Download Semiconductor Diodes Semiconductors

2/8/2014
Semiconductor Diodes
Peter Mathys
ECEN 1400
Semiconductors
• Modern diodes and transistors are made from
semi-conductive materials (conductivity in
range of 103 to 10-8 S per cm).
• Typical semi-conductive materials are silicon
(Si), germanium (Ge), gallium arsenide (GaAs),
and silicon carbide (SiC).
• Silicon, atomic number 14, is the eighth most
common element by mass in the universe.
1
2/8/2014
Periodic Table of Elements
Semiconductors
Silicon
• To use silicon in semiconductor devices, it
must be grown in crystals of very high purity.
• The crystals are then cut into wafers which are
doped to alter their conductive states.
Atomic configuration
of silicon (Si), atomic
number 14, four
valence electrons
2
2/8/2014
Pure (intrinsic) Silicon
• Pure silicon has no free electrons. Applying a
voltage produces almost no electron flow.
All electrons are locked
up in covalent bonds
between neighboring
atoms.
Doping Silicon
• Doping is a process where a controlled
amount of impurities is added to change the
conductivity of selected areas of the Si wafer.
Impurity with additional
valence electron
Impurity with “missing”
valence electron
3
2/8/2014
n-Type Silicon
• Addition of atoms with 5 valence electrons.
Additional electron is not
bonded and can be easily
moved around.
p-Type Silicon
• Addition of atoms with 3 valence electrons.
Missing electron acts like
positive charge that can
be easily moved.
4
2/8/2014
Diodes
• Symbol:
• Forward Biased: Anode is more positive than
cathode => Positive charge flows A -> C
• Reverse Biased: Cathode is more positive than
anode => No charge flows through diode.
pn-Junction Diodes
• pn junction diodes are formed by sandwiching ntype and p-type silicon together.
• For “one-way gate”, make charge carriers (free
electrons and holes) in n, p regions interact such
that current flows only in one direction.
5
2/8/2014
Forward Biased
• Electrons and holes are forced toward pn
junction by electric field supplied by battery.
Electrons and
holes combine
and current
can flow.
Reverse Biased
• Holes are attracted to – and electrons are
attracted to + of battery.
This creates a zone
(called depletion
region) at the pn
junction which is free
of charge carriers
(holes and electrons)
and no current can
flow.
6
2/8/2014
Shockley Equation for iD
Real pn Junction Diodes
• A real pn junction has resistance, from the
semiconductor to metal contacts at both ends,
and in the semiconductor material itself.
• SPICE model example:
.model 1N4148 D(Is=2.52n Rs=.568 N=1.752)
7
2/8/2014
i-v Characteristic
Rs·iD
Diode Applications
• Half wave rectifier:
8
2/8/2014
Diode Applications
• Full wave rectifier:
Simple Diode Model
• A simple model for a pn junction diode in
forward biased mode is shown below.
• For silicon vF is approximately 0.7 V.
9
2/8/2014
Solar Cell
• A solar cell is essentially a pn junction with a
thin transparent n layer facing the light source
and a large surface area.
Equiv. Circuit, i-v Char.
10
2/8/2014
LED: Light Emitting Diode
11