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