Download Electrical Properties - Lecture Slides | CHEM 584 and more Study notes Chemistry in PDF only on Docsity!
ISSUES TO ADDRESS...
- How are electrical conductance and resistance characterized?
- What are the physical phenomena that distinguish conductors, semiconductors, and insulators?
- For metals, how is conductivity affected by imperfections, T , and deformation?
- For semiconductors, how is conductivity affected by impurities (doping) and T?
Electrical Properties
Electrical Conduction
- Resistivity, and Conductivity, : geometry-independent forms of Ohm's Law
E : electric field intensity
resistivity (Ohm-m) I/A ≡ J : current density
Resistivity is a material property & is independent of sample
A
I
L
V
- Ohm's Law: V = I R voltage drop (volts = J/C) C = Coulomb
resistance (Ohms) current (amps = C/s) e - I
A (cross sect. area) V L
conductivity ^
1
A
L A
L R
Electrical Properties
Which will conduct more electricity?
Analogous to flow of water in a pipe
So resistance depends on sample geometry, etc.
D
2 D
I
RA VA
Definitions
Further definitions
J = <= another way to state Ohm’s law
J current density
electric field potential = V / or ( V / )
likeaflux surfacearea
current A
I
Current carriers
- electrons in most solids
- ions can also carry (particularly in liquid solutions)
Electron flux conductivity voltage gradient
J = ( V / )
Band Structure
Valence band – filled – highest occupied energy levels Conduction band – empty – lowest unoccupied energy levels
valence band
Conduction band
Adapted from Fig. 18.3, Callister 7e.
Conduction & Electron Transport
- Metals (Conductors): Thermal energy puts many e -^ into accessible higher energy states.
e-
filled band
Energy
partly filled valence band
empty band GAP
filled states
Energy
filled band
filled valence band
empty band
filled states
Energy States: Insulators & Semiconductors
- Insulators: Higher energy states not accessible due to gap (> 2 eV).
valence
Energy
filled band
filled
band
empty band
filled states
GAP
- Semiconductors: Higher energy states separated by smaller gap (< 2 eV).
Energy
filled band
filled valence band
empty band
filled states
GAP
Charge Carriers
Two charge carrying mechanisms
Electron – negative charge Hole – equal & opposite positive charge
Move at different speeds
- electron vs. hole drift velocity
Higher temp. promotes more electrons into the conduction band as T Electrons scattered by impurities, grain boundaries, etc.
Intrinsic Semiconductors
Pure material semiconductors: e.g., silicon & germanium Group IVA materials
- Compound semiconductors III-V compounds - Ex: GaAs & InSb II-VI compounds - Ex: CdS & ZnTe The wider the electronegativity difference between the elements the wider the energy gap.
Conduction: Electron and Hole Migration
- Electrical Conductivity given by:
electrons/m^3
electron mobility: μ (^) e in m 2 /V-s
holes/m 3
hole mobility: μh in m 2 /V-s
electric field electric field electric field
electron hole pair creation
no applied applied
valence electron Si atom
applied
electron hole pair migration
- Intrinsic :
electrons = # holes ( n = p )
case for pure Si - Extrinsic : n ≠ p occurs when impurities are added with a different # valence electrons than the host (e.g., Si atoms)
Intrinsic vs Extrinsic Conduction
n -type Extrinsic: ( n >> p )
no applied electric field
5+
4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+
4+ 4+
Phosphorus atom
valence electron Si atom
conduction electron
hole
n e e
p -type Extrinsic: ( p >> n )
no applied electric field
Boron atom
3+
4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+ 4+
4+ 4+ p^ e h
n-type extrinsic donor: high energy e- high-lying HOMO reductant e.g., P
Intrinsic vs Extrinsic Conduction
p-type extrinisic donor: low energy hole low-lying LUMO oxidant e.g., B
acceptor state h +
Si Oxidation
Doped Semiconductor: Conductivity vs. T
- Data for Doped Silicon: increases w/ doping reason: imperfection sites lower the activation energy to produce mobile electrons.
doped 0.0013at%B
0.0052at%B
electrical conductivity,
(Ohm-m)
50 100 1000
10 -
10 -
10 0
10 1
10 2
10 3
10 4
pure (undoped)
T(K)
- Comparison: intrinsic vs extrinsic conduction... extrinsic doping level: 1021 /m^3 of a n -type donor impurity (such as P). for T < 100 K: "freeze-out“, thermal energy insufficient to excite electrons. for 150 K < T < 450 K: "extrinsic" for T >> 450 K: "intrinsic"
conduction electronconcentration (
21 /m
3 )
0 200400600^ T (K)
0
1
2
3
freeze-outextrinsicintrinsic
doped undoped
Doped Semiconductor: Conductivity vs. T
Number of Charge Carriers
Intrinsic Conductivity
= n | e | e + p | e | e
n
e e n
10 ^6 ( m)^1
(1.6 x 10 ^19 C)(0.85 0.45 m^2 /V s)
For GaAs n = 4.8 x 10^24 m - For Si n = 1.3 x 10^16 m -
- for intrinsic semiconductor n = p
= n | e |( e + n )
Basic Semiconductor Devices
p - n Rectifying Junction: Barrier Potential
p-type
n-type
p-type Depletionn-type zone
Barrier Potential
“Initial”
Real: after e- diffusion
p - n Junction: Band Bending & Barrier Potential
Level bands: NOT in equilibrium Fermi Levels not equal
Band Bending: In equilibrium Fermi Levels equal
Forward Bias in p - n Rectifying Junction
potential drives majority carriers: strong current
p - n Rectifying Junction
p - n Rectifying Junction
p - n Rectifying Junction: LED
p-type
n-type
resistor
light emission
-ve
+ve I
LED from p - n Rectifying Junction
Transistor MOSFET
Transistor MOSFET
Transistor MOSFET
Silicon Purification
