ECE100 Part 3: Introduction to Semiconductor materials.
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Section III
Introduction to Semi-conductor devices and
circuits
Objectives:
1. To describe the characteristics and operation
of the diode
2. To understand the electrical conduction in
Semiconductor materials and to study the
characteristics of P-N junctions
3. To model the P-N junction (Diode) as a circuit
element
4. To analyze circuits comprising diodes
5. To study the Bipolar Junction Transistors
characteristics and analyze transistor circuits.
ECE100 Part 3: Introduction to Semiconductor materials.
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1. Diodes
• A diode is a non-linear electrical component that
conducts electricity in one direction but not the other.
It is made of semiconductor material (Silicon or
Germanium)
• The diode has two terminals-an anode A terminal and a
cathode K terminal. The symbol and the characteristic
of an ideal diode are shown below
Fig. 1: The symbol and the characteristic of an ideal diode
• Diodes play an important role in electrical and
electronic circuits. They are used mainly in rectifiers
(to convert AC to fixed DC voltages), filters and
waveshaping circuits.
ECE100 Part 3: Introduction to Semiconductor materials.
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2. Electrical conduction in Semiconductors
• Semiconductors (Silicon or Germanium) are materials
that have electrical properties falling between those
of conducting and insulating materials.
• At very low temperatures, semiconductors have the
property of an insulator (no free electrons). However,
when a thermal energy is supplied to the lattice
structure of the silicon material, for example, some
electrons break their bonds with the lattice and
become free electrons leaving a positive hole in their
place.
• Both free electrons and holes contribute to the
conduction process.
• Electrons are negative charges, and holes are positive
charges.
Valence electrons
Covalent
bonds
ECE100 Part 3: Introduction to Semiconductor materials.
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Fig.2: Arrangement of atoms in a silicon crystal (2-dimensional)
• Doping process: To control the number of free
carriers
(electrons
and/or
holes)
inside
a
semiconductor and thus achieve better conduction,
semiconductors go through doping process.
• Doping means adding some impurities to the structure
of the semiconductor.
• When a material is doped with extra holes, the
material
becomes
a
positive
type
(p-type)
semiconductor.
• When extra electrons are added the material becomes
a negative type (N-type).
¾ N-type, the majority carriers are electrons and
the minority are holes.
¾ P-type, the majority carriers are holes and the
minority are electrons.
ECE100 Part 3: Introduction to Semiconductor materials.
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Fig.3: Doped semiconductors (N-type and P-type silicon)
3. The P-N Junction (Diode)
• The P-N junction is an N-type material brought in
contact with a P-type material.
• At the contact surface and the surrounding area, the
electrons and the holes combine to neutralize this
zone and almost no carriers will be found in this area.
This area is referred to as depletion region.
• A contact potential appears across this region and in
Silicon P-N junction this potential has a value of 0.7V.
This potential is called the offset voltage (Vγ).
P
N
P
N
Depletion region
Fig. 4: The P-N Junction diode
ECE100 Part 3: Introduction to Semiconductor materials.
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4. I-V characteristics of P-N junction (Diode
Characteristics)
• When the diode is forward biased, conduction starts
at VD = Vγ , where VD is the voltage across the diode
(Anode-Cathode) voltage. The diode behaves like a
switch in the “on” position.
• When the diode is reverse biased, very small current
(order of nA) flows which is neglected and the diode
considered to be a switch in its “off” position.
• Therefore, the diode is either conducting (forward
biased) or not conducting (reverse biased).
Fig. 5: Forward and Reverse Biasing of P-N Junctions
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ECE100 Part 3: Introduction to Semiconductor materials.
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Fig. 6: I-V characteristics of P-N junction Diodes
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ECE100 Part 3: Introduction to Semiconductor materials.
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5. Diode equivalent circuit model:
5.1. Ideal diode
Fig. 7: Equivalent circuit model of an ideal diode
5.2. Real diode
Fig. 8: Equivalent circuit model of a real diode
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ECE100 Part 3: Introduction to Semiconductor materials.
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6. Diode Circuit Analysis
6.1 Diodes in DC circuits
To analyze diode circuits, the state of the diode (on or
off) must first be found. The diode can then be replaced
by its equivalent circuit model and carry on the analysis.
Procedure:
1. Assume a state for each diode you have in the circuit.
(Either “on” or “off”). You need 2N states for N
diodes.
2. Analyze the circuit to determine the current through
the diodes assumed to be “on” and the voltage across
the diodes assumed to be “off”.
3. Check to see if the result is consistent with the
assumed state for each diode.
¾ Current must flow in the forward direction for
the diodes assumed to be on.
¾ Voltage across the diodes assumed to be “off”
must be negative (reverse biased).
If the results are consistent the analysis is finished,
otherwise return to step 1 and choose different states.
ECE100 Part 3: Introduction to Semiconductor materials.
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6.2 Diodes in AC circuits
AC circuits have a voltage that varies with time.
Therefore, there may be times when the AC voltage
forward-biases a diode and times when it reverse-biases
the same diode. Circuit Analysis can be done separately
for positive and negative half-cycles. It must be noted
when the voltage polarity across the diode forwardbiases it and when it reverse-biases it. The diode can
then be replaced by its equivalent circuit model.
6.3 Applications
¾
Half wave rectifiers
Fig. 9: Half wave rectifier circuit and waveform
ECE100 Part 3: Introduction to Semiconductor materials.
¾
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Full Wave rectifier Bridge
Fig. 10: Full wave rectifier circuit and waveform
¾
Wave shaping
Fig. 11: Clipper circuit
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ECE100 Part 3: Introduction to Semiconductor materials.
4.3 Examples
Back to the blackboard
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