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CHAPTER 1
Introduction
To
Diodes
OBJECTIVES
Describe and Analyze:
• Function of Diodes
• Some Physics of Diodes
• Diode Models
Introduction
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•Diodes let current flow one way, but not the other
•Conventional current flows from anode to cathode
•Electrons flow from cathode to anode
Diodes are Important
• The humble silicon diode is the simplest of all the
semiconductor devices. It is also one of the most
important. Without diodes, you could not build
electronic equipment
• Applications for diodes range from power supplies to
cell phones and everything in between.
Diodes are Important
• It’s important to understand what a diode is and what
makes it work.
• Diodes use a PN junction. Later, we will see how PN
junctions play a key role in transistors.
Bias: Forward & Reverse
• A forward-biased diode conducts.
• A diode is forward-biased when the voltage on the
anode is positive with respect to the cathode.
• A reverse-biased diode does not conduct.
• A diode is reverse-biased when the voltage on the
anode is negative with respect to the cathode.
• A diode acts like a voltage-controlled switch.
Valence Electrons
• The outer band of electrons in an atom is called the
valence band.
• Atoms in a conductor (e.g. copper) have valence
electrons that can move freely through the material.
• The valence electrons of insulators are bound to the
atoms and can not move freely.
Covalent Bonds
• A covalent bond is formed when atoms can share
valence electrons with adjacent atoms. The result is
crystalline material such as silicon.
• Covalent bonds are very strong. They are what
make diamonds hard.
Doping
Adding different atoms to a crystal is called doping.
• Donor atoms (e.g. arsenic) add movable electrons to the
crystal’s valence band.
• Acceptor atoms (e.g. gallium) add movable “holes” : open
spaces in the valence band to accept electrons. Holes act like
positive charge carriers.
Semiconductors: N & P
• Doped silicon becomes a semiconductor.
• Current can flow through a semiconductor, but not as
easily as through metal conductors.
• N-Material is silicon that has been been doped with
donor atoms.
• P-Material is silicon that has been been doped with
acceptor atoms.
• When a single crystal has N-material on one side and
P-material on the other side, things get interesting.
The PN Junction
• Anode is P-material.
• Cathode is N-material.
• The interface is the PN junction, which is a diode.
Reverse-Biased PN Junction
The electrons and holes are drawn away from the
junction, leaving a depletion region devoid of charge
carriers. No current can flow across the junction.
Forward-Biased PN Junction
• Electron are forced to move across the junction and fall
into the holes on the other side. Current is flowing.
• The energy required to make the electrons and holes
combine shows up as a 0.7 Volt drop across the junction.
Other Semiconductors
• Silicon is not the only material used to make
semiconductors.
• Germanium was used originally back in the 1950s,
and is used today in high frequency applications.
• Germanium has a 0.3 Volt drop (approximately)
across a PN junction.
• Other kinds of semiconductor materials can have PN
drops up to 1.5 Volts or even up to 3 Volts.
Ideal Diode Model
A perfect diode would behave as shown.
Practical Diode Model
Closer to how real diodes behave.
Detailed Diode Model
Very close to how real diodes behave.
Ideal vs. Practical vs. Detailed
• The only difference between the ideal model and the
practical model is the 0.7 Volt battery. That may be
important when working with circuits that use, for
example, a 1.5 Volt battery.
• The detailed model includes the reverse leakage
current and the diode’s internal resistance.
• Leakage current is not usually a problem with silicon.
• Usually, the diode’s resistance is only important when
the applied forward-bias is a low voltage.
Choosing a Model
• The ideal model shows the key feature of a diode:
one-way conduction of current.
• For most purposes, the practical model is sufficient.
• The detailed model may be needed when low
voltages and small currents need to be analyzed.
Summary
• Doping changes silicon from an insulator to a
semiconductor.
• Donor atoms make silicon into N-material.
• Acceptor atoms make silicon into P-material.
• A PN junction makes a diode.
Summary
• Diodes let current flow when anode to cathode is
positive about 0.7 Volts.
• Diodes block current when anode to cathode voltage
is negative.
• Diodes can be modeled by combining basic circuit
elements: switch, battery, resistor.