EEE132 Electronic Devices Prof Syed Idris Syed Hassan Mr Arjuna Marzuki Mrs Norlaili Mohd Noh.
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Transcript of EEE132 Electronic Devices Prof Syed Idris Syed Hassan Mr Arjuna Marzuki Mrs Norlaili Mohd Noh.
EEE132 Electronic Devices
Prof Syed Idris Syed HassanMr Arjuna Marzuki
Mrs Norlaili Mohd Noh
Course outcome 1st topic
To understand the semiconductor materials that are suitable for electronic devices
To study the properties of materials for electronic devices
To understand the biasing of diode Able to design Diode circuit Able to apply some types of diodes
Assessments/Books
3 Tests 15% Assignment/Tutorials 15% Examination 70%
Text books: Electronic devices and circuit theory by Robert L. Boylestad and Louis Nashelsky
Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator.
They are fall into two classes : single crystal and compound
Single crystal e.g Ge and Si Compound e.g GaAs , CdS, GaN and
GaAsPGe-germanium; Si-silicon; Ga-gallium ;As –arsenide; Cd-cadmium; S-sulfide; N-nitride; P-phosphide
Semiconductor Material
Group → 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
↓ Period
11H
2He
23Li
4Be
5B
6C
7N
8O
9F
10Ne
311Na
12Mg
13Al
14Si
15P
16S
17Cl
18Ar
419K
20Ca
21Sc
22Ti
23V
24Cr
25Mn
26Fe
27Co
28Ni
29Cu
30Zn
31Ga
32Ge
33As
34Se
35Br
36Kr
537Rb
38Sr
39Y
40Zr
41Nb
42Mo
43Tc
44Ru
45Rh
46Pd
47Ag
48Cd
49In
50Sn
51Sb
52Te
53I
54Xe
655Cs
56Ba
* 72Hf
73Ta
74W
75Re
76Os
77Ir
78Pt
79Au
80Hg
81Tl
82Pb
83Bi
84Po
85At
86Rn
787Fr
88Ra
** 104Rf
105Db
106Sg
107Bh
108Hs
109Mt
110Ds
111Rg
112Uub
113Uut
114Uuq
115Uup
116Uuh
117Uus
118Uuo
* Lanthanides57La
58Ce
59Pr
60Nd
61Pm
62Sm
63Eu
64Gd
65Tb
66Dy
67Ho
68Er
69Tm
70Yb
71Lu
** Actinides89Ac
90Th
91Pa
92U
93Np
94Pu
95Am
96Cm
97Bk
98Cf
99Es
100Fm
101Md
102No
103Lr
Historical Diode , in 1939 was using Ge Transistor, in 1947 was using Ge In1954 Si was used in Transistor because Si is
less temperature sensitive and abundantly available.
High speed transistor was using GaAs in 1970 (which is 5 times faster compared to Si)
Si, Ge and GaAs are the semiconductor of choice
Atomic structure
+
Valence shell (4 valence electrons)
Valenceelectron
Nucleus
orbitingelectrons
Silicon
shells
+
Valence shell (4 valence electrons)
Valenceelectron
orbitingelectrons
Germanium
14 orbiting electrons (Tetravalent)
32 orbiting electrons (tetravalent)
Atomic structure
Nucleus
shells
+
Valence shell (3 valence electrons)
Valenceelectron
orbitingelectrons
Gallium
Nucleus
shells
+
Valence shell (5 valence electrons)
Valenceelectron
orbitingelectrons
Arsenic
31 orbiting electrons (trivalent)
33 orbiting electrons (pentavalent)
ENERGY LEVELS Each and every isolated electron has
a specific energy level associated with shell and orbiting electron.
The farther an electron from the nucleus, the higher is the energy state.
Electron left its parent atom has a higher energy state than any electron in the atomic structure.
Band Theory
Analogy to atoms–From chemistry, we are familiar with the idea of “electron clouds” orbiting the nucleus. –The energy of the different clouds, or levels, is discrete. Adding energy can cause an electron to “jump” into a higher level. In the same way, an electron can lose energy and emit a specific wavelength of light when falling to a lower energy level. (Atomic spectra)–Pauli Exclusion Principle: no two electrons can occupy the same exact state at the same time. This is why electrons fill the energy levels in the way they do.–Valence electrons are the electrons bound farthest from the nucleus
Covalent bonding
This bonding of atoms, strengthened by the sharing of electrons, is called covalent bonding
Covalent bonding of Si crystalCovalent bonding of GaAs crystal
Exercise 1
Name the materials that suitable for electronic devices
What are the advantages and disadvantages of each materials?
What is a covalent bonding? Describe it.
Free state Energy from external natural cause the
valence electrons to absorb sufficient kinetic energy and break the covalent bond. This is assume to be free state.
Electron that has separated from the fixed lattice structure is called free carriers.
At room temperature, approximately 1.5X 1010 free carriers in 1 cm3 of intrinsic silicon material
*intrinsic means material has been refined to a very low level of impurities.
Table: Intrinsic carriers
Semiconductor Intrinsic carriers/cm3
GaAs 1.7 X 106
Si 1.5 X 1010
Ge 2.5 X 1013
Relative mobility n
Relative mobility is the ability of free carriers to move throughout the material.
Semiconductor n (cm2/V-s)
Si 1500
Ge 3900
GaAs 8500
Effect on temperature
Conductor – increase resistance with increase in heat ( number of carrier do not increase)- is said to have a positive temperature coefficient.
Semiconductor- increase conductivity with increase in heat ( number of carrier increase)- is said to have a negative temperature coefficient.
Band Theory
What is a crystalline solid? –A volume of atoms covalently bonded in a periodic structure with well defined symmetries. –Example: Silicon•Face-Centered Cubic (FCC) structure•Group-IV elements (4 valence electrons)
Where are the electrons?–Covalent bonds share electrons. The e-are delocalized, they can move around the crystal, orbit any atom, as long as there is an open state (cannot violate Pauli Exclusion)–This forms discrete energy bands. Solving Schroedinger’s Equation in the specific periodic structure reveals these bands.
In an atom, electrons orbit in their shell, at a given energy.
•In a crystal, many electrons occupy a small energy band. There is a width to the energy band, which is why Pauli Exclusion is not violated.
•Within the band, electrons can move easily if there are available states, because the difference in energy is tiny.
•Between bands, electrons must get energy from another source, because the band gap can be significant.
Specifics of Crystals
Fermi Energy
The highest energy an electron reached if you were to fill the solid with the intrinsic number of electrons at absolute zero. (No added thermal energy)•Meaningful! There is a sea of electrons sitting beneath this energy.–If you bring two solids together with different Fermi energies, the electrons will move around to reach an equilibrium. (Foreshadowing: PN junction)–If you try to put a lower energy electron into a solid (at absolute zero) with a higher Fermi energy, it won’t fit. It cannot be done due to Pauli Exclusion.•If the highest energy electron exactly fills a band, the Fermi Energy is near the center of the bands.
The level of a single quantum number n and the energies are given as
22
22
2n
mL
hEn
Fermi Energy
22
22
2/2
NmL
hE f
Fermi energy for N particles and the spin is half , thus
h=plank’s constant; n =quantum no. , m mass ;L =side length
Fermi level in three dimension
3/13
N
n f
3/222 3
2
V
N
m
hE f
3/2
2
222
2
22 3
22
N
mL
hn
mL
hE ff
3
3
4
8
12 fnN
2
2
22
2n
mL
hEn
Where is a vector quantum staten
Number of fermions is given by
Therefore
Then the Fermi energy is given by
By replacing L2 with V2/3
V=volume
Typical Fermi Energy of Nucleus
3/1151025.1 AmR
MeV30eV1030
1
1063
26
3/2
3
4322
mm
hE
pf
344
3102.1
3
4 m
R
An
A is a nucleons (proton and neutron)
Number density of nucleons in nucleus is
Fermi Energy of a Nucleus
Beyond 0 K: Fermi-Dirac Statistics
•Fermi Energy: The energy state whose probability of being occupied is exactly 1/2 .•Electrons obey Fermi-Dirac-statistics, which describe the probability of an electron being present in an allowed energy state.•Note that if there are no states at a given energy (i.e., in the band gap) there will be no electrons, even if there is finite probability.
kT
EEF
e
Ef
1
1Fermi function
k=Boltzmann’s constant, T=temp, E =energy , Ef=Fermi energy
Different Types of Solids
Fermi level falls between bands, with a large band gap. SiO2: 9 eV.
>5eV
Fermi level falls between bands, with a small band gap.Si: 1.11 eV, Ge:0.67 eV, GaAs: 1.43
Fermi level falls inside the energy band. Easy for electrons to move around
Fermi level
Transport in Semiconductors
Electrons that get excited into the conduction band carry current.•The space left behind in the valence band is called a hole.•Holes also conduct current. In reality, it’s the movement of all the other electrons. The hole allows this motion. (Bubbles)•Holes can easily travel “up” in energy.•Holes have positive charge.•Current flows in the same direction as the holes move.•Holes have different mass (effective mass) and mobility compared to electrons.
Intrinsic Semiconductor
Fermi Level: All solids are characterized by an energy that describes the highest energy electron at 0K, the level which has1/2 probability of being occupied at finite temperature.•Semiconductors: A solid with its Fermi level exactly between bands, with a band gap small enough to be overcome at room temperature.•Both electrons and holes carry current.
Silicon bond model: electrons and holes
Si is in Column IV of periodic table:
Electronic structure of Si atom:• 10 core electrons (tightly bound)• 4 valence electrons (loosely bound, responsible for mostchemical properties)Other semiconductors:• Ge, C (diamond form), SiGe• GaAs, InP, InGaAs, InGaAsP, ZnSe, CdTe(on average, 4 valence electrons per atom)
Silicon crystal structure
• Silicon is a crystalline material:– long range atomic arrangement• Diamond lattice:– atoms tetrahedrally bonded by sharing valence electrons(covalent bonding)• Each atom shares 8 electrons:– low energy and stable situation• Si atomic density: 5 × 1022 cm−3
Controlling the properties of aSemiconductor
Silicon: 4 valence electrons. Each Si atom bonds to four
others. Doping
•Replace some Si atoms with atoms that do not have four valence electrons. •These atoms will have an extra electron (group IV), or an extra hole (group III).•Doping increases the number of carriers and changes the Fermi level.
Extrinsic materials
A semiconductor material that has been subjected to the doping process is called an extrinsic material.
Type of materials n-type p-type
Phosphorus Doping (n-type)
Phosphorus has 5 valence electrons. •P atoms will sit in the location of a Si atom in the lattice, to avoid breaking symmetry, but each will have an extra electron that does not bond in the same way.•These electrons form their own band. Exactly where depends on the amounts of the two materials.•This new band is located closer to the conduction band, because these extra electrons are easier to excite (and can move around more easily)
Eg. Of n-type material doping
Doping with Sb
Sb - antimony
One electron loosely bound and freely to move in the crystal structure. The atoms (in this case is antimony (Sb)) with five valence electrons are called the donor atoms. Similarly with Phosphorus (P) which also a donor.
Boron Doping (p-type)
Boron has 3 valence electrons.•B will sit at a lattice site, but the adjacent Si atoms lack an electron to fill its shell. This creates a hole.•These holes form their own energy band.•This band is located closer to the valence band, because these extra holes are easy to “excite down” into the valence band.
Eg of p-type material
Boron (B)
•In this case, an insufficient number of electrons to complete the covalent bonds.•The impurities with three valence electrons are called acceptor atoms. E.g of materials are Gallium (Ga), Indium (In)•Void is called hole
Doping
• Doping involves in adding dopant atoms to an intrinsic semiconductor.
• n-type materials: Doping Si with a Group V element, providing extra electrons (n for negative) and moving the Fermi level up.
• p-type materials: Doping Si with a Group III element, providing extra holes (p for positive) and moving the Fermi level down.
Dopant
Group V (n-type) –usually antimony(Sb), arsenic(A), phosphorus(P)
Group III (p-type)-usually Boron(B) , Gallium (Ga) and Indium (In)
Electron versus hole flow
•The valence electron acquires sufficient kinetic energy to break its covalent bond and fills the void created by hole•When the electron move to fill the hole therefore a transfer of holes to the left and electrons to the right •This flow is known as conventional flow.
Majority and Minority Carriers
•N-type material, the electron is called majority carrier and hole the minority carrier•P-type material, the hole is called majority carrier and electron the minority carrier.
Equilibrium Concentrations: electrons
statesofdensityeffective*2
22/3
2
h
kTmN nC
no=equilibrium electron carrier concentration
N(E)=density of states
kTEE
CkTEEC
CCE
oFC
FCeN
e
NEfNdEENEfn
C
/
/1
f(E) =fermi function k=Boltzmann’s const.
Equilibrium Concentrations: holes
statesofdensityeffective*2
2
2/3
2
h
kTmN p
po=equilibrium hole carrier concentration
N(E)=density of states
kTEE
kTEEC
E
oF
FeN
e
NEfNdEENEfp /
/111
f(E) =fermi function k=Boltzmann’s const.
Intrinsic Semiconductors
• In intrinsic semiconductors (no doping) the electron and hole concentrations are equal because carriers are created in pairs
kTEEi
kTEE
CiiFC eNpeNn
//
kTEEkTEE
CooFFC eNeNpn
//
kTEEio
Fienp/
kTEE
ioiFenn/
2//
iiikTE
C
kTEE
C npneNNeNN gC
•This allows us to write
–As the Fermi level moves closer to the conduction [valence] band, the n0[p0] increases exponentially
ni=no. of electrons (intrinsic) pi= no. of holes (intrinsic)
Temperature Dependence of Carrier Concentrations
310 /105.1 cmni
•The intrinsic concentration depends exponentially on temperature. The T3dependence is negligible.•Ionization: only a few donors [acceptors] are ionized. •Extrinsic: All donors [acceptors] are ionized•Intrinsic: As the temperature increases past the point where it is high enough to excite carriers across the full band gap, intrinsic carriers eventually contribute more.•At room temp (300K), the intrinsic carrier concentration of silicon is:
kTEEkTEpno
iFg eemmh
kTn
/2/4/32/3
2**
22
Moving Carriers (i.e., current)
There are two mechanisms by which mobile carriers move in semiconductors –resulting in current flow–Drift•Carrier movement is induced by a force of some type–Diffusion•Carriers move (diffuse) from a place of higher concentration to a place of lower concentration
Drude Model of Conductivity
The current is the charge*number of electrons*area*velocity in a unit of time. For j = current density, divide by the area. The drift velocity (vd) is a function of charge mobility (μn) and electric field (E).•At equilibrium, there is no net motion of charge, vavg= 0.•With an applied electric field, there is a net drift of electrons [holes] against [with] the electric field resulting in an average velocity.•This model allows us to apply Newton’s equations, but with an effective mass. The effective mass takes the interactions with the rest of the solid into account.
•Electrons are assumed to move in a direct path, free of interactions with the lattice or other electrons, until it collides.•This collision abruptly alters its velocity and momentum.•The probability of a collision occurring in time dtis simply dt/, whereis the mean free time is the average amount of time it takes for an electron to collide. nga qnqnJ
DrudeModel
ato maqEF m
qEa
Em
qt ga
*
EpnqJ pn
Em
nqEJ
*
2
qn
*m
qEga
*
2
m
nq
•Consider an electron just after a collision. The velocity it acquires before the next collision will be acceleration*time
•We want the average velocity of all the electrons, which can be obtained by simply averaging the time, which we already know is •We can also write this in terms of mobility:
•Taking both holes and electrons into account, we end up with the following formula for current density due to drift.
Hall Effect
BqF
qn
BJE zxy
Moving electrons experience a force due to a perpendicular B field
yresistivit
qn
1
An electric field develops in response to this force.•The sign of this field perpendicular to the flow of current determines the carrier type. •Density and mobility can also be calculated.
Diffusion
•Diffusion results in a net flux of particles from the region of higher concentration to the region of lower concentration–This flux leads to current (movement of charged particles)–Magnitude of current depends on the gradient of concentration
dx
xdnqDxJ nn
)()(diffusion,
q
kTDD
p
p
n
n
–Dnis the diffusivity coefficient
•Diffusivity is related to mobility by Einstein’s relationship
–Typical values for Si at room temp•Dn= 34 cm2/s and Dp= 13 cm2/s