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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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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


Relationship between Resistivity and Dopant Density

DOPANT DENSITY cm-3

P-type

N-type

RESISTIVITY (cm)

= 1/


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?


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

.


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 .


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


Device Fabrication

Oxidation

Lithography &

Etching

Ion Implantation

Annealing &

Diffusion


Beginning from a silicon wafer

Side View

Top View


Thermal Oxidation

Side View

Top View


Spin-on Photo Resist (PR)

Side View

Top View



Oxide Etched

Side View

Top View


Remove Photo Resist (PR)

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



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



Field Oxide Etched

Side View

Top View


Field Oxide Etched

Side View

Top View



Spin-on Photo Resist (PR)

Side View

Top View



Aluminum Etched

Side View

Top View



– +

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.


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.


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.


LED Materials and Structure



Forward biased

V = 0

I

V

Reverse bias

Forward bias

Reverse biased

SchottkyDiodes


MOS: Metal-Oxide-Semiconductor

Vg

Vg

gate

gate

metal

SiO2

SiO2

N+

N+

P-body

Si body

MOS transistor

MOS capacitor


Gauss’s Law

Surface Accumulation

Vg <Vt


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


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


+ for P-body,

– for N-body

Threshold Voltage


>

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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