Basics of Semiconductor Physics - KMUTTwebstaff.kmutt.ac.th/~werapon.chi/M2_3/1_2015/ENE... · Basics of Semiconductor Physics Outline • Electrons & Electron Devices. • Si dtBiSemiconductor

8/10/2015

Module Electronics 1

Basics of Semiconductor Physics

Outline• Electrons & Electron Devices.

S i d t B i• Semiconductor Basics. • PN Junction and Its Electrical Characteristics.

KMUTT: Electronics 1

Basics of Semiconductor Physics

Outline• Electrons, electron devices & basic required functions

I t d i l t d f i d• Introducing electrons and friends • Free electron generation

• Diode & triode (vacuum state)

• Semiconductor basics

• PN junction

• Biasing• Biasing

• Electrical characteristics of PN junction

• Semiconductor diodes• Review questions

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Vacuum Tubes (Thermionic Valves)

Various types of miniature and subminiature tubes

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Uses of Vacuum Tubes

Prototypes of Fleming valve (diode) between 1904 and 19191904 and 1919

•Vacuum tubes may be used for rectification, amplification, switching, or similar processing or creation of electrical signals.

• Vacuum tubes rely on thermionic emission of electrons from a hot filament or cathode.

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Introducing Electrons and Friends

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Introducing Electrons and Friends (II)

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Introducing Electrons and Friends (III)

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Ions and Ionization

• The process of producing ions is known as ionization

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Kinetic Energy- the Energy of Motion

• When you cause that ball to move, you give it a certain amount of energy.

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Bombardment

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Electron Emission


Electron Emission by Energy Conversion


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Photoelectric Emission


Secondary Emission


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Cold Cathode Emission


Thermionic Emission

• Thermionic emission is the heat induced flow of charge• Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier.

• Charge carriers can be electrons (in case of thermionic emission) or ions.


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Diode tubes

I

• Current I flows in onedirection.


Inside the Diode


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Diode: Effects of External Voltage

A. High-speed electrons arriving at the plate even no potential is applied

h l

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to the plate.

B. No plate current when the plate potential is ().

C. Plate current is greater than that of (A) when the plate potential is (+).

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V-I Characteristic of Diode

V I h t i ti i i t t• V-I characteristic is very important.

• Cathode temperature greatly affects electrical characteristics of diode tubes.

• Number of electrons is limited at a certain plate voltage


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AC Signal Applied to Diode Plate (Anode)

input output

• Only the positive part of AC signal passes through the diode. The negative part is blocked.

•This action called rectification (AC to DC converter).KMUTT: Electronics 21

Problems of Diode

• Diode can be used as a switch (Fully ON & Fully OFF)• We are looking for the devices that have controllable abilityability.• The controllable ability must be done in electronically forms, i.e., voltage or current controlled one.


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Audion or Triode Tubes

18 3 1961

• Dr. Lee De Forest (Yale University) was granted a US patent in 1908 as Space Telegraphy (US Patent 879,532).• Triode was a much more sensitive detector of radio waves.

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1873-1961

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Control Grid : Use of the Third Electrode / Element

US879532

• A third electrode, a grid is placed between plate and cathode in modern triodes.

• Triodes can be used as amplifier or switch.24KMUTT: Electronics

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The Triode Operating Potentials

• The grid voltage has more influence than the plate voltage.• Why ?


Inside a Triode


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Inside a Triode (II)


Static Characteristics of a Triode


- (Right) Plate characteristic curves- (Left) Transfer (mutual) characteristic curves show the

transconductance gm in the slope.

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Controlled Sources: Linear Application

• The variation in the grid-cathode voltage at the input section results the variation in plate current at the output section.


Electronic Switch: Computer Applications

• Vacuum tubes, which could be used for switching, have made electronic computing possible for the first time. • But the cost and relatively short mean time between failures of tubes were limiting factors (Failures are due to ( a u es a e due ofilament burnt-out, losses of vacuum) .

The 1946 ENIAC computer used 17,468 vacuum tubes and consumed 150kW of power.


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Cathode Ray Tube: CRT

• We can use the principle of controlled electron for display devices.• The screen is coated with phosphors which fluoresces (emitting visible light) when bombarded with electrons .• CRT are used in TV, oscilloscopes and displays . Compared with LEDs and plasma tubes , CRT’s have better light quality, contrast and view angle.


Problems of Vacuum Tubes

• Vacuum tubes require large heat (thermal energy) to generate free electrons.• Vacuum tubes have limited life spanVacuum tubes have limited life span.

- About 600 hrs for large high power types- About 100,000 hrs for small types

• Big sizes of vacuum tubes


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Basic Semiconductor Physics

Materials’ ability to conduct electricity fall between 2 extremes :

-Conductor : supports a large flow of charges when a voltage source of limited magnitude is applied across its terminals (Conductors are mostly metals) - Insulator : has a very low conductivity under an applied voltage. Examples of insulators are mica, rubber, glass, and etc.

Key factor is the conductance G of a material (A/V) or which is reciprocal of resistance, 1/R


2 Forms of Semiconductors

Materials :- Elements : C, Si, and GeGe

- Compounds GaAs, SiC, etc.


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Resistivity : ρ (Greek letter “rho” )

If the material has a cube shape having 1-cm2 area and 1-cm long then


Typical Resistivity of 3 Types of Materials

- Purity of materials is very important.

- Resistivity of conductors and insulators differ by 1018 times.

- Modern semiconductor manufacturing methods can result in a very high purity.1 unwanted atoms per 10 billion (1:10,000,000,000) Si atoms for intrinsic semiconductors(1:10,000,000,000) Si atoms for intrinsic semiconductors

- Light & heat can greatly influence resistivity of semiconductors, but not metals or insulators.


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Comparison Between Atoms ofMetals and Semiconductors

Si atom has 4 valence electrons

Silicon, Si Copper, Cu

-Si atom has 4 valence electrons

-Cu atom has only one valence electron


Simplified Model of Covalent Bonding

-Each Si atom, having 4 valence electrons, shares it electrons with 4 adjacent Si atoms. -Silicon crystal has 3-dimensional diamond structure.


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Temperature Affects Resistance

NTC

• Semiconductor materials such as Ge and Si that

show a reduction in resistance with increase in temperature are said to have a negative temperature coefficient (NTC).( )

• Metals has positive temperature coefficients (increase in resistance with increase in temperature) • Why?


Energy Level Diagram

•The larger the distance of an electron is from the nucleus, the higher the energy state of that electron,• Any electron that has left its parent atom has a higher energy state than electrons confined in the atomi.

Quantum physics 40KMUTT: Electronics

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Energy Band Model of Conductor, Insulator, Semiconductor

Fermi level

- Valence electrons are bound by attraction with nucleus and are inside an atom. They have energy in the energy range/band called “valence energy band - VB”.They have energy in the energy range/band called valence energy band VB .

- Electrons that conduct electricity must escape from an atom. They become

“free electrons or conduction electrons”. They have higher energy and are in

“ conduction energy band – CB”.

- Input energy is needed to push valence electrons to escape an atom. Or we

say that we need input energy to excite electrons from VB to CB.

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Energy Band Model of Conductor, Insulator, Semiconductor

Fermi level

C d t h- Conductors have no energy gap or narrow energy gap.

-Insulator has large energy band gap (higher than 5 eV). At room ordinary , no electrons can reach the conduction band.

- In semiconductors, thermal energy can excite electrons to cross the energy gap, about 0.5-1.5 eV).

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Intrinsic Semiconductor Materials

• At very low temperature, the covalence bond is complete, not broken. • At room temperature, thermal energy ( ib ti f t i l t t id(vibration of atoms, moving electrons outside atoms) breaks covalent bonds and create free electrons (charge carriers).• Conductivity increases or resistivity decrease with increasing temperature• The absence of electron creates “Hole”.• If an electron moves away from one directionIf an electron moves away from one direction, this is equivalent to a hole moving in opposite direction.• Recombination Process (electrons and holes recombine)


Model of Atom Structure of Intrinsic Materials

* electron-hole pair, No. of hole = No. of free electron !

-The vacancy is called a hole and is represented by a small circle or positive sign due to the absence of a negative charge.

- Recombination occurs when an electron in CB losses energy, falls back into a hole in VB. Electron and hole then recombine.


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Electron Flow - Hole Flow – Current Flow

• Hole will be created in the covalent bond that has released the electronthe electron.• The direction to be used is that of a conventional flow, which is indicated by the direction of a hole flow. Electron flows is in opposite direction of conventional current flow(positive charge flow).


Extrinsic Materials

• Pure or intrinsic semiconductor has low carrier concentration (ni = 1010 cm-3 at 300 K). In Si there is about 1free electron for every 1012 atoms (1 to 109 for Ge). •But it is a good starting point.g g• A semiconductor material that selected impurities has been added, or subjected to the doping process, is called an extrinsic semiconductor.• There are 2 types of extrinsic semiconductors.• n-type and p-type materials n => negative charge carriers p => positive charge carriers

S f G G• Selected impurities are from Group III or Group V elements.

• Group V atoms donate electrons, and called donors.• Group III atoms accept electrons, and called acceptors.


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Model of Atom Structure of Extrinsic Materials

n-type p-type

• n-type: Donor atoms have 5 valence electrons , e.g. Sb, As, and P.

• p-type: Acceptor atoms have 3 valence electrons ,e.g. B, Ga, and In.• On average, one donor atom will create one extra electron, one acceptor create one hole.

Or one impurity atom create one extra charge carrier. 47KMUTT: Electronics

Effect of Donor Impurities on Energy Band Structure

• Donors create a new energy level below bottom of a conduction bandband.

• Acceptors create a new energy level above top of a valence band.

• Impurities also change Fermi energy EF.

EF is an indicator on highest energy of electrons in atom


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Effect of Donor Impurities on Semiconductor Charge Carriers and Conductivity

• Si has about 5 x1022 atoms/cm-3. At room temperature, Si

has about 1.45x1010 conduction electrons/cm-3.

• If we dope Si with donors at a doping level of 1 in 10 million

(1:107), i.e. one donor atom for 10,000,000 Si atoms,

there will be 1015 donor atoms / cm-3 , and 1015 additional conduction electrons cm-3. (1022 / 107 = 1015 )

• Conduction electrons (charge carriers) increase from

1.45x1010/cm-3 to (1.45x1010/cm-3 )+( 1015 /cm-3 ) , an increase of 1015 times.

• Conductivity increases by 1015 times.

• Doping can greatly influence semiconductor conductivity. 49KMUTT: Electronics

Majority and Minority Carriers

• In n-type material, the electron is the majority carrier, and the hole the minority carrier. y

There are more electrons than holes. More (-ve) carriers than (+ve) carriers.

• In p-type material, the hole is the majority carrier and the electron is the minority carrier.


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Diffusion and Drift (I)

• Diffusion is associated with random motion due to thermal agitationthermal agitation.

• Carriers such as holes (and electrons) will diffuse from the region of high concentration to the region of lowconcentration.


Diffusion and Drift (II)

n-Type

• Carrier drift occurs when an electric field is applied ppacross a piece of silicon.


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Just a Resistor

R

- We cannot made useful electronic devices with only single n type nor p type materials!single n-type nor p-type materials!


P-N Junction

• PN junction made by chemical processes not mechanicalPN junction made by chemical processes not mechanical ones.

• An area near the junction that has no electrical carriers, called “Depletion region” or “Space-charge region”.


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PN Junction in Thermal EquilibriumID

IS

• (-) sign in p-type and (+) in n-type materials are “uncovered / ionized”

impurity atoms. They are not neutral anymore and exhibit net charges.

• Potential difference across depletion region is called Built-in Potential

Barrier Vbi.

• Diffusion Current ID equals to Drift Current IS.55KMUTT: Electronics

Energy Diagrams for P-N Junction

The energy level CB in n region decreases- The energy level CB in n-region decreases.

- There is an "energy hill / energy barrier " that electrons in an n-region must climb to get into the p region

- Energy gap between the VB and the CB remains the same.


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P-N Junction Biasing

Forward bias Reverse bias

- Biasing is a method to apply an external voltage across the p-n junction.

- 2 types of biasing:2 types of biasing:

: Forward Bias when p-type side is connected to a higher voltage than n-type, and

: Reverse Bias when p-type side is connected to a lower voltage than n-type


Reverse Bias

• (-) of VR pull holes in p type while (+) of VR pull free electrons in n type

material.

• Reverse bias increases the depletion region.


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Reverse Bias (continued)

• In practice, current source or voltage source with series resistor is used to test the pn junction.

• IS – ID = I


Forward Bias

• Electric field EA induced by vD is in opposite direction to E A y D ppfield in depletion region.

• The net electric field inside the depletion region is always from n- to p-region.


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Forward Bias (continue)

• To test forward bias in pn junction, current limit is a must.• ID - IS = I


V-I Characteristic of pn Junction

- Normally in reverse bias,value of leakage current is very small.- However, special cases called “break down” occur if reverse bias voltage is lower than a certain value (VBR). Large amount current can be conducted.•Zener effect•Avalanche multiplication•Mixed


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V-I Characteristic of Sources

- V-I curve of voltage sources (VS) have a slope of infinite value.value.

- V-I curve of current sources (CS) have a slope of zero value.

- V-I curve of resistor has the range of slope between VS and CS, and passes the origin.


Thermal Effect on VI Characteristics

- In Forward Bias, forward voltage drop decreases with increasing temperature.

Siincreasing temperature.

This is desirable property

- However, increasing temperature also increases the leakage current in Reverse Bias.

This is undesirable effect.


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Semiconductor Diode Constructions

• Diode is the most common application of a pn junction.

• Diode for very high frequency applications still use point contact structure.• Modern diodes use planar construction.


Semiconductor Diode Constructions (cont.)

• Small piece of semiconductor is used


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Packages of Practical Diodes

• There are large varieties of diode packages.• Large power diodes have metal tap (heat sink connections).


Light-Emitting Diodes

• When an electron recombines/ meets a hole, it falls into a lower energy level .• Energy is released in the form of a photon.

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http://en.wikipedia.org/wiki/Light-emitting_diode

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Light-Emitting Diodes (continued)

• The wavelength of the light emitted, and thus its color, depends

on the band gap energy of the materials forming the pn junction.

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http://en.wikipedia.org/wiki/Light-emitting_diode

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Red LED (GaP) Inventor

• Invented by Nick Holonyak,Jr. (University of Illinois at Urbana-Champaign) at GE Lab. in 1962.• He also invents red-light semiconductor laser, usually called laser diode.• p-n-p-n switch• With Milton Feng, he announced the “laser transistor” for “optical computers”, which is 100 or more p ,times faster than present computers.

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1928 - present

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Review Questions

• What are electronic switch and controlled sources?• What is a semiconductor?• What are element & compound semiconductors ?• What are element & compound semiconductors ?• What is a hole?• What are 2 categories of semiconductors ?• What is doping?• Why is the need to add impurities?• How is a pn junction formed?• What is biasing?• What is the temperature effect to on a pn junction?


Documents

Basics of Semiconductor Physics - KMUTTwebstaff.kmutt.ac.th/~werapon.chi/M2_3/1_2015/ENE... · Basics of Semiconductor Physics Outline • Electrons & Electron Devices. • Si dtBiSemiconductor