Upload
maharjansumir
View
257
Download
4
Embed Size (px)
Citation preview
Chapter 18 - 1
ISSUES TO ADDRESS...• How are electrical conductivity and resistivity characterized?
• What electronic structure distinguishesconductors, semiconductors, and insulators?
• How is conductivity in metals affected byimpurities, temperature, and deformation?
• How is conductivity in semiconductors affectedby impurities (doping) and temperature?
Chapter 18: Electrical Properties II: Intrinsic & Extrinsic Semiconductors
Class 21 – Chapter 18: Conductivity II – Intrinsic & Extrinsic Semic - Topic #7.2
Chapter 18 -
Intrinsic Elemental vs. Compound Semiconductors
Elemental Semiconductors
Chapter 18 -
Conductivity in Semiconductors
Chapter 18 -
Conduction in Terms of Electron & Hole Migration
Adapted from Fig. 18.11, Callister 7e.
electric field electric field electric field
• Electrical Conductivity given by:
# electrons/m3 electron mobility
# holes/m 3
hole mobilityhe epen
• Concept of electrons and holes:
+-
electron holepair creation
+-
no applied applied
valence electron Si atom
applied
electron holepair migration
Chapter 18 -
Intrinsic Semiconductor Conductivity Increases with T
Chapter 18 - 6
Intrinsic Semiconductors: Conductivity vs Tand Also Plotted as ln Conductivity vs. 1/T
• Data for Pure Silicon:-- increases with T-- opposite to metals
materialSiGeGaPCdS
band gap (eV)1.110.672.252.40
Selected values from Table 18.3, Callister & Rethwisch 8e.
ni eEgap / kT
ni e e h
• Data for Pure Silicon:-- ln decreases vs. 1/T
Chapter 18 - 7
• Intrinsic:-- case for pure Si-- # electrons = # holes (n = p)
• Extrinsic:-- electrical behavior is determined by presence of impurities
that introduce excess electrons or holes-- n ≠ p
Intrinsic vs Extrinsic Conduction
3+
• p-type Extrinsic: (p >> n)
no applied electric field
Boron atom
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+ hep
hole
• 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
conductionelectron
een
Adapted from Figs. 18.12(a) & 18.14(a), Callister & Rethwisch 8e.
Chapter 18 -
What Elements Make Si an n-type Extrinsic Semiconductor?
• Majority carriers are electrons• Minority carriers are electron holes
Donor energy level
Chapter 18 -
How Does T Affect an n-type Semiconductor?
e e e e
Chapter 18 -
What Elements Make Si a p-type Extrinsic Semiconductor?
• Majority carriers are electron holes• Minority carriers are electrons
Chapter 18 -
Activity - How Does T Affect a p-type Semiconductor?
Chapter 18 -
Activity – Electrical Property Concept Questions1. What will increase the conductivity of polycrystalline silicon?
1. decreasing temperature 2. increasing the grain size3. decreasing the grain size
2. Will adding 1% As to silicon increase or decrease its conductivity? 1. Increase because the As will contribute electrons 2. Decrease because As scatters electrons3. Increase because arsenic is a better conductor than silicon 4. Decrease because arsenic is a worse conductor than silicon5. Decrease because As takes more thermal energy for the electrons to move
3. Electrical conductivity will decrease when : 1. a metal has impurities added2. a semiconductor has impurities added3. a metal has its temperature increased4. a semiconductor has its temperature increased5. a metal is annealed
4. The addition of boron (III) to silicon (IV): 1. provides donors2. gives electron hole minority carriers3. makes it n-type4. makes it p-type5. reduces the energy gap
Chapter 18 -
Summary of Band Gaps in Intrinsic vs Extrinsic Semic.
Chapter 18 -14
Extrinsic Semiconductors: Conductivity vs. Temperature
• Data for Doped Silicon:-- increases doping-- reason: imperfection sites
lower the activation energy toproduce mobile electrons.
• Comparison: intrinsic vsextrinsic conduction...
-- extrinsic doping level:1021/m3 of a n-type donorimpurity (such as P).
-- for T < 100 K: "freeze-out“,thermal energy insufficient toexcite electrons.
-- for 150 K < T < 450 K: "extrinsic"-- for T >> 450 K: "intrinsic"
Adapted from Fig. 18.17, Callister & Rethwisch8e. (Fig. 18.17 from S.M. Sze, Semiconductor Devices, Physics, and Technology, Bell Telephone Laboratories, Inc., 1985.)
Con
duct
ion
elec
tron
conc
entra
tion
(1021
/m3 )
T (K)6004002000
0
1
2
3
freez
e-ou
t
extri
nsic
intri
nsic
dopedundoped
Chapter 18 -15
• Allows flow of electrons in one direction only (e.g., usefulto convert alternating current to direct current).
• Processing: diffuse P into one side of a B-doped crystal.
-- No applied potential:no net current flow.
-- Forward bias: carriersflow through p-type andn-type regions; holes andelectrons recombine atp-n junction; current flows.
-- Reverse bias: carriersflow away from p-n junction;junction region depleted of carriers; little current flow.
Building Block for Devices: p-n Rectifying Junction
++
+ ++
- ---
-p-type n-type
+ -
++ +
++
--
--
-
p-type n-typeAdapted from Fig. 18.21 Callister & Rethwisch8e.
+++
+
+
---
--
p-type n-type- +
Chapter 18 -16
p - n Rectifying Junction: AC to DC Conversion
Fig. 18.22, Callister & Rethwisch 8e. Fig. 18.23, Callister & Rethwisch 8e.
Silicon controlled rectifier
Chapter 18 -17
p - n Rectifying Junction: Avalanche Diode
Fig. 18.22, Callister & Rethwisch 8e.
Chapter 18 -18
• For intrinsic semiconductors, conductivity is increased by-- increasing temperature
• For extrinsic semiconductors conductivity is increased by -- doping by adding B (III) to Si (IV) (p-type)-- doping by adding P to Si (n-type)]
• p – n junctions can be used to make devices such as-- rectifiers to convert AC to DC current-- avalanche diodes for surge protectors
Summary
Chapter 18 -
HW# 21 Electrical Properties II: Intrinsic & Extrinsic Semiconductors
5. Explain how the avalanche diode in the surge protector plug-in outlet strip willprotect your computer from damage by the voltage surge.
Chapter 18 -
Points of Reflection on Today’s Class
Please briefly describe your insights on the following points from today’s class.
• Point of Interest: Describe what you found most interesting in today’s class.How Interesting? (circle) Little Bit 1 2 3 4 5 Very Much
• Muddiest Point: Describe what was confusing or needed more detail.How Muddy? (circle) Little Bit 1 2 3 4 5 Very Much
• Learning Point: Describe what you learned about how you learn?
Letter + 4 digit number ______________ F MClass Topic: _______________________Date: ________________
20