This document provides an overview of diodes and semiconductor physics. It defines key concepts like intrinsic and extrinsic semiconductors, n-type and p-type materials, and pn junctions. It describes how a diode allows current to flow in one direction due to the structure of the pn junction. The diode is modeled as an ideal diode in parallel with a 0.7V voltage source to account for its forward voltage drop. Diodes act as uni-directional switches compared to mechanical switches which conduct current bidirectionally.

1. ENGN2218 Electronic Systems and Design1
Reading for LECTURE 02
Revision of Diode
Dr. Salman Durrani
School of Engineering,
The Australian National University.
http://people.cecs.anu.edu.au/asp/academics/bio

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Revision Slides

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• The convention used in this course is
• Commonly used symbols in this course are
Terminology

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• An electric circuit consists of circuit elements (such as
voltage sources, resistors, capacitors and inductors)
connected in closed paths by conductors.
• A node in an electrical circuit is a point at which two or
more circuit elements are joined together (an essential
node is where three or more circuit elements join).
• A loop in an electrical circuit is a closed path whose last
node is the same as the starting node.
• A mesh is a special case of a loop. A mesh is a loop that
does not contain any other loops.
Electric Circuit Vocabulary

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• Kirchhoff’s Current Law (KCL):
• Kirchhoff’s Voltage Law (KVL):
Kirchhoff’s Laws

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• Conventional current flows from +ve terminal of battery
to –ve terminal. Flow of electrons is in opposite direction
to conventional current direction.
• The terminal where conventional current enters an
element is assigned +ve polarity and the terminal
where conventional current leaves an element is
assigned -ve polarity.
• Following conventional current in a circuit, if we move
from +ve polarity towards –ve polarity (voltage drop),
the element absorbs energy that appears as heat or
stored chemical energy. If we move from –ve polarity
towards +ve polarity (voltage rise), the element supplies
energy.
Current and Voltage Conventions

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• The extreme value of R =infinity Ω is given special name
of open circuit.
– Using analogy of a switch, it corresponds to the case when
switch is open (i.e. OFF)
• The extreme value of R =0Ω is given special name of
short circuit.
– Using analogy of a switch, it corresponds to the case when
switch is closed (i.e. ON)
Open and Short Circuit Concepts

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Summary of Open and Short Circuit

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• A semiconductor is neither a good conductor nor a
good insulator.
Semiconductors

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• Electronic devices such as diodes and transistors are
made of a semiconductor material.
• A basic knowledge of semiconductor physics helps us to
understand diode and bipolar junction transistor
behaviour.
• Common semiconductor materials are
– Silicon (Si) [single-element semiconductor]
– Germanium (Ge) [single-element semiconductor]
– Carbon (C) [single-element semiconductor]
– Gallium Arsenide (GaAs) [compound semiconductor]
Basic Semiconductor Concepts

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• Silicon is the most widely used material in diodes,
transistors other semiconductor devices.
• Intrinsic (pure) Silicon is a crystalline material with Si
atoms held together by covalent bonds.
Intrinsic Silicon

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• Extrinsic (impure) semiconductors are formed by
adding small amounts of suitable impurities to intrinsic
semiconductors in order to improve conductivity.
• The process of adding controlled impurities to intrinsic
(pure) semiconductor materials is called doping.
• Two types of extrinsic semiconductor materials are:
– An n-type semiconductor material consists of Si
atoms and pentavalent impurity atoms.
– A p-type semiconductor material consists of Si atoms
and trivalent impurity atoms.
Extrinsic Semiconductors

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• An n-type semiconductor material consists of Si atoms
and pentavalent impurity atoms such as Arsenic (As),
Phosphorus (P), Bismuth (Bi) and Antimony (Sb).
N-Type Semiconductors

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• A p-type semiconductor material consists of Si atoms
and trivalent impurity atoms such as Boron (B), Indium
(I) and Gallium (Ga).
P-Type Semiconductors

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• A pn junction consists of a single crystal of
semiconductor material that is doped to produce n-type
material on one side and p-type on the other side.
• The pn junction is fundamental to the operation of
devices such as diodes and transistors.
• Barrier Potential is the
amount of voltage required
to move electrons through
the pn junction.
(0.7V for Si and 0.3V for Ge)
PN Junction

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• Diode is a device that allows current to flow in one
direction but blocks current in reverse direction.
• An diode is a two terminal device (the anode or “p” and
the cathode or “n”) commonly made from Silicon.
• The arrow in the symbol points in the direction of
conventional current flow.
Diode

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• Bias is the use of a dc voltage to establish certain
operating conditions for an electronic device.
• A diode is said to be forward biased if the p-side of the
diode is more +ve with respect to the n-side of the diode.
• A diode is said to be reverse biased if the n-side of the
diode is more +ve with respect to the p-side of the diode.
Basic Definitions

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• Diode is a non-linear device.
• Diode is usually operated in the forward bias and reverse
bias regions, without entering the breakdown region.
Volt-ampere Characteristics

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• V-I characteristics for a typical Si diode.
Volt-ampere Characteristics (contd.)

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• The ideal diode acts as a short circuit for forward
currents and as an open circuit with reverse voltage
applied.
Ideal Diode Model

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• The practical diode model adds a 0.7V voltage source in
series with an ideal diode model.
Practical Diode Model

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• A mechanical switch in “ON” state allows current to
flow in either direction.
• A diode in “ON” state allows current to flow in one
direction only (uni-directional switch). The arrow in the
symbol points in the direction of conventional current
flow.
• Practically, we assume a 0V drop across a mechanical
switch in “ON” state while we assume a 0.7V drop
across a diode in “ON” state.
Comparison with Mechanical Switch

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Comparison of Diode Models