SEMICONDUCTOR PPt

1
Review of Semiconductor Physics, PN Junction
Diodes and Resistors

• Semiconductor fundamentals
• Doping
• Pn junction
• The Diode Equation
• Zener diode
• LED
• Resistors

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What Is a Semiconductor?
3
Semiconductors

• A material whose properties are such that it is
  not quite a conductor, not quite an insulator
• Some common semiconductors
• elemental
• Si - Silicon (most common)
• Ge - Germanium
• compound
• GaAs - Gallium arsenide
• GaP - Gallium phosphide
• AlAs - Aluminum arsenide
• AlP - Aluminum phosphide
• InP - Indium Phosphide

4
Crystalline Solids

• In a crystalline solid, the periodic arrangement
  of atoms is repeated over the entire crystal
• Silicon crystal has a diamond lattice

5
Crystalline Nature of Silicon

• Silicon as utilized in integrated circuits is
  crystalline in nature
• As with all crystalline material, silicon
  consists of a repeating basic unit structure
  called a unit cell
• For silicon, the unit cell consists of an atom
  surrounded by four equidistant nearest neighbors
  which lie at the corners of the tetrahedron

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Whats so special about Silicon?

• Cheap and abundant
• Amazing mechanical, chemical and electronic
  properties
• The material is very well-known to mankind
• SiO2 sand, glass

Si is column IV of the periodic table Similar to
the carbon (C) and the germanium (Ge) Has 3s² and
3p² valence electrons
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Nature of Intrinsic Silicon

• Silicon that is free of doping impurities is
  called intrinsic
• Silicon has a valence of 4 and forms covalent
  bonds with four other neighboring silicon atoms

8
Semiconductor Crystalline Structure

• Semiconductors have a regular crystalline
  structure
• for monocrystal, extends through entire structure
• for polycrystal, structure is interrupted at
  irregular boundaries
• Monocrystal has uniform 3-dimensional structure
• Atoms occupy fixed positions relative to one
  another, butare in constant vibration about
  equilibrium

9
Semiconductor Crystalline Structure

• Silicon atoms have 4 electrons in outer shell
• inner electrons are very closely bound to atom
• These electrons are shared with neighbor atoms on
  both sides to fill the shell
• resulting structure is very stable
• electrons are fairly tightly bound
• no loose electrons
• at room temperature, if battery applied, very
  little electric current flows

10
Conduction in Crystal Lattices

• Semiconductors (Si and Ge) have 4 electrons in
  their outer shell
• 2 in the s subshell
• 2 in the p subshell
• As the distance between atoms decreases the
  discrete subshells spread out into bands
• As the distance decreases further, the bands
  overlap and then separate
• the subshell model doesnt hold anymore, and the
  electrons can be thought of as being part of the
  crystal, not part of the atom
• 4 possible electrons in the lower band (valence
  band)
• 4 possible electrons in the upper band
  (conduction band)

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Energy Bands in Semiconductors

• The space between the bands is the energy gap, or
  forbidden band

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Insulators, Semiconductors, and Metals

• This separation of the valence and conduction
  bands determines the electrical properties of the
  material
• Insulators have a large energy gap
• electrons cant jump from valence to conduction
  bands
• no current flows
• Conductors (metals) have a very small (or
  nonexistent) energy gap
• electrons easily jump to conduction bands due to
  thermal excitation
• current flows easily
• Semiconductors have a moderate energy gap
• only a few electrons can jump to the conduction
  band
• leaving holes
• only a little current can flow

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Insulators, Semiconductors, and Metals (continued)
Conduction Band
Valence Band
Conductor
Semiconductor
Insulator
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Hole - Electron Pairs

• Sometimes thermal energy is enough to cause an
  electron to jump from the valence band to the
  conduction band
• produces a hole - electron pair
• Electrons also fall back out of the conduction
  band into the valence band, combining with a hole

pair elimination
pair creation
hole
electron
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Improving Conduction by Doping

• To make semiconductors better conductors, add
  impurities (dopants) to contribute extra
  electrons or extra holes
• elements with 5 outer electrons contribute an
  extra electron to the lattice (donor dopant)
• elements with 3 outer electrons accept an
  electron from the silicon (acceptor dopant)

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Improving Conduction by Doping (cont.)

• Phosphorus and arsenic are donor dopants
• if phosphorus is introduced into the silicon
  lattice, there is an extra electron free to
  move around and contribute to electric current
• very loosely bound to atom and can easily jump to
  conduction band
• produces n type silicon
• sometimes use symbol to indicate heavier
  doping, so n silicon
• phosphorus becomes positive ion after giving up
  electron

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Improving Conduction by Doping (cont.)

• Boron has 3 electrons in its outer shell, so it
  contributes a hole if it displaces a silicon atom
• boron is an acceptor dopant
• yields p type silicon
• boron becomes negative ion after accepting an
  electron

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Epitaxial Growth of Silicon

• Epitaxy grows silicon on top of existing silicon
• uses chemical vapor deposition
• new silicon has same crystal structure as
  original
• Silicon is placed in chamber at high temperature
• 1200 o C (2150 o F)
• Appropriate gases are fed into the chamber
• other gases add impurities to the mix
• Can grow n type, then switch to p type very
  quickly

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Diffusion of Dopants

• It is also possible to introduce dopants into
  silicon by heating them so they diffuse into the
  silicon
• no new silicon is added
• high heat causes diffusion
• Can be done with constant concentration in
  atmosphere
• close to straight line concentration gradient
• Or with constant number of atoms per unit area
• predeposition
• bell-shaped gradient
• Diffusion causes spreading of doped areas

top
side
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Diffusion of Dopants (continued)
Concentration of dopant in surrounding atmosphere
kept constant per unit volume
Dopant deposited on surface - constant amount per
unit area
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Ion Implantation of Dopants

• One way to reduce the spreading found with
  diffusion is to use ion implantation
• also gives better uniformity of dopant
• yields faster devices
• lower temperature process
• Ions are accelerated from 5 Kev to 10 Mev and
  directed at silicon
• higher energy gives greater depth penetration
• total dose is measured by flux
• number of ions per cm2
• typically 1012 per cm2 - 1016 per cm2
• Flux is over entire surface of silicon
• use masks to cover areas where implantation is
  not wanted
• Heat afterward to work into crystal lattice

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Hole and Electron Concentrations

• To produce reasonable levels of conduction
  doesnt require much doping
• silicon has about 5 x 1022 atoms/cm3
• typical dopant levels are about 1015 atoms/cm3
• In undoped (intrinsic) silicon, the number of
  holes and number of free electrons is equal, and
  their product equals a constant
• actually, ni increases with increasing
  temperature
• This equation holds true for doped silicon as
  well, so increasing the number of free electrons
  decreases the number of holes

np ni2
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INTRINSIC (PURE) SILICON

• At 0 Kelvin Silicon density is 510²³
  particles/cm³
• Silicon has 4 valence electrons, it covalently
  bonds with four adjacent atoms in the crystal
  lattice

• Higher temperatures create free charge carriers.
• A hole is created in the absence of an
  electron.
• At 23C there are 10¹º particles/cm³ of free
  carriers

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DOPING
There are two types of doping N-type and P-type.

• The N in N-type stands for negative.
• A column V ion is inserted.
• The extra valence electron is free to move about
  the lattice

• The P in P-type stands for positive.
• A column III ion is inserted.
• Electrons from the surrounding Silicon move to
  fill the hole.

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Energy-band Diagram

• A very important concept in the study of
  semiconductors is the energy-band diagram
• It is used to represent the range of energy a
  valence electron can have
• For semiconductors the electrons can have any one
  value of a continuous range of energy levels
  while they occupy the valence shell of the atom
• That band of energy levels is called the valence
  band
• Within the same valence shell, but at a slightly
  higher energy level, is yet another band of
  continuously variable, allowed energy levels
• This is the conduction band

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Band Gap

• Between the valence and the conduction band is a
  range of energy levels where there are no allowed
  states for an electron
• This is the band gap
• In silicon at room temperature in electron
  volts
• Electron volt is an atomic measurement unit, 1 eV
  energy is necessary to decrease of the potential
  of the electron with 1 V.

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Impurities

• Silicon crystal in pure form is good insulator -
  all electrons are bonded to silicon atom
• Replacement of Si atoms can alter electrical
  properties of semiconductor
• Group number - indicates number of electrons in
  valence level (Si - Group IV)

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Impurities

• Replace Si atom in crystal with Group V atom
• substitution of 5 electrons for 4 electrons in
  outer shell
• extra electron not needed for crystal bonding
  structure
• can move to other areas of semiconductor
• current flows more easily - resistivity decreases
• many extra electrons --gt donor or n-type
  material
• Replace Si atom with Group III atom
• substitution of 3 electrons for 4 electrons
• extra electron now needed for crystal bonding
  structure
• hole created (missing electron)
• hole can move to other areas of semiconductor if
  electrons continually fill holes
• again, current flows more easily - resistivity
  decreases
• electrons needed --gt acceptor or p-type material

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COUNTER DOPING

• Insert more than one type of Ion
• The extra electron and the extra hole cancel out

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A LITTLE MATH

• n number of free electrons
• pnumber of holes
• ninumber of electrons in intrinsic
  silicon10¹º/cm³
• pi-number of holes in intrinsic silicon 10¹º/cm³
• Mobile negative charge -1.610-19 Coulombs
• Mobile positive charge 1.610-19 Coulombs
• At thermal equilibrium (no applied voltage)
  np(ni)2 (room temperature
  approximation)
• The substrate is called n-type when it has more
  than 10¹º free electrons (similar for p-type)

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P-N Junction

• Also known as a diode
• One of the basics of semiconductor technology -
• Created by placing n-type and p-type material in
  close contact
• Diffusion - mobile charges (holes) in p-type
  combine with mobile charges (electrons) in n-type

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P-N Junction

• Region of charges left behind (dopants fixed in
  crystal lattice)
• Group III in p-type (one less proton than Si-
  negative charge)
• Group IV in n-type (one more proton than Si -
  positive charge)
• Region is totally depleted of mobile charges -
  depletion region
• Electric field forms due to fixed charges in the
  depletion region
• Depletion region has high resistance due to lack
  of mobile charges

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THE P-N JUNCTION
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The Junction
?
The potential or voltage across the silicon
changes in the depletion region and goes from
in the n region to in the p region
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Biasing the P-N Diode
THINK OF THE DIODE AS A SWITCH
Forward Bias Applies - voltage to the n region
and voltage to the p region CURRENT!
Reverse Bias Applies voltage to n region and
voltage to p region NO CURRENT
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P-N Junction Reverse Bias

• positive voltage placed on n-type material
• electrons in n-type move closer to positive
  terminal, holes in p-type move closer to negative
  terminal
• width of depletion region increases
• allowed current is essentially zero (small
  drift current)

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P-N Junction Forward Bias

• positive voltage placed on p-type material
• holes in p-type move away from positive terminal,
  electrons in n-type move further from negative
  terminal
• depletion region becomes smaller - resistance of
  device decreases
• voltage increased until critical voltage is
  reached, depletion region disappears, current can
  flow freely

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P-N Junction - V-I characteristics

• Voltage-Current relationship for a p-n junction
  (diode)

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Current-Voltage Characteristics
THE IDEAL DIODE
Positive voltage yields finite current Negative
voltage yields zero current
REAL DIODE
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The Ideal Diode Equation
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Semiconductor diode - opened region

• The p-side is the cathode, the n-side is the
  anode
• The dropped voltage, VD is measured from the
  cathode to the anode
• Opened VD ? VF
• VD VF
• ID circuit limited, in our model the VD cannot
  exceed VF

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Semiconductor diode - cut-off region

• Cut-off 0 lt VD lt VF
• ID ? 0 mA

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Semiconductor diode - closed region

• Closed VF lt VD ? 0
• VD is determined by the circuit, ID 0 mA
• Typical values of VF 0.5 0.7 V

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Zener Effect

• Zener break down VD lt VZ
• VD VZ, ID is determined by the circuit.
• In case of standard diode the typical values of
  the break down voltage VZ of the Zener effect -20
  ... -100 V
• Zener diode
• Utilization of the Zener effect
• Typical break down values of VZ -4.5 ... -15 V

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LED

• Light emitting diode, made from GaAs
• VF1.6 V
• IF gt 6 mA

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Resistor in an Integrated Circuit
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