2/8/2014 1 Semiconductor Diodes Peter Mathys ECEN 1400 Semiconductors • Modern diodes and transistors are made from semi-conductive materials (conductivity in range of 10 3 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.
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Semiconductor Diodes
Peter MathysECEN 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.
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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
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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
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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.
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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 n-type 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.
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Forward Biased
• Electrons and holes are forced toward pnjunction 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 pnjunction which is free of charge carriers (holes and electrons) and no current can flow.
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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)
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i-v Characteristic
Rs·iD
Diode Applications
• Half wave rectifier:
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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.
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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.