Discuss properties of insulators, conductors, and semiconductors

Discuss covalent bonding

Describe the conductions in semiconductorDiscuss N-type and P-type semiconductor

Discuss basic structures of atoms

Discuss the diode

Discuss the bias of a diode

1.1 Atomic structure1.2 Semiconductor, conductors and

insulators1.3 Covalent bonding1.4 Conduction in semiconductors1.5 N-type and P-type

semiconductors1.6 Diode1.7 Biasing the diode1.8 Voltage-current characteristic of

a diode1.9 Diode models1.10 Testing a diode

• Move information not things - Phone, fax, internet - Takes much less energy and money

• Electronics are easy to move/control - Easy to move/control electrons than real physical stuff

Basicstructure

Atomicnumber

Electron shells

Valence electron

Free electron

Ionization

ATOM

smallest particle of an element contain 3 basic particles:

Protons (positive charge)

Neutrons (uncharged)

Nucleus(core of atom)

Electrons(negative charge)

ATOM

This model was proposed by Niels Bohr in 1915.

-electrons circle the nucleus that consists of protons and neutrons.

Figure 1.1 Bohr model of Figure 1.1 Bohr model of an atoman atom

Atomic Number- Element in periodic table are arranged according to atomic

number- Atomic number = number of protons in nucleus

Electron Shells and Orbits- Electrons near the nucleus have less energy than those in more distant orbits.- Each distance (orbits) from the nucleus corresponding to a certain energy level.- In an atom, the orbits are group into energy bands – shells- Diff. in energy level within a shell << diff. in energy between shells.

Valence Electrons- Electrons with the highest energy levels exist in the outermost shell and loosely bound to the atom. The outermost shell –

valence shell. - Electron in the valence shell called valence electrons.

Ionization- When atoms absorb energy (e.g heat source) – losing valence electrons called ionization. - Escape electron called free electron.

The Number of Electrons in Each Shell

- The maximum number of electrons (Ne) in each shell is calculated using formula below:

- n = number of shell- Example for 2nd shell

22nNe

8)2(22 22 nNe

•Atom can be represented by the valence shell and a core•A core consists of all the inner shell and the nucleus

Carbon atom:-valence shell – 4 e-inner shell – 2 e

Nucleus:-6 protons-6 neutrons

+6 for the nucleus and -2 for the two inner-shell electrons(net charge +4)

Conductors

• material that easily conducts electrical current.

• The best conductors are single-element material (e.g copper, silver, gold, aluminum)

• Only one valence electron very loosely bound to the atom- free electron

Insulators

• material does not conduct electrical current

• valence electron are tightly bound to the atom – very few free electron

Semiconductors

• material between conductors and insulators in its ability to conduct electric current

• in its pure (intrinsic) state is neither a good conductor nor a good insulator

• most common semiconductor- silicon(Si), germanium(Ge), and carbon(C) which contains four valence electrons.

Energy Bands

1.2 Semiconductors, Conductors, and Insulators (cont.)

Energy Bands

1-2 Semiconductors, Conductors, and Insulators (cont.)

• Energy gap-the difference between the energy levels of any two orbital shells• Band-another name for an orbital shell (valence shell=valence band)• Conduction band –the band outside the valence shell where it has free electrons.

Comparison of a Semiconductor Atom & Conductor Atom

A Copper atom:• Only 1 valence electron• A good conductor• Electron conf.:2:8:18:1

A Silicon atom:• 4 valence electrons• A semiconductor• Electron conf.: 2:8:4

14 protons14 nucleus10 electrons in inner shell

29 protons29 nucleus28 electrons ininner shell

Covalent bonding – holding atoms together by sharing valence electrons

To form Si crystalsharing of valence electronproduce the covalent bond

1-3 Covalent Bonding

Result of the bonding:

1. The atom are held together forming a solid substrate.

2. The atoms are all electrically stable, because

their valence shells are complete.3. The complete valence shells cause the

silicon to act as an insulator-intrinsic (pure) silicon.

In other word, it is a very poor conductor.

• Covalent bonding in an intrinsic or pure silicon crystal. An intrinsic crystal has no impurities.

Covalent bonds in a 3-D silicon crystal

Figure 1-10 Energy band diagram for a pure (intrinsic) silicon crystal with unexcited (no external energy such as heat) atoms. There are no electrons in the conduction band. This condition occurs only at a temperature of absolute 0 Kelvin.

Figure 1-11 Creation of electron-hole pairs in a silicon crystal. Electrons in the conduction band are free (also called conduction electrons).

Absorbs enough energy (thermal energy)to jumps

a free electron andits matching valence band hole – electron-hole pair

Recombination-when a conduction electron loses energy and fall back into hole in valence band

Figure 1-12 Electron-hole pairs in a silicon crystal. Free electrons are being generated continuously while some recombine with holes.

Figure 1-13 Electron current in intrinsic silicon is produced by the movement of thermally generated free electrons.

Electron current

Apply voltage

freeelectrons

When a voltage is applied, free electrons are free to move randomly and attracted toward +ve end. The movement of electrons is one type of current in semiconductor and is called electron current.

Figure 1-14 Hole current in intrinsic silicon.

movementof holes

Trivalent Impurities:

• Aluminum (Al)

• Gallium (Ga)

• Boron (B)

• Indium (In)

Pentavalent Impurites:

• Phosphorus (P)

• Arsenic (As)

• Antimony (Sb)

• Bismuth (Bi)

Doping - The process of creating N and P type materials

- By adding impurity atoms to intrinsic Si or Ge to improve the

conductivity of the semiconductor

- Two types of doping – trivalent (3 valence e-) & pentavalent (5 valence e-)

p-type material – a semiconductor that has added trivalent impurities

n-type material – a semiconductor that has added pentavalent

impurities

N-type semiconductor:Pentavalent impurities are added to Si or Ge, the result is an increase of free electrons1 extra electrons becomes a conduction electrons because it is not attached to any atomNo. of conduction electrons can be controlled by the no. of impurity atomsPentavalent atom gives up an electron -call a donor atomCurrent carries in n-type are electrons – majority carriersHoles – minority carriers (holes created in Si when generation of electron- holes pair.

Pentavalent impurity atom in a Si crystal

Sb impurity atom

P-type semiconductor:- Trivalent impurities are added to Si or Ge to increase number of holes.- Boron, indium and gallium have 3 valence e- form covalent bond with 4 adjacent silicon atom. A hole created when each trivalent atom is added.- The no. of holes can be controlled by the no. of trivalent impurity atoms- The trivalent atom can take an electron- acceptor atom- Current carries in p-type are holes – majority carries- electrons – minority carries (created during electron-holes pairs generation).

Trivalent impurity atom in a Si crystal

B impurity atom

- Diode is a device that conducts current only in one direction.

- n-type material & p-type material become extremely useful when

joined together to form a pn junction – then diode is created

- before the pn junction is formed -no net charge (neutral) since no of proton and electron is equal in both n-type and p-type.

-p region: holes (majority carriers), e- (minority carriers)

-n region: e- (majority carriers), holes (minority carriers)

Summary:When an n-type material is joined with a p-type material:1. A small amount of diffusion occurs across the junction.2. When e- diffuse into p-region, they give up their energy and fall

into the holes near the junction.3. Since the n-region loses electrons, it creates a layer of +ve

charges (pentavalent ions).4. p-region loses holes since holes combine with electron and will

creates layer of –ve charges (trivalent ion). These two layers form depletion region.

5 Depletion region establish equilibrium (no further diffusion) when total –ve charge in the region repels any further diffusion of electrons into p-region.

Barrier Potential:

In depletion region, many +ve and –ve charges on opposite sides of pn junction.

The forces between the opposite charges form a “field of forces "called an electric field.

This electric field is a barrier to the free electrons in the n-region, need more energy to move an e- through the electric field.

The potential difference of electric field across the depletion region is the amount of voltage required to move e- through the electric field. This potential difference is called barrier potential. [ unit: V ]

Depends on: type of semicon. material, amount of doping and temperature. (e.g : 0.7V for Si and 0.3 V for Ge at 25°C).

Overlapping

Energy level for n-type (Valence and Cond. Band) << p- type material (difference in atomic characteristic : pentavalent & trivalent) and significant amount of overlapping.

Free e- in upper part conduction band in n-region can easily diffuse across junction and temporarily become free e- in lower part conduction band in p-region. After crossing the junction, the e- loose energy quickly & fall into the holes in p-region valence band.

As the diffusion continues, the depletion region begins to form and the energy level of n-region conduction band decreases due to loss of higher-energy e- that diffused across junction to p-region.

Soon, no more electrons left in n-region conduction band with enough energy to cross the junction to p-region conduction band.

Figure (b), the junction is at equilibrium state, the depletion region is complete diffusion has ceased (stop). Create an energy gradient which act as energy ‘hill’ where electron at n-region must climb to get to the p-region.

The energy gap between valence & cond. band – remains the same

No electron move through the pn-junction at equilibrium state.

Bias is a potential applied (dc voltage) to a pn junction to obtain a desired mode of operation – control the width of the depletion layer.

Two bias conditions : forward bias & reverse bias

Depletion Layer Width

Junction Resistance

Junction Current

Min Min Max

Max Max Min

The relationship between the width of depletion layer & the junction current

1. Voltage source or bias connections are + to the p region and – to the n region.

2. Bias voltage must be greater than barrier potential (0 .3 V for Germanium or 0.7 V for Silicon).› The depletion region

narrows.› R – limits the current

which can prevent damage to the diode

Diode connection

The negative side of the bias voltage push the free electrons in the n-region -> pn junction. Flow of free electron is called electron current.

Also provide a continuous flow of electron through the external connection into n-region.

Bias voltage imparts energy to the free e- to move to p-region.

Electrons in p-region loss

energy-combine with holes in valence band.

1.7 Biasing The Diode (cont.)Forward bias

Since unlike charges attract, positive side of bias voltage source attracts the e- left end of p-region.

Holes in p-region act as medium or pathway for these e- to move through the p-region.

e- move from one hole to the next toward the left.

The holes move to right toward the junction. This effective flow is called hole current.

Flow of majority carries and the voltage across

the depletion region

As more electrons flow into the depletion region, the no. of +ve ion is reduced.

As more holes flow into the depletion region on the other side of pn junction, the no. of –ve ions is reduced.

Reduction in +ve & -ve ions – causes the depletion region to narrow.

Electric field between +ve & -ve ions in depletion region creates “energy hill” that prevent free e- from diffusing at equilibrium state -> barrier potential

When apply forward bias – free e- provided enough energy to climb the hill and cross the depletion region.

Electron got the same energy = barrier potential to cross the depletion region.

An add. small voltage drop occurs across the p and n regions due to internal resistance of material – called dynamic resistance – very small and can be neglected

Reverse bias - Condition that prevents current through the diode

Voltage source or bias connections are – to the p material and + to the n material

Current flow is negligible in most cases. The depletion region widens than in forward bias.

Diode connection

+ side of bias pulls the free electrons in the n-region away from pn junction cause add. +ve ions are created, widening the depletion region.

In the p-region, e- from – side of the voltage source enter as valence electrons e- and move from hole to hole toward the depletion region, then created add. –ve ions.

As the depletion region widens, the availability of majority carriers decrease.

• Extremely small current exist – after the transition current dies out caused by the minority carries in n & p regions that are produced by thermally generated electron hole pairs. • Small number of free minority e- in p region are “pushed toward the pn junction by the –ve bias voltage.• e- reach wide depletion region, they “fall down the energy hill” combine with minority holes in n -region as valence e- and flow towards the +ve bias voltage – create small hole current.• The cond. band in p region is at higher energy level compare to cond. band in n-region e- easily pass through the depletion region because they require no additional energy.

-When a forward bias voltage is applied, there is current called forward current, IF .

-In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small.

-Increase forward bias voltage – current also increase.

FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

- With the applied voltage exceeding the barrier potential (0.7V), forward current begins increasing rapidly.

- But the voltage across the diode increase only gradually above 0.7 V. this is due to voltage drop across internal dynamic resistance of semicon material.

1.8 Voltage-Current Characteristic of a Diode (cont.)V-I Characteristic for Forward Bias

FIGURE 1-26 Forward-bias measurements show general changes in VF and IF as VBIAS is increased.

-Plot the result of measurement in Figure 1-26, you get the V-I characteristic curve for a forward bias diode

- Increase to the right

- increase upward

-After 0.7V, voltage remains at 0.7V but IF increase rapidly.

-Normal operation for a forward-biased diode is above the knee of the curve.

1.8 Voltage-Current Characteristic of a Diode (cont.)V-I Characteristic for Forward Bias

FFd IVr /'

dynamic resistance r’d decreases as you move

up the curve

FV

FI

VVF 7.0

zerobias

VVF 7.0

Below knee, resistance is greatest since current increase very little for given voltage,Resistance become smallest above knee where a large change in current for given change in voltage.

1.8 Voltage-Current Characteristic of a Diode (cont.)

V-I Characteristic for Reverse Bias

Reverse Current

- VR increase to the left along x-axis while IR increase downward along y-axis.

- When VR reaches VBR , IR begin to increase rapidly.Breakdown voltage, VBR.

- not a normal operation of pn junction devices.

- the value can be vary for typical Si.

- Cause overheating and possible damage to diode.

1.8 Voltage-Current Characteristic of a Diode (cont.)

The Complete V-I Characteristic Curve

Combine-Forward bias & Reverse bias CompleteV-I characteristic curve

1.8 Voltage-Current Characteristic of a Diode (cont.)Temperature Effects on the Diode V-I Characteristic

Forward biased diode : for a given value of

Barrier potential decrease as T increase.

For reverse-biased, T increase, IR increase.

Reverse current breakdown – small & can be neglected

FI,T

FV

Direction of current

cathodeanode

DIODE MODEL

The Ideal Diode Model

The Complete Diode Model

The Practical Diode Model

Ideal model of diode- simple switch:

•Closed (on) switch -> FB

•Open (off) switch -> RB

• Barrier potential, dynamic resistance and reverse current all neglected.

• Assume to have zero voltage across diode when FB.

VVF 0

LIMIT

BIASF R

VI

•Forward current determined by Ohm’s law

BIASR

R

VV

AI

0

•Adds the barrier potential to the ideal switch model

• ‘ is neglected

•From figure (c):

The forward current [by applying Kirchhoff’s voltage law to figure (a)]

By Ohm’s Law:

dr '

•Equivalent to close switch in series with a small equivalent voltage source equal to the barrier potential 0.7V

•Represent by produced across the pn junction

FV

•Open circuit, same as ideal diode model.

•Barrier potential doesn’t affect RB

)(3.0

)(7.0

GeVV

SiVV

F

F

0LIMITRFBIAS VVV

LIMITFR RIVLIMIT

LIMIT

FBIASF R

VVI

BIASR

R

VV

AI

0

Complete model of diode consists:

•Barrier potential

•Dynamic resistance,

•Internal reverse resistance,

•The forward voltage consists of barrier potential & voltage drop across r’d :

•The forward current:

dr '

Rr '•acts as closed switch in series with barrier potential and small

dr '

Rr '

•acts as open switch in parallel with the large '7.0 dFF rIVV

'

7.0

dLIMIT

BIASF rR

VVI

10V10V

1.0kΩ1.0kΩ

5V5V

1.0kΩ1.0kΩ

(1) Determine the forward voltage and forward current [forward bias] for each of the diode model also find the voltage across the limiting resistor in each cases. Assumed rd’ = 10 at the determined value of forward current.

a)a) Ideal ModelIdeal Model::

b)b) Practical ModelPractical Model::

(c) (c) Complete model:Complete model:

VARIV

mAV

R

VI

V

LIMITFR

BIASF

F

LIMIT10)101)(1010(

101000

10

0

33

VARIV

mAVV

R

VVI

VV

LIMITFR

LIMIT

FBIASF

F

LIMIT3.9)101)(103.9(

3.91000

7.010)(

7.0

33

VkmARIV

mVmAVrIVV

mAk

VV

rR

VVI

LIMITFR

dFF

dLIMIT

BIASF

LIMIT21.9)1)(21.9(

792)10)(21.9(7.07.0

21.9101

7.0107.0

'

'

Diodes come in a variety of sizes and shapes. The design and structure is

determined by what type of circuit they will be used in.

- Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode.

K A A K

- With the diode check function a good diode will show approximately 0.7 V or 0.3 V when forward biased. - When checking in reverse bias, reading based on meter’s internal voltage source. 2.6V is typical value that indicate diode has extremely high reverse resistance.

-When diode is failed open, open reading voltage is 2.6V or “OL” indication for forward and reverse bias.

-If diode is shorted, meter reads 0V in both tests. If the diode exhibit a small resistance, the meter reading is less than 2.6V.

Select OHMs range

Good diode:

Forward-bias: get low resistance reading (10 to 100 ohm)

Reverse-bias: get high reading (0 or infinity)

P-materials are doped with trivalent impurities

N-materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction.

A diode is nothing more than a PN junction.

At the junction a depletion region is formed. This creates barrier which requires approximately 0.3 V for a Germanium and 0.7 V for Silicon for conduction to take place.

Diodes, transistors, and integrated circuits are all made of semiconductor material.

The voltage at which avalanche current occurs is called reverse breakdown voltage. Reverse breakdown voltage for diode is typically greater than 50V.

There are three ways of analyzing a diode. These are ideal, practical, and complete. Typically we use a practical diode model.

A diode conducts when forward biased and does not conduct when reverse biased

There once was a wise man that was known throughout the land for his wisdom. One day a young boy wanted to test him to prove that the wise man a fake.

He thought to himself, “I will bring one live bird to test the old man. I will ask him whether the bird in my hand is dead or alive. If he says that it is alive, I will squeeze hard to kill the bird to prove that he is wrong.

On the other hand if he says that it is dead, I will let the bird fly off, proving that he is wrong. Either way the wise man will be wrong.”

With that idea in mind, he approached the wise man and asked, “Oh wise man, I have a bird in my hand. Can you tell me if the bird is dead or alive?”.

The wise man paused for a moment and replied, “Young man, you indeed have a lot t learn. That which you hold in your hand, it is what you make of it. The life of the bird is in your hand.

If you wish it to be dead, then it will die. On the other hand if you desire it to live, it will surely live”. The young boy finally realized that the answer given was indeed that of a man of wisdom.

Our dreams are very fragile, just like the little bird. It is our own decision, if we decide to kill it, or allow others to steal it away from us. However, it is also our own choice to nurture it and let it grow to fruition. Success comes to those who allow their dreams to fly high, just like the little bird, which will soar into the sky if the young boy released it from his grasp.