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Institute of Radio Physics and Electronics

University of Calcutta

A Diamond Jubilee Celebration program

Organized byRadio Physics and Electronics Association (1960)

CAS in Radio Physics and Electronics (1963)

S. K. Mitra Centre for Space weather (2004)Centre fur TeleInFrastructur (CTIF) -India (2007)UGC Networking Resource Centre in Physical Sciences (2008)

Centre for Research & Training in Microwave & Millimeterwave Technolog

National MEMS Design Centre (2009)

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Workshop onFrontiers of Electronics and Communication

atNorth Eastern Regional Institute of Science andTechnology

(NERIST),Nirjuli, Arunachal Pradesh

August 08, 2007

Modern Semiconductor Materials And Devices Physics, Technology and Challenges

P. K. Basu

Director, UGC Networking Resource Centre for Physical Science Institute of Radio Physics and Electronics

University of Calcutta

A Lecture under IEEE National Distinguished LectureProgramme

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DEVICE PHYSICS

NanoDev - 1

P. K. Basu

Institute of Radio Physics and Electronics92 Acharya Prafulla Chandra Road

Kolkata 700 009

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Current Voltage Relation

J

Current density J = Charge crossing unit area per sec.

Consider a bar of cross section area = 1 cm2, length =v cm = velocity of electrons. Electrons in volumev x 1 cm3 will cross unit area per unit time.Total no of electrons =n x v x 1; Total charge =nev

J = nev = neE; v = E; = mobility of electrons , E = Electric field =V/length. I = J xA = neE x A= neV(A/L)

V = (1 / ne )( L/A ) I = RI : Ohms law R = L/A ; =(1/)= specific resistance; = conductivity = ne.

v

LA

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What is a Semiconductor?

Has conductivity between metal(10 7 ) and insulator(10 -7

) A band gap separates the conduction and valencebands

Band gap varies from 0.1 3.0 eV; higher gapscorresponds to insulators

Typical Semiconductors :

Ge : 0.7 eV : Elemental

Si : 1.1 eV : Elemental

GaAs: 1.43 eV : III-V compound

ZnS : 2.6 : II-VI Compound

In x Ga 1-x As : III-V alloy

Si is the most widely used material in electronics/ VLSI

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Semiconductor

III-V II-VI IV-VI IV-IV

AlAs,AlPAlSbAlN

BAsBNBPBSb

GaAsGaPGaSbGaN

ZnSZnSeZnTeZnO

HgSHgSeHgTe

CdSCdTeCdSe

MISC

InAsInPInSbInN

MgSMgSeMnSeMnTe

SnTeEuTeYbTeSnTe

PbTePbSePbS

GeTe

SiGeC

SiC

TeGaSeCuCl

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Semiconductor Alloys

Binary Alloy: Si1-xGex (0 < x

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Amorphous Polycrystals Single crystal

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Growth of single crystalof Si

View of finished wafer of Si

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Hydrogen bond Si: Atomic No. 14 : 2 + 8+ 4electrons. Covalent bonding in Si.4 outermost electrons shared by 4nearest neighbours

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Pure Si crystal at T > 0K. A bond is broken by thermal energy.An EHP is created. An el goes from VB to CB. An emptystate(hole) is created in Si crystal.ni = p i = Intrinsic carrier

concentration.

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What is a Semiconductor?

Has conductivity between metal(10 7) and insulator(10 -7)

A band gap separates the conduction and valence bands

Band gap varies from 0.1 3.0 eV; higher gaps corresponds to insulators

The upper band, the CB is empty at 0K and VB is full at 0K.

Electric field > no e in CB > no flow of e > no current. Similarly at VB all states are full. No ecan gain energy from E-field> no movement of charge> no current. Semiconductor is aninsulator at 0k.

At higher temperature electrons go from VB to CB. There is an EHP. Now e and h areaccelerated by E-field and current flows.

J = e(n n +p p )E = E Ohms law > Current is proportional to electric field = mobilityconductivity.

Intrinsic (pure) semiconductor: n =p; n ~ exp(-E g /k BT).

Intrinsic n or p increases with decrease in gap, increase in T

N =1.5 x 10 10cm -3 at 300 K. No control on n or p or conductivity.

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Current Flow and Conductivity

Current density J = E

To calculate conductivity of intrinsic Si atroom temperature

Conductivity = e(n i n + p i p ) ; n = 1500 n = 500;

n = 1.5 x 10 10 ; conductivity = 4.8 x 10 -6 mho/cm very low and cannot be controlled.

HOW TO INCREASE AND CONTROLCONDUCTIVITY?

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Doped /Extrinsic Semiconductors

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Donor and Donor Binding EnergyConsider pure Si doped with Gr. V atom like P, As etc.

P atom substitutes a Si atom. 4 electrons out of 5 take part inbonding process.

Extra 5 th electron rotates around parent nucleus. H atomproblem Bohr theory needed.

Electron mass = effective mass; material permittivity inCoulomb force.

Binding energy = 13.6 ( m*/m 0 ) (eps 0 /eps) 2 eV.

m* = 0.12 m 0 ; eps = 16 eps 0

Binding energy ~ 6 meV. < kT at 300 (26 meV)

P atom easily ionised P + ions + 1 electron overall chargeneutrality

The e goes to CB > one e per P atom

If P atom = 10 15 cm -3 , same number of free electrons.

Conductivity increases by 5 orders!

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Carrier Concentration

n = N C exp[- ( E C E F )/k BT ]

N C = [42 (m*k BT)3/2]/h3 : Effective Density of States p = N v exp[- ( E F E V )/k BT ]

N V = [42 (m*k B T)3/2 ]/h3 : Effective Density of States

np = n i2

Whenn (or N D ) = N C , Fermi level touches CB edge: conditionfor degeneracy

dE E E m

dE E S dN cc

c2/1

2/3

22)(

2

2

1)(

==

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Band Structure Calculation Kronig Penney model Tight binding approximation Nearly free electron model Pseudopotential method

k.p perturbation (useful near the bandextrema of semiconductors)

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X1

X4

25

15

2L3

L1

L3

0 (100)(111)

0

-2

24

6

HHLH

k=(000)X5

X1X3

L3

L3

L1 15 1

15

E(eV)

-2

2

4

k=/a(111) k=2/a(100)SO

k x

k y

k z

Band Structure of Si

Band Structure of GaAs

Constant energy surfaces inCB of Si

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E E

Eg Eg

q

k k 00

Conduction band

Valence band

Direct and Indirect Gap Semiconductors

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Scattering MechanismsBulk: Impurity, Phonons, Defects (Alloy Disorder)

QW: Remote Impurity Scattering in MD structures,Surface Roughness

QWR: Reduced scattering rate for 1 DEG

QD: Phonon bottleneck

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Hot Carrier Phenomena

Velocity saturation, ndr, velocity overshoot and ballistic transport

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DEVICE PHYSICS: Fundamentals

P. K. Basu

Institute of Radio Physics and Electronics92 Acharya Prafulla Chandra Road

Kolkata 700 009

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p-n junction

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METAL OXIDE SEMICONDUCTOR FIELD

EFFECT TRANISTORS

MOSFETs

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Classification

Enhancement mode : n channel , p channel

Depletion mode: n channel, p channel

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CMOS INVERTER

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I sub = 0 C ox (W/L)(m-1)(V T )2

exp[(V G V T )/mkT] x [ 1 exp(- V ds /kT)] m = 1 + (C dm /C ox )

Lower V T increases subthreshold leakage current. V ds has little effect onsubthreshold current.

T d f l lt g f CMOS

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Trends of power supply voltage of CMOS

MOSFET: The Leaky Switch

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y

Subthreshold leakage (I sub )Dominant when device isOFF.Enhanced by reduced V T due to process scaling

Gate tunneling leakage (I gate )Due to aggressive scalingof the gate oxide layer

thickness (Tox)A super exponentialfunction of ToxComparable to I sub at 90nmtechnology

Power consumption vs supply for a CMOS

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Power consumption vs supply for a CMOSgate using bulk and SOI CMOS devices

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Power Explosion

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Power Explosion

IEDM 2003

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Leakage Power

Leakage power limits Vt scalingLeakage power limits Vt scaling

A. Grove, IEDM 2002

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Leakage Current Components

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Leakage Current Components

I1 = pn reverse bias current

I2 = Weak Inversion

I3 = Gate oxide tunneling

I4 = Hot carrier injection

I5 = GIDL

I6 = DIBL

Cross section of bulk and SOI MOS

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Cross section of bulk and SOI MOSdevices

Quantum SizeEff

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Q Effect Free Electron, ~ e ikx

Let us take E ~ 10 meVDe Broglie Wavelength,

m2

1h

m2

k E 2

222

=

mE2h=

~ 500 m = 0.067 m o

(2) Confine the electron

(Particle in a boxL

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Band bending and subbands at Si-SiO 2 interface

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High gate voltage triangular potential e motion quantized predicted bySchrieffer verified in Si MOSFET byIBM in 1966

To CB of SiO2

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Different Tunneling Processes through ThinGate

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Different Forms of FETs

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(a) MOSFET (b) FIN FET (c ) Nano Wire FET (d) Vertical NWFET: completeuniform wrap-around gate (e) Array of Vertical NWFETs as in (d).

C. Thelander, Materials Today, vol. 9, no. 10, p. 28 (2006)

High Electron Mobility Transistors (HEMTs)

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High Electron Mobility Transistors (HEMTs)

The 2DEG in GaN is separated from impurity ions in AlGaN. Reduced Couscattering leads to mobility enhancement. The text-book examples deal withAlGaAs/GaAs modulation doped single heterojunction.

Modulation doped

Or AlGaAs

or GaAs

Resonant Tunneling Diodes

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QCA The Four Dot DeviceQCA The Four Dot Device

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Uses electrons in cells tostoreandtransmitdata Electrons move between different positions via

electron tunneling Logic functionsperformed byCoulombic interactions

Carbon Nanotubes between Metal Electrodes

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References (contd)

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M Balkanski and R F Wallis (2000)Semiconductor Physics and Applications, OxfordUniv Presss, Oxford UK.

P K Basu (2003)Theory of Optical Processes in Semiconductors: Bulk and Microstructures, Clarendon Press, Oxford, UK.

G Bastard (1988)Wave mechanics Applied to Semiconductor Heterostructures, LesEditions de Physique, Les Ulis.

C Weisbuch and B Vinter (1991)Quantum Semiconductor Structures, Academic, SanDiego

V Mitin, M A Strocio and Kochelap (1999)Quantum Heterostructures , John Wiley,

NY. B K Ridley (2000)Quantum Processes in Semiconductors, 5th edition, ClarendonPress, Oxford,

Paul Harrison (2000)Quantum Wells, Wires and Dots: Theoretical and Computational Physics, John Wiley & Sons, Ltd, Chichester, UK.

Omar Manasreh (2005).Semiconductor Heterojunctions and Nanostructures, McGrawHill, NY.

John H. Davies (1998)The Physics of Low-Dimensional Semiconductors: An Introduction, Cambridge Univ Press , Cambridge, UK.

Jasprit Singh (2003) Electronic and optoelectronic properties of semiconductor structures , Cambridge Univ. Press, Cambridge, New York.

R A Smith (1964)S i d t Cambridge Univ Press Cambridge UK