physical basis of semiconductors - lecture notes - week 0

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Semiconductor Devices 2015

I. Shih

Physical Basis of Semiconductor Devices

ECSE 533

- Evolution of electronic devices- Crystal structures- Directions and planes


I. Shih

Electrical and Computer Engineering

Physical Basis of Semiconductor Devices Course ECSE 533 Winter 2015

Instructor : I. Shih Phone : 398-7147Office: MC707E-mail: [email protected]

Lecture: ENGTR 0070, M W 08:35 – 09:55

Prerequisite: ECSE 330, ECSE 351, and PHYS 271

TA: TBD (Office hour: Th 13:30-14:30 MC707)

Course web: webCTCourse Description:Quantitative analysis of diodes and transistors. Semiconductor fundamentals, equilibrium and non-equilibrium carrier transport, and Fermi levels. PN junction diodes, the ideal diode, and diode switching. Bipolar Junction Transistors (BJT), physics of the ideal BJT, the Ebers-Moll model. Field effect transistors, metal-oxide semiconductor structures, static and dynamic behaviour, small-signal models.

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I. Shih

Electrical and Computer Engineering


Course ECSE 533 Winter 2015

Text book:

Semiconductor Physics and Devices, IrwinDonald Neamen


I. Shih

Grading:

ECSE 533

Assignments 15%

Mid-term 30%

Final Exam --

Project* 55%

*Project topics will be announced in late January

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Main areas of semiconductor devices and circuits

-- Semiconductor materials

-- Processing technology

-- Electronic devices

-- Circuit design


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Prior to 1950s, the electronic industry was dominated by vacuum technology. Components for vacuum technology require:

- Hot electrode (about 800oC)- Vacuum container (<10-5 torr)- High operating voltages (200 volts)- High power consumption (about 1 W)

- Low packaging density (diameter about 1 cm)- Low functionality

Results:

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Non-linear device

Linear device

Voltage

Current

Non-linear devices are required for signal amplification and processing:


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The operation of a vacuum tube is based on the emission of electrons from a conductor.

Energy

Distance0

Work function

Principles of a vacuum tube:

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Low T Metal

High T Metal

Non-symmetrical I-V characteristics are obtained when the voltage is reversed.

e-

+ -

V

I

Rate of electron emission from a hot metal surface is higher than that from a cold surface (under same electric field).


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This is because the same device needs to act as a “source of electrons” and a “sink of electrons” to achieve charge manipulation.

I

V

Source of electrons

Sink of electrons

Non-linear “characteristics” are required for active devices, for electronic data manipulation.

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The electrons near the metal surfaces need to gain an energy equal to or greater than the “work function” in order to escape the metal surface. Total energy of electrons in the hot W is greater => larger emission rate.

Cold W Hot W

electrons

(emitter)Thermionic Emission of electrons from a metal


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Pressure less than 10-5 torrTemperature of heater > 700oC

I

V

V

I

+

-Heater

Ni anode

W cathode

Glass vacuum tube

~ 2 cm

A vacuum diode:

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Vacuum tubes for high frequency operation

Audio tube

High power tube


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d 0

V decreases

T1 T2

Appropriate vacuum packages can not be developed easily.

Solutions: Solid state devices

[1] Semiconductor devices => electronic and optoelectronic devices

[2] Superconductor devices

-- Can not be implemented by batch fabrication processes.

-- Operating voltage can not be reduced easily

Main problem with the vacuum tubes:for constant

E

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- Light weight

- Low power consumption

- High operation speed

- No maintenance

- Low ownership cost

Advantages of semiconductor devices:


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- P-N junctions

- JFETs

- MOSFETs

- BJTs

- ICs

- Opto-electronic devices

- Sensors

Semiconductor electronic devices

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- Semiconductors

- P-N junctions

- MOSFETs

- BJTs

- Introduction to ICs

Main Objectives


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Crystalline solids

Structures: S.C.F.C.C.B.C.C.Diamond

Crystal growth

Energy bands

MetalsInsulatorsSemiconductors

Random velocityn, vd, μ

Fabrication technology

(ECSE 485, 545)

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Equilibrium conditions

Non-equilibrium conditions

Intrinsic semiconductor

Extrinsic semiconductor

Carrier recombination

ni, Efi, NC, NV

Hydrogen modelImpurity effects(donors and acceptors)Ionization energy

Direct Indirect

Carrier lifetime

Distribution of electrons in semiconductors (Fermi-Dirac function):


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Intrinsic semiconductor

Extrinsic semiconductor

Carrier recombination

ni, Efi, NC, NVHydrogen modelImpurity effects Direct Indirect

PN junctions BJTsLasers MOSFETs

Integrated circuits

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Introduction to fabrication technology

PN junction electrostatics and I-V

BJTs MOSFETs MESFETsOptoelectronic devices

Fabrication technology

Process simulation

Superconductor electronics

Circuit and system design

Vacuum microelectronics

Electronic displays

Electron Devices & Basic Electronic Physics


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ECSE 533

- Evolution of electronic devices- Crystal structures- Directions and planes

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Single crystals are required for semiconductor devices and

integrated circuits !

Three forms of solids:

amorphous

poly crystal

single crystal


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Electrical properties and uniformity of a single crystal can be controlled more precisely than amorphous materials or poly-crystals:

Carrier densityCarrier mobilityConductivityLifetime

Single crystals are needed for VLSI circuit fabrication.

Amorphous Si and poly-crystal films are used for large area semiconductor devices such as photovoltaic solar cells and electronic displays.

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1 2

light heat temp. resistance

1

2

light

light carriers voltage or current

Optical sensors


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Homogeneous Solids

Crystalline solids Amorphous solids

Single crystals

Poly-crystals Consist of several crystallites

Useful electronic devices and circuits are fabricated using single crystals

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[1] Amorphous Solid

[3] Monocrystalline Solid

[2] Polycrystalline Solid

Aggregation of atoms without long range order.

Atoms with long range order but with grains.

With long range order and without grains.

Three forms of solids:


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Amorphous S.C.

Poly-crystal

Single crystals

Grain boundaries

No grain boundaries

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A

BC

A BCOrientation

Surface Free Energy-- Atoms can deposit

more easily on a surface with a low free energy.

-- Crystals with definite shape are formed from melt or vapour sources.


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- Selection of materials- Purification of materials- Selection of growth methods- Preparation of samples (surfaces)

- Fabrication of devices- Circuits- Systems- Applications

Growth of single crystals

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SiO2 + impurities

distillation

Si + impurities

SiCl4 + impurities

SiCl4

+H2

Si Solid

99.99995% pure

Crystal Growth

SiMaterial

purification

95% pure Poly-crystals

Single crystals

Single crystals for devices


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Requirements:

[1] Controlled and stabilized temperature distribution (gradient).

[2] Precision pulling mechanism.

T

Tm Crystal

Melt

v

Czochralski method

hr

cmv 21

Methods for crystal growth:

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Si crystal

Si crystal with 30 cm diameter


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- A single crystal consists of a large No. of atoms arranged in special and regular 3-D order => crystal lattices

- The types of crystal lattices are limited

- Crystals formed by elements in the same group of the periodic table may have the same lattice structures

- The lattice type of a given crystal can be specified by the nature of symmetry (operations)

IVCSiGeSn

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[1] Rotational Symmetry

They are classified by structural symmetries:

No. of rotational operations in 360o is called the No. of rotational symmetry. (4-fold rotational symmetry)

1 2

3 4 1

2

3

4

90o

Equivalent !

The No. of categories of crystal structures is limited.

Crystal axis is defined by the number of rotational symmetry.


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ReflectionInversion

1 2

3 4

a a

1 2

3 4Equivalent !

[2] Translation Symmetry

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A crystal often contains very large No. of atoms.

The structural properties of these atoms can be described using a small set of basic No. of atoms.

A generalized primitive unit cell.

atoms

a

bc

a, b, and c are the lattice constants.

Unit cell

Crystals structures important to semiconductor devices:


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Unit Cell – 2D

Unit cell is the smallest portion of a crystal that could be used to reproduce the entire crystal.

¼ of an atom on each of the four corners.

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Unit Cell – 3D

3-D unit cell for a simple cubic crystal.

1/8 atom x 8 corners = 1 atom


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[1] Cubic

[2] Tetragonal

-----------

[3] Hexagonal

Based on the nature of symmetries, crystals can be classified into 7 groups:

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(1) Cubic (2) Tetragonal (3) Orthorhombic

aa

ac

aa b

a

c

One atom at each corner.

Seven crystal structures:


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Simple Cubic

Face-centered Cubic

Body-centered Cubic

Crystals with cubic structures

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Crystal sample

X-ray source

X-ray

Diffracted x-ray beams

X-ray film

Laue x-ray diffraction set-up


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Principles of X-ray diffraction

d1

d

sin2:conditionBragg dn

Orientation of a crystal can be given by “planes” or “directions”.3-D lattice

Constructive interference

Destructive interference

The Miller indices are used to designate the planes and directions.

Bright spots

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Single crystalPolycrystalAmorphous

Laue X-ray diffraction results


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[1-1] Simple Cubic

This is because both Si and GaAs have structures derived from the simple cubic.

One atom at each corner of a simple cubic.

Define a unit cell of a given crystal structure.

No. of atoms in the unit cell is ___.

Simple cubic is the crystal structure most important to semiconductor devices.

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[1-2] Face-centered Cubic

Face-centered cubic

Face-centered cubic is a derivation from the simple cubic. In addition to the eight atoms at the corners, there is one atom at center of each face.



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[1-3] Body-centered Cubic

Body-centered cubic

Body-centered cubic is also a derivation from the simple cubic. In addition to the eight atoms at the corners, there is one atom at center of body.


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Unit Cell – 3D

1/8 atom x 8 corners = 1 atom


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Diamond Structure

[1] 1st FFC lattice

[2] 2nd FCC lattice

The diamond structure consists of two interpenetrating face-centered cubic lattices.

[3] 2nd FCC is displaced by ¼ of the body diagonal

[4] 8 atoms at corners of the cube, 6 atoms at centers of faces and 4 atoms inside the cube.

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Diamond Structure[1] 1st FFC lattice

[2] 2nd FCC lattice

[4] 8 atoms at corners of the cube, 6 atoms at centers of faces and 4 atoms inside the cube.

[5] Each atom has four nearest neighbors: 4 covalent bonds structure for crystals of group elements or 3-5 compounds.


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Each atom has 4 nearest neighbors => 4 covalent bonds.Structure for crystals of group 4 elements or 3-5 (III-V) compounds

Diamond Structure

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Diamond Crystal StructurePeriodic Table

III IV V

B C N

Al Si P

Ga Ge As

In Sn Sb

The most useful elemental semiconductors are from group IV.The most important compound semiconductors are from III-V.All of them have the diamond crystal structure.


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InP III-V compound S.C. wafer

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Crystals with the same structure may be insulators, semiconductors or metals.

Group IV

C

Si

Ge

Sn

Crystals of C insulator

Crystals of Si, Ge semiconductors

Crystals of Sn metal


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Knowledge of crystal orientation is important to S.C. device and ckt performance.

[1] The surface density of atoms is determined by orientation.

[2] For surfaces with different atomic density, the electronic or optical properties is different.

[3] Orientation of a crystal can be determined by X-ray diffraction.

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Principles of X-ray diffraction

d1

d

sin2:conditionBragg dn

Orientation of a crystal can be given by “planes” or “directions”.3-D lattice

Constructive interference

Destructive interference

The Miller indices are used to designate the planes and directions.


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Single crystalPolycrystalAmorphous

Laue X-ray diffraction results

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Miller indices - plane z

y

x

z=

y=x=a

Procedure to obtain indices for a given plane:

[2] Determine intercept of the plane with each axis.

[3] Invert the intercept values.

[4] Convert to the smallest integers.

[5] Enclose the No. in curvilinear brackets.

[1] Establish the coordinate system for the crystal.

x y z

1 0 0 (100)

a

111

a

Plane


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Miller indices – plane[1] Determine intercept of the plane with each axis.



x y z2a 2a 2a

1/2a 1/2a 1/2a

1 1 1 [4] Enclose the No. in curvilinear brackets.

(111)

z

y

x

Plane

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Miller indices – plane with (-) interceptsz

y

x[4] Enclose the No. in curvilinear brackets.

(111)

[1] Determine intercept of the plane with each axis.



x y za -a a

1/a -1/a 1/a

1 -1 1

Plane


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Miller indices – plane summary

On (100), (010), (001), (100), (010) and (001) planes, the density and arrangement of atoms are the same.

Hence, the crystallography, physical and electrical properties are the same.

z

yx

(100) plane

(010) plane

(001) plane

All these planes are equivalent.

{100} is used to represent all of the above equivalent planes.

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Si wafer for VLSI fabrication

z

yx

(100) plane (surface)

Flat edge

[011] direction

Directions in a crystal must be defined


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(100) Si (111) Si

(100) plane

Minor flat

Minor flat

Major flat

Major flat

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Miller indices - directionsz

y

x

Directions in a crystal[1] Draw a vector and take components

[2] Reduce to simplest integers

[3] Enclose the No. set by square brackets

0 2a 2a

0 1 1

[0 1 1] Direction


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Miller indices – directions with (-) indicesz

y

x

[1] Draw a vector and take components



0 -a 2a

0 -1 2

[0 1 2] Direction

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Miller indices – direction summary

x y z1: a 0 02: 0 a 03: 0 0 a

1: 1 0 02: 0 1 03: 0 0 1

1: [100]2: [010]3: [001]

Directions in a crystal[1] Draw a vector and take components



z

yx

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Miller indices – direction summary

1: [100]2: [010]3: [001]

Equivalent directions

The above directions are equivalent due to crystal symmetry

Triangular brackets are used to represent equivalent directions: <100>

[100][010][001]

z

yx

12

3

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- Fermi distribution function

- Transport processes in S.C.

- Effects of impurities

- Formation of a PN junction

- Bipolar junction transistor

- Uni-polar transistors

- Integrated circuits

To understand and design an electronic device and circuit, it is necessary to study carrier processes:


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Miller indices – plane and direction

z

y

x

A

B

C

D

E

F

G

HEFGH plane

(010)

y-direction[010]

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temp

z

y

x


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Orientation of a Si wafer

z

yx

(100) surface

Flat edge

[011] direction

physical basis of semiconductors - lecture notes - week 0

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