Topic 1 ee201[1]

Topic 1: INTRODUCTION TO SEMICONDUCTOR

EE201

SEMICONDUCTOR DEVICES

Reference’s Book: “Electronic Devices and

Circuit Theory” Robert L.Boylestad and Louis Nashelsky

Eight Edition

DEU 2BDEU 2B

LEARNING OUTCOMES Upon completion of this Topic 1.0 : Introduction to Semiconductor, students should be able to:

Understand the Characteristics and electrical properties of Semiconductors

Identify the effects when a PN junction is supplied with forward biased voltage and reverse biased voltage

Explain why breakdown occurs when P-N junction is reverse biased

1.1

Understand the characteristic of P-N junction and Its reaction towards voltage biasing

1.2

1.2.4

1.2.2

1.2.3

Ilustrate the meaning of Forward biased voltage and reverse biased voltage supplied across a P-N junction

EE201 SEMICONDUCTOR DEVICES

• A basic understanding of atomic activity is necessary to understand the operation and application of semiconductor devices in electronic circuits.

• Semiconductor devices, such as transistors and diodes, form the basis of nearly all modern electronic systems.

• Basic Atomic Theory

Review


•Review of Basic Atomic Model

• Atoms are comprised of electrons, neutrons, and protons.

• Electrons are found orbiting the nucleus of an atom at specific intervals, based upon their energy levels.

• The outermost orbit is the valence orbit.

Review


VIDEO ON COVALENT BAND

•Energy Levels• Valence band

electrons are the furthest from the nucleus and have higher energy levels than electrons in lower orbits.

• The region beyond the valence band is called the conduction band.

• Electrons in the conduction band are easily made to be free electrons.

Review


• Classifications of Material

• Materials can be classified in many ways.• One way of classification is into solid, liquid, or gas

states. The materials in this section are all classed as solid-state.

• Other methods of classification include: electrical conductivity, color, density, hardness, composition, and so on.

• Classes of material according to conductivity are: insulators, conductors, semiconductors, and superconductors.

Review


• The term CONDUCTOR is applied to any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals.

• A SEMICONDUCTOR, therefore, is a material that has a conductivity level somewhere between the extremes of an insulator and a conductor.

• An INSULATOR is a material that offers a very low level of conductivity under pressure from an applied voltage source

•Definition The label Semiconductor itself provides a hint as to its

characteristic. The prefix semi –is normally applied to a range of levels midway between two limits.


1.1 The Characteristics and electrical properties of Semiconductor

• Conductors: have low resistance which allows electrical current flow.

• Good conductors have low resistance so electrons flow through them with ease.

• Semiconductors: can allow or suppress electrical current flow.

• Semiconductors are materials that essentially can be conditioned to act as good conductors, or good insulators, or any thing in between.

• Insulators: have high resistance which suppresses electrical current flow.

• Insulators have a high resistance so current does not flow in them.

The goal of electronic materials is to generate and control the flow of an electrical current. Electronic materials include:


•Materials1.1 The Characteristics and electrical properties of Semiconductor

• Best element conductors include: Copper, silver, gold, aluminum, & nickel

• Alloys are also good conductors: Brass & steel

• Good conductors can also be liquid: Salt water

• Common elements such as carbon, silicon, and germanium are semiconductors.

• Silicon is the best and most widely used semiconductor.

• Good insulators include: Glass, ceramic, plastics, & wood

• Most insulators are compounds of several elements.

• The atoms are tightly bound to one another so electrons are difficult to strip away for current flow.


•Materials1.1 The Characteristics and electrical properties of Semiconductor

Copper Atom

Conductor Atomic Structure• The atomic structure of good

conductors usually includes only one electron in their outer shell.

• It is called a valence electron. • It is easily striped from the atom,

producing current flow.

• The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

Semiconductor Valence Orbit

•Atomic Structure



•Intrinsic Semiconductors

• Silicon, germanium, and gallium arsenide are the primary materials used in semiconductor devices.

• Silicon and germanium are elements and are intrinsic semiconductors.

• In pure form, silicon and germanium do not exhibit the characteristics needed for practical solid-state devices.



•Isolated Semiconductor Atoms

• Silicon and Germanium are electrically neutral; that is, each has the same number of orbiting electrons as protons.

• Both silicon and germanium have four valence band electrons, and so they are referred to as tetravalent atoms. This is an important characteristic of semiconductor atoms.



•Semiconductor Crystals

• Tetravalent atoms such as silicon, gallium arsenide, and germanium bond together to form a crystal or crystal lattice.

• Because of the crystalline structure of semiconductor materials, valence electrons are shared between atoms.

• This sharing of valence electrons is called covalent bonding. Covalent bonding makes it more difficult for materials to move their electrons into the conduction band.



•The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice

•The atoms link together with one another sharing their outer electrons.

•These links are called covalent bonds

•Crystal Lattice Structure



• If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow.

• Good insulating semiconductor material is referred to as intrinsic.

• Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to get free for current flow.• Silicon in this form is a great insulator.• Semiconductor material is often used as an insulator.

•Semiconductors can be Insulators




Impurities are added to intrinsic semiconductor materials to improve the electrical properties of the material.

Impurities” are different elements.

This process is called doping and the resulting material is called extrinsic semiconductor.

There are two major classifications of doping materials. i. Trivalent - aluminum, gallium,boron ii. Pentavalent - antimony, arsenic, phosphorous

1.1 The Characteristics and electrical properties of SemiconductorSemiconductor

Doping

•Trivalent Doping• SiliconSilicon is the most widely used semiconductor material.

• By adding a trivalenttrivalent material to the crystal structure, holesholes are introduced and provide a mechanism for conduction.

• Because trivalent materials can accept an additional electron, they are called acceptor atoms.

• A silicon crystal doped with trivalent material is called p-type material.



•Trivalent Doping



•Pentavalent Doping

• Doping silicon with pentavalentpentavalent material results in extra electrons extra electrons being available, improving the conduction characteristics.

• Pentavalent materials donate electrons, and therefore are called donor atoms.

• Once a silicon crystal has been doped with pentavalent materials, it is called n-type semiconductor material.



•Pentavalent Doping




•Semiconductors can be Conductors

• An impurity, or element like arsenic, has 5 valence electrons.

• Adding arsenic (doping) will allow four of the arsenic valence electrons to bond with the neighbouring silicon atoms.

• The one electron left over for each arsenic atom becomes available to conduct current flow.


•Types of Semiconductor Materials


• The silicon doped with extra electrons is called an “N type” semiconductor.

• “N” is for negative, which is the charge of an electron.

• Silicon doped with material missing electrons that produce locations called holes is called “P type” semiconductor.

• “P” is for positive, which is the charge of a hole.


•Energy Levels1.1 The Characteristics and electrical properties of Semiconductor

In SummaryIn its pure state, semiconductor material is an excellent excellent insulator.insulator.

The commonly used semiconductor material is silicon.silicon.

Semiconductor materials can be doped can be doped with other atoms to add or subtract electrons.

An N- typeN- type semiconductor material has extra electronsextra electrons.

A P- typeP- type semiconductor material has a shortage of electronsshortage of electrons with vacancies called holes.holes.

The heavier the dopingheavier the doping, the greater the conductivity greater the conductivity or the lower the resistance.

By controlling the doping of silicon the semiconductor material can be made as conductive as desired.




Current Flow in a Semiconductor

• When a doped semiconductor has a voltage applied to it, current will flow from negative to positive, regardless of whether it is p- or n-type material.

• The current flow is radically different for the two types of material.


• Current Flow in N-type Semiconductors


• The DC voltage source has The DC voltage source has a positive terminal that a positive terminal that attracts the free electrons in attracts the free electrons in the semiconductor and the semiconductor and pulls them away from their pulls them away from their atoms leaving the atoms atoms leaving the atoms charged positively.charged positively.

• Electrons from the negative Electrons from the negative terminal of the supply enter terminal of the supply enter the semiconductor material the semiconductor material and are attracted by the and are attracted by the positive charge of the positive charge of the atoms missing one of their atoms missing one of their electrons.electrons.

• Current (refer to electrons) Current (refer to electrons) flows from the positive flows from the positive terminal to the negative terminal to the negative terminalterminal..



Current Flow Through N-Type Material

• N-type material has many conduction band electrons.

• If a voltage is connected across n-type crystal, free electrons will move toward the positive terminal.

• As electrons are moved from one atom towards the positive terminal, a hole is left behind, allowing more electrons to shift towards the source voltage.


• Current Flow in P-type Semiconductors


• Electrons from the negative supply terminal are attracted to the positive holes and fill them.

• The positive terminal of the supply pulls the electrons from the holes leaving the holes to attract more electrons.

• Current (electrons) flows from the negative terminal to the positive terminal.

• Inside the semiconductor, current flow is actually by the movement of the holes from positive to negative.



Current Flow Through P-Type Material

Current flow in p-type material causes the shift of “holes” towards the negative terminal because of the shifting of the covalent electrons.

Hole flow moves from positive to negative in a p-type semiconductor material.

Actual current flow is still electron current flow from negative to positive.



Electron versus Hole Flow• Electron flow in p-type Electron flow in p-type

material occurs in the material occurs in the valence bandvalence band; ; electron movement in n-type material occurs in the conduction band

• Electrons are the majority carriers in n-type material; they are holes they are holes in p-type material.in p-type material.



1.2 The Characteristics of P-N junction and its reaction towards voltage biasing.

Semiconductor Junctions

• When p-type material meets n-type material within a single silicon crystal, a PN junction is formed.



Unbiased Junction

• The pn junction is formed in the process of creating the semiconductor device.

• Before carrier migration, there are equal numbers of holes and electrons on either side of the junction.

• Because of random thermal energy, some electrons will pass across the pn junction mating with holes on the other side. This is called recombinationrecombination.

•Formation Of Depletion Region/Layer



• As soon as the junction is formed, free electrons and holes cross through the junction by the process of diffusion.

• During this process , the electrons crossing the junction from N- region into P-region , recombine with holes in the P-region very close to the junction.

• Similarly holes crossing the junction from the P- region into the N-region, recombine with electrons in the N-region very close to the junction.

• Depletion Region/Layer



• After a time, the region will be depleted of charge carriers because of the migration of electrons and holes.

• This leaves an area known as the depletion region/ layerdepletion region/ layer in the pn junction, which does not have any mobile charge very close to the junction.



In this region, on the left side of the junction, the acceptor acceptor atoms become negative ions atoms become negative ions (an atom gains electrons, (an atom gains electrons, number of electrons exceed number of electrons exceed from the number of proton from the number of proton and hence an atom gets and hence an atom gets negatively charged)negatively charged)

and on the right side of the junction, the donor atoms donor atoms become positive ionsbecome positive ions (an (an atom looses electrons the atom looses electrons the total number of protons total number of protons become more than the become more than the number of electrons and the number of electrons and the atom becomes positively atom becomes positively charged)charged)

• Depletion Region/Layer

• Function of Depletion Region of Pn Junction :



• An electric field is set up, between the donor and acceptor ions in the depletion layer of the pn junction .

• The potential at the N-side is higher than the potential at P-side.Therefore electrons in the N- side are prevented to go to the lower potential of P-side.

• Similarly, holes in the P-side find themselves at a lower potential and are prevented to cross to the N- side.

• Function of Depletion Region of Pn Junction :



• Thus, there is a barrier at the junction which opposes the movement of the majority charge carriers. The difference of potential from one side of the barrier to the other side of the barrier is called potential barrier.potential barrier.• In silicon, the potential is 0.6–0.7 Vsilicon, the potential is 0.6–0.7 V; in germanium, it is 0.2–0.3 Vgermanium, it is 0.2–0.3 V.(Threshold Voltage, VT)

• The distance from one side of the barrier to the other side is called the width of the barrier, which depends on the nature of the material.

A PN junction is present in every semiconductor device.

N-type

P-type

Donors

V

I

Reverse bias Forward bias

N P

V

I

diodesymbol

– +

• PN Junctions1.2 The Characteristics of P-N junction and its reaction towards voltage biasing.


•Lattice Bands

•Conduction

• In order for an electron to become free and participate in current flow, it must gain enough energy to jump over the forbidden band.

• For semiconductors at room temperature, there is not enough energy to conduct.

• As temperature increases more electrons have the energy to jump the forbidden band

• Resistivity decreasesResistivity decreases• This is the opposite behavior of conductorsThis is the opposite behavior of conductors

•The PN Junction DiodeStart with a P and N type material. Note that there is excess negatives in the n-type and excess positives in the p-type

Merge the two – some of the negatives migrate over to the p-type, filling in the holes. The yellow region is called the depletion zonedepletion zone.

More positive than rest of N

More negative than rest of P

•Biasing the JunctionApply a voltage as indicated. The free charge carriers (negative charges in the N material and positive charges in the P material) are attracted to the ends of the crystal. No charge flows across the junction and the depletion zone grows. This is called reverse bias.

Switch polarity. Now the negative charges are driven toward the junction in the N material and the positive charges also are driven toward the junction in the P material. The depletion zone shrinks and will disappear if the voltage exceeds a threshold. This is called forward bias.

• Forward Biased and Reverse Biased Voltage Supplied




1.2 The Characteristics of P-N junction and its reaction towards voltage biasing. • Forward Biased

Junction• An external source can either oppose or aid the barrier

potential.• If the positive side of the voltage is connected to the p-

type material, and the negative side to the n-type material, then the junction is said to be forward biasedforward biased.



• Forward Biased

Junction

• In a forward biased junction, the following conditions exist:

• Forward bias overcomes barrier potential.• Forward bias narrows the depletion region.• There is maximum current flow with forward bias.


1.2 The Characteristics of P-N junction and its reaction towards voltage biasing. •Reverse Biased

Junction• Reverse bias occurs

when the negative negative source is source is connected to the p-connected to the p-typetype material and the positive source is positive source is connected to the n-connected to the n-type materialtype material.

• Reverse bias strengthens the barrier potential.

• Reverse bias widens the depletion region.

• Current flow is minimum.

a) Forward biased PN junction, b) Corresponding diode schematic symbol c) Silicon Diode I vs V characteristic curve.

• Forward Biased

Junction

• Characteristics of Reverse Bias PN Junction:



• From the characteristic curve, it can be concluded that, as voltage is increased from zero, reverse current (Ir) (in the order of microamperes) increases and reaches the maximum value at a small value of reverse voltage (Vr). • When the voltage is further increased, the current is almost independent of the reverse voltage up to a certain critical value.• This reverse current is known as the reverse saturation current or leakage current. • This current is due to the minority charge carriers, which depends on junction temperature.


1.2 The Characteristics of P-N junction and its reaction towards voltage biasing. •Reverse Biased

Junction

• A reversed biased junction has zero current flow (ideally).

• Reverse current is temperature dependent.• If reverse biased is increased enough, the reverse

current increases dramatically.• This breakdown is called junction breakdownjunction breakdown. The

voltage required to reach this point is the reverse reverse breakdown voltagebreakdown voltage..

• As the breakdown occurs, avalancheavalanche may occur and destroy the device if uncontrolled.


1.2 The Characteristics of P-N junction and its reaction towards voltage biasing. •Review…..

• Breakdowns in Reverse

Biased


• There are two mechanisms by which breakdown can occur at a reverse biased P-N junction : avalanche breakdown and Zener/Tunneling breakdown.

Diode I vs V characteristic curve.)


Junction Breakdown

A Zener diode is designed to operate in the breakdown mode.

V

I

VB

, breakdown

Forward Current

Small leakageCurrent

voltage



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