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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
ENGN2218 Electronic Systems and Design3
• The convention used in this course is
• Commonly used symbols in this course are
Terminology
ENGN2218 Electronic Systems and Design4
• 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
ENGN2218 Electronic Systems and Design5
• Kirchhoff’s Current Law (KCL):
• Kirchhoff’s Voltage Law (KVL):
Kirchhoff’s Laws
ENGN2218 Electronic Systems and Design6
• 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
ENGN2218 Electronic Systems and Design7
• 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
ENGN2218 Electronic Systems and Design9
• A semiconductor is neither a good conductor nor a good insulator.
Semiconductors
ENGN2218 Electronic Systems and Design10
• 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
ENGN2218 Electronic Systems and Design11
• 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
ENGN2218 Electronic Systems and Design12
• 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
ENGN2218 Electronic Systems and Design13
• 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
ENGN2218 Electronic Systems and Design14
• 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
ENGN2218 Electronic Systems and Design15
• 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
ENGN2218 Electronic Systems and Design16
• 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
ENGN2218 Electronic Systems and Design17
• 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
ENGN2218 Electronic Systems and Design18
• 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
ENGN2218 Electronic Systems and Design19
• V-I characteristics for a typical Si diode.
Volt-ampere Characteristics (contd.)
ENGN2218 Electronic Systems and Design20
• The ideal diode acts as a short circuit for forward currents and as an open circuit with reverse voltage applied.
Ideal Diode Model
ENGN2218 Electronic Systems and Design21
• The practical diode model adds a 0.7V voltage source in series with an ideal diode model.
Practical Diode Model
ENGN2218 Electronic Systems and Design22
• 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