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ECE685 Nanoelectronics – Semiconductor Devices. Lecture given by Qiliang Li. Silicon Structure. Unit cell of silicon crystal is cubic. Each Si atom has 4 nearest neighbors . Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Dopants, Electrons and holes. As. B.

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Presentation Transcript
slide2
Silicon Structure
  • Unit cell of silicon crystal is cubic.
  • Each Si atom has 4 nearest neighbors.
slide3
Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Si

Dopants, Electrons and holes

As

B

slide4
Relationship between Resistivity and Dopant Density

DOPANT DENSITY cm-3

P-type

N-type

RESISTIVITY (cm)

= 1/

slide5
GaAs, III-V Compound Semiconductors, and Their Dopants

Ga

As

Ga

As

Ga

As

Ga

As

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?

slide6
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

.

slide7
Energy Band Diagram

Conduction band

E

c

Band gap

E

g

E

v

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 .

slide8
Conduction Band

E

c

E

Donor Level

d

Donor ionization energy

Acceptor ionization energy

Acceptor Level

E

a

E

v

Valence Band

Donor and Acceptor in the Band Model

Ionization energy of selected donors and acceptors in silicon

slide9
Device Fabrication

Oxidation

Lithography &

Etching

Ion Implantation

Annealing &

Diffusion

slide11
Thermal Oxidation

Side View

Top View

slide12
Spin-on Photo Resist (PR)

Side View

Top View

slide14
Oxide Etched

Side View

Top View

slide15
Remove Photo Resist (PR)

Side View

Top View

slide18
Spin-on Photo Resist (PR)

Side View

Top View

slide20
Oxide Etched

Side View

Top View

slide21
Remove Photo Resist (PR)

Side View

Top View

slide22
Grow Gate Oxide (dry)

Side View

Top View

slide23
Spin-on Photo Resist (PR)

Side View

Top View

slide25
Field Oxide Etched

Side View

Top View

slide26
Field Oxide Etched

Side View

Top View

slide28
Spin-on Photo Resist (PR)

Side View

Top View

slide30
Aluminum Etched

Side View

Top View

slide32
– +

V

Donor ions

I

I

N

P

N-type

V

P-type

Forward bias

Reverse bias

PN Junction

PN junction is present in perhaps every semiconductor device.

slide33
N-region

P-region

Ef

(a)

Ec

Ec

Ef

(b)

Ev

Ev

Ec

Ef

(c)

Ev

Neutral

Neutral

Depletion

layer

P-region

N-region

Ec

Ef

(d)

Ev

Energy Band Diagram of a PN Junction

Ef is constant at equilibrium

Ec and Ev are known relative to Ef

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

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

slide34
Ec

Non-radiative recombination through traps

Radiative recombination

Ev

  • Light emitting diodes (LEDs)
  • LEDs are made of compound semiconductors such as InP and GaN.
  • Light is emitted when electron and hole undergoradiative recombination.
slide37
Forward biased

V = 0

I

V

Reverse bias

Forward bias

Reverse biased

SchottkyDiodes

slide38
MOS: Metal-Oxide-Semiconductor

Vg

Vg

gate

gate

metal

SiO2

SiO2

N+

N+

P-body

Si body

MOS transistor

MOS capacitor

slide39
Gauss’s Law

Surface Accumulation

Vg

slide40
Surface Depletion ( )

V

>

V

g

fb

qV

ox

E

c

f

q

s

gate

E

-

+ + + + + +

f

-

-

E

-

SiO

v

qV

2

- - - - - - -

g

- - - - - - -

W

- - - - - - -

E

E

dep

V

c

,

f

depletion layer

depletion

charge,

Q

region

dep

E

v

P-Si body

M

O

S

slide41
E

c

f

st

E

i

=

f

q

A

C

B

E

f

D

B

E

=

qV

qV

v

g

t

E

E

c

,

f

E

v

M

O

S

Threshold Condition and Threshold Voltage

Threshold (of inversion):

ns = Na, or

(Ec–Ef)surface= (Ef –Ev)bulk , or

A=B, and C = D

slide42
+ for P-body,

– for N-body

Threshold Voltage

slide43
>

V

V

g

t

E

c

-

E

gate

f

-

-

E

-

v

++++++++++

-

-

SiO

2

qV

g

-

-

-

-

-

-

-

-

- - - - - - -

V

E

E

c

,

f

Q

Q

dep

inv

E

-

Si substrate

P

v

M

O

S

Strong Inversion–Beyond Threshold

Vg > Vt

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