review of semiconductor physics, pn junction diodes and...
TRANSCRIPT
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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?
•Many materials, such as most metals, allow electrical current to
flow through them
•These are known as conductors
•Materials that do not allow electrical current to flow through
them are called insulators
•Pure silicon, the base material of most transistors, is considered
a semiconductor because its conductivity can be modulated by
the introduction of impurities
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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
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Crystalline Solids
� In a crystalline solid, the periodic arrangement of atoms is
repeated over the entire crystal
� Silicon crystal has a diamond lattice
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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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What’s 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
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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, but
are in constant vibration about
equilibrium
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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
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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 doesn’t 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 can’t 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
hole electron
pair creation
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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 doesn’t 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 5*10²³ 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
�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
There are two types of doping
N-type and P-type.
�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.
EG
E eVG ==== 11.
1eV 1.602 10 joule19==== ×××× −−−−
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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 --> “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 --> “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
p=number of holes
ni=number of electrons in intrinsic silicon=10¹º/cm³
pi-number of holes in intrinsic silicon= 10¹º/cm³
Mobile negative charge = -1.6*10-19 Coulombs
Mobile positive charge = 1.6*10-19 Coulombs
At thermal equilibrium (no applied voltage) n*p=(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
Forward BiasForward BiasForward BiasForward Bias
Applies - voltage to the n region and + voltage to the p region
CURRENT!
Reverse BiasReverse BiasReverse BiasReverse Bias
Applies + voltage to n region and –voltage to p region
NO CURRENT
THINK OF THE DIODE AS A SWITCH
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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
I IqV
kT
where
I diode current with reverse bias
q coulomb the electronic ch e
keV
KBoltzmann s cons t
====
−−−−
====
==== ××××
==== ××××
−−−−
−−−−
0
0
19
5
1
1602 10
8 62 10
exp ,
. , arg
. , ' tan
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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 < VD < VF:
ID ≅ 0 mA
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Semiconductor diode - closed region
� Closed: VF < 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 <= 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
– VF=1.6 V
– IF >= 6 mA
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Resistor in an Integrated Circuit
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