ECE685 Nanoelectronics – Semiconductor Devices

Lecture given by Qiliang Li

• Unit cell of silicon crystal is cubic.

• Each Si atom has 4 nearest neighbors.

Silicon Structure

Si Si Si

Si Si

Si Si Si

Si Si Si

Si Si

Si Si Si

As B

Dopants, Electrons and holes

N-type

P-type

Relationship between Resistivity and Dopant Density

= 1/

DOPA

NT D

ENSI

TY c

m-3

RESISTIVITY

(cm)

GaAs, III-V Compound Semiconductors, and Their Dopants

As AsGa

Ga

· GaAs has the same crystal structure as Si.· GaAs, GaP, GaN are III-V compound semiconductors, important for optoelectronics.· Wich group of elements are candidates for donors? acceptors?

GaAs

AsGa Ga

Energy Band Model

· Energy states of Si atom (a) expand into energy bands of Si crystal (b).· The lower bands are filled and higher bands are empty in a semiconductor.· The highest filled band is the valence band.· The lowest empty band is the conduction band .

2p

2s

(a) (b)

conduction band)(

(valence band)

Filled lower bands

} Empty upper bands

}

Energy Band Diagram

Conduction band Ec

Ev

Eg

Band gap

Valence band

· Energy band diagram shows the bottom edge of conduction band, Ec , and top edge of valence band, Ev .

·

Ec and Ev are separated by the band gap energy, Eg .

Donor and Acceptor in the Band Model

Conduction Band Ec

EvValence Band

Donor Level

Acceptor Level

Ed

Ea

Donor ionization energy

Acceptor ionization energy

Ionization energy of selected donors and acceptors in silicon

AcceptorsDopant Sb P As B Al In

Ionization energy, E c –E d or E a –E v (meV) 39 44 54 45 57 160

Donors

Device Fabrication

Oxidation

Lithography &Etching

Ion Implantation

Annealing & Diffusion

Side View Top View

Beginning from a silicon wafer

Side View Top View

Thermal Oxidation

Side View Top View

Spin-on Photo Resist (PR)

Side View Top View

Alignment, UV Expose and Develop Photo Resist (PR)

Side View Top View

Oxide Etched

Side View Top View

Remove Photo Resist (PR)

Side View Top View

Doping (implantation or diffusion)

Side View Top View

Grow Field Oxide (wet/dry) and dopant diffusion

Side View Top View

Spin-on Photo Resist (PR)

Side View Top View

Alignment, UV Expose and Develop Photo Resist (PR)

Side View Top View

Oxide Etched

Side View Top View

Remove Photo Resist (PR)

Side View Top View

Grow Gate Oxide (dry)

Side View Top View

Spin-on Photo Resist (PR)

Side View Top View

Alignment, UV Expose and Develop Photo Resist (PR)

Side View Top View

Field Oxide Etched

Side View Top View

Field Oxide Etched

Side View Top View

Metal (e.g., Aluminum) deposition

Side View Top View

Spin-on Photo Resist (PR)

Side View Top View

Alignment, UV Expose and Develop Photo Resist (PR)

Side View Top View

Aluminum Etched

Side View Top View

Remove Photo Resist (PR), annealing - complete

PN junction is present in perhaps every semiconductor device.

N P

VI

– +

PN Junction

V

I

Reverse bias Forward bias

Donor ions

N-type

P-type

Energy Band Diagram of a PN Junction

A depletion layer exists at the PN junction where n 0 and p 0.

Ef is constant at equilibrium

Ec and Ev are smooth, the exact shape to be determined.

Ec and Ev are known

relative to Ef

N-region P-region(a) Ef

(c)

Ec

Ev

Ef

(b)

Ec

Ef

Ev

Ev

Ec

(d)

Depletionlayer

Neutral P-region

NeutralN-region

Ec

Ev

Ef

Light emitting diodes (LEDs)•LEDs are made of compound semiconductors such as InP and GaN.• Light is emitted when electron and hole undergo radiative recombination.

Ec

Ev

Radiative recombination

Non-radiative recombination through traps

LED Materials and Structure

)(24.1

energy photon24.1 m) ( h wavelengtLED

eVEg

Common LEDs

AlInGaP Quantun Well

V

I

Reverse bias Forward bias

V = 0

Forward biased

Reverse biased

Schottky Diodes

MOS: Metal-Oxide-Semiconductor

SiO2

metal

gate

Si body

Vg

gate

P-body

N+

MOS capacitor MOS transistor

Vg

SiO2

N+

Surface Accumulation

fbgox VVV

)( fbgoxacc VVCQ

oxsox CQV /

oxaccox CQV /Gauss’s LawVg <Vt

ox

ssa

ox

depa

ox

dep

ox

sox C

qNC

WqNCQ

CQV

2

Surface Depletion ( )gV > Vfb

Ec, Ef

Ev

Ec

Ef Ev

M O S

qVg

depletion region

qs

Wdep

qVox

- - --SiO

2

gate

P-Si body

+ + + + + + - - - - - - -

V - - - - - - -

depletion layer charge, Q dep

- - - - - - -

Threshold Condition and Threshold Voltage

Threshold (of inversion):ns = Na , or

(Ec–Ef)surface= (Ef – Ev)bulk , or

A=B, and C = D

i

aBst n

Nq

kT ln22

i

a

a

v

i

vbulkvf

gB n

Nq

kTNN

qkT

nN

qkTEE

Eq lnlnln|)(

2

Ec, Ef

M O S

Ev

Ef

Ei

Ec

A

B

C = qB

Ev

DqVg

= qVt

st

ox

BsaBfbgt C

qNVthresholdatVV

222

Threshold Voltage

ox

BssubBfbt C

qNVV

222 + for P-body,

– for N-body

(a)

(b)

Tox = 20nm

V t(V), N

+ gat

e/P-bo

dy

V t(V), P

+ ga

te/N-

body

Body Doping Density (cm-3)

Body Doping Density (cm-3)

V t(V), P

+ ga

te/N-

body

V t(V), N

+ ga

te/P-b

ody

Tox = 20nm

Strong Inversion–Beyond Threshold

Ec, Ef

Ev

Ec

Ef Ev

M O S

qVg

-

-

- - --

a

Bsdmaxdep qN

WW 22Vg > Vt

SiO2

gate

P - Si substrate

++++++++++

V

Vg > Vt

- - - - - --- - - - - - - -

Q dep Qinv

Basic MOSFET structure and IV characteristics

+ +

)(0 dstgsnsoxeds

ds mVVVCL

WdVdI

m

VVV tgs

dsat