Transcript Lecture 3 Diode

The Devices:
Diode
EE314 Basic EE II
1.Semiconductors
2.Doping concept
3.n & p-type semiconductors
4.Si diode
5.Forward & reversed bias
6.Examples
7.Diode Characteristic
Chapter 10: Diodes
Engineer-In-Training Reference Manual
http://www.amazon.com/Engineer-Training-Reference-Michael-Lindeburg/dp/0912045566
EE314 Basic EE II
Outline
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Motivation and Goals
Semiconductor Basics
Diode Structure
Operation
» Static model
EE314 Basic EE II
Atom
Composed of 3 Basic particles:
Protons, Electrons & Neutrons.
An Atom requires balance, an equal No. of Protons &
Electrons.
When an atom has one more particle (protons or electrons)
it acquires a charge:
+ Ion has more Protons than Electrons,
- Ion has more Electrons than Protons.
EE314 Basic EE II
What do we know about an atomic structure?
EE314 Basic EE II
Semiconductor Basics  I
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Electrons in intrinsic (pure) Silicon
»
»
»
»
»
covalently bonded to atoms
“juggled” between neighbors
thermally activated: density  eT
move around the lattice, if free
leave a positively charged `hole’ behind
EE314 Basic EE II
http://www.masstech.org/cleanenergy/solar_info/images/crystal.gif
Semiconductor Basics  II
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Two types of intrinsic carriers
»
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Electrons (ni) and holes (pi)
In an intrinsic (no doping) material, ni=pi
At 300K, ni=pi is low (1010cm-3)
Use doping to improve conductivity
EE314 Basic EE II
Semiconductor Basics  III
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Extrinsic carriers
» Also two types of dopants (donors or acceptors)
– Donors bring electron (n-type) and become ive ions
– Acceptors bring holes (p-type) and become ive ions
» Substantially higher densities (1015cm-3)
» Majority and minority carriers
– if n>>p (n-type) electrons majority and holes minority
– Random recombination and thermal generation
EE314 Basic EE II
Conduction
Conductor;
Has loosely bound electrons in its outer or Valence ring,
they are easily displaced.
Insulator;
Has tightly bound electrons in its outer or Valence ring,
they cannot be easily displaced.
Semiconductor;
Has at least 4 electrons in the outer or Valence ring, it is
neither a conductor nor an insulator.
In its pure state it makes a better insulator than conductor.
4 electrons allows easy bonding w/ other materials.
EE314 Basic EE II
Semiconductor Basics  I

Electrons in intrinsic (pure) Silicon
»
»
»
»
covalently bonded to atoms
“juggled” between neighbors
thermally activated: density  eT
move around the lattice, if free
» leave a positively charged `hole’ behind
EE314 Basic EE II
Semiconductor Basics  II

Two types of intrinsic carriers
»
»
»
»

Electrons (ni) and holes (pi)
In an intrinsic (no doping) material, ni=pi
At 300K, ni=pi is low (1010cm-3)
Use doping to improve conductivity
Extrinsic carriers
» Also two types of dopants (donors or acceptors)
– Donors bring electron (n-type) and become ive ions
– Acceptors bring holes (p-type) and become ive ions
» Substantially higher densities (1015cm-3)
» Majority and minority carriers
– if n>>p (n-type) electrons majority and holes minority
– Random recombination and thermal generation
EE314 Basic EE II
The Diode
B
A
Al
SiO
2
p
n
Cross section of pn-junction in an IC process
EE314 Basic EE II
N-type region
P-type region
doped with donor
impurities
(phosphorus,
arsenic)
doped with
acceptor
impurities (boron)
The Diode
Simplified structure
A
p
Al
A
n
The pn
region is
assumed to
be thin (step
or abrupt
junction)
EE314 Basic EE II
B
One-dimensional
representation
B
diode symbol
Different concentrations of
electrons (and holes) of the p and ntype regions cause a concentration
gradient at the boundary
Depletion Region
•Concentration Gradient causes electrons to diffuse from n to p,
and holes to diffuse from p to n
•This produces immobile ions in the vicinity of the boundary
•Region at the junction with the charged ions is called the
depletion region or space-charge region
•Charges create electric field that attracts the carriers, causing
them to drift
•Drift counteracts diffusion causing equilibrium ( Idrift = -Idiffusion )
hole diffusion
electron diffusion
p
n
hole drift
electron drift
EE314 Basic EE II
Depletion Region
•Zero bias conditions
hole diffusion
electron diffusion
p
•p more heavily doped
than n (NA > NB)
•Electric field gives rise
to potential difference in
the junction, known as
the built-in potential
(a) Current flow.
n
hole drift
electron drift
Charge
Density

+
x
Distance
-
Electrical
Field
(b) Charge density.

x
(c) Electric field.
V
Potential
-W 1
EE314 Basic EE II

W2
x
(d) Electrostatic
potential.
Forward Bias
hole diffusion
electron diffusion
p
n
hole drift
electron drift
+
-
•Applied potential lowers the potential barrier, Idiffusion > I drift
•Mobile carriers drift through the dep. region into neutral regions
•become excess minority carriers and diffuse towards terminals
EE314 Basic EE II
Reverse Bias
hole diffusion
electron diffusion
p
n
hole drift
electron drift
-
+
•Applied potential increases the potential barrier
•Diffusion current is reduced
•Diode works in the reverse bias with a very small drift current
EE314 Basic EE II
Diode Current
Ideal diode equation:
EE314 Basic EE II