
• Current is thus formed through the n region by the movement of majority electrons to the pn junction
• Once the conduction electrons enter the p region and combine with holes, they become valence electrons
• They move as valence electrons from hole to hole to the positive terminal of the voltage source
• Thus the current in the p region is formed by the movement of holes toward the pn junction
![Page 18: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/18.jpg)
6. Introduction to Semiconductor Devices
18
DIODE: REVERSE BIAS (1)
• Prevents current flow
• Negative terminal of
DC voltage source
connected to p region,
positive to n region
• Negative terminal attracts
holes in p region away from pn junction
• The positive terminal attracts electrons away from the
pn junction
• This causes the depletion region to widen
• More positive ions created in the n region, and more
negative ions created in the p region
• The depletion region widens until the voltage across it
equals the source bias, and at this point the holes and
electrons stop moving away from the pn junction
• When reversed bias, depletion region acts as an
insulator between layers of oppositely charged ions
• The depletion region widens with increased reverse
bias voltage
• Majority current becomes zero with reverse bias
• Small amount of minority current is leaked (nA)
• Some electrons manage to diffuse across the pn
junction
![Page 19: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/19.jpg)
6. Introduction to Semiconductor Devices
19
DIODE: REVERSE BIAS (2)
• As electrons and holes move away from the pn
junction, the depletion region widens
• More positive ions are created in the n region, and
more negative ions in the p region
• Initial flow of majority carriers away from the pn junction
is called transient current and lasts only for a very
short time on application if reverse bias
![Page 20: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/20.jpg)
6. Introduction to Semiconductor Devices
20
REVERSE BREAKDOWN
• If the external reverse bias voltage is increased to a
large enough value, reverse breakdown occurs
• Most diodes normally are not operated in reverse
breakdown
• Diodes can be damaged when reverse breakdown
occurs
• Zener diodes are specifically designed for reverse
breakdown operation
![Page 21: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/21.jpg)
6. Introduction to Semiconductor Devices
21
DIODE SYMBOL
• The arrowhead in the symbol points not in the direction
of electron flow, but in the direction of conventional
current flow
• Metal contacts are connected to each region, anode to
the p region, cathode to the n region
• When the anode is positive with respect to the cathode,
diode is forward biased and current IF
is from cathode
to anode
• When the diode is forward biased, the barrier potential,
VB, always appears between the anode and cathode
• When the anode is negative with respect to the
cathode, diode is reverse biased, and there is no
current flow
VBIAS
+ VB -
R
+ VB -
RVBIAS
IF
I = 0
![Page 22: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/22.jpg)
6. Introduction to Semiconductor Devices
22
IDEAL DIODE MODEL
IF
VR VF
IR
![Page 23: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/23.jpg)
6. Introduction to Semiconductor Devices
23
PRACTICAL DIODE MODEL (1)
• The ideal model on the previous slide neglected the
effect of the barrier potential, the internal resistances
and other parameters
• The practical diode model offers more accuracy
• The forward bias diode is represented as a closed
switch in series with a small battery equal to the barrier
potential, VB
(0.7V for silicon)
• The positive end of the battery is towards the anode
• Remember that the barrier potential is not a voltage
source, and can’t be measured with a voltmeter
• It only has the effect of a battery when forward bias is
applied because the forward bias voltage must
overcome the barrier potential for the diode to conduct
• The reverse biased diode is represented by an open
switch (as ideal case) because the barrier potential
does not affect reverse bias
![Page 24: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/24.jpg)
6. Introduction to Semiconductor Devices
24
PRACTICAL DIODE MODEL (2)
![Page 25: ATOMS - City University Londondanny/6_introsc.pdf · • Such atoms have three valence electrons, such as aluminium (Al), boron (B), Gallium (Ga), and are known as acceptor atoms](https://reader034.vdocuments.site/reader034/viewer/2022042105/5e837877f25e0f4ade7f830b/html5/thumbnails/25.jpg)
6. Introduction to Semiconductor Devices
25
COMPLETE DIODE MODEL
IF
VR VF
IR
0.7V
Slope due to low forward
resistance
Small reverse current due
to high reverse resistance