CHAPTER 1 : Introduction To Diodes ppt

• A diode is forward-biased when the voltage on the anode is positive with respect to the cathode.. • A diode is reverse-biased when the voltage on the anode is negative with respect to

CHAPTER 1

Introduction

To Diodes

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

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

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

• 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

• 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