Week01 diode revision [revision]

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 Design2

Revision Slides

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 Design8

Summary of Open and Short Circuit

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

ENGN2218 Electronic Systems and Design23

Comparison of Diode Models