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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.

Equiv. Circuit, i-v Char.

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LED: Light Emitting Diode