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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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ECE685 Nanoelectronics – Semiconductor Devices

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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


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


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

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


AlInGaP Quantun Well

Common LEDs


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


Basic MOSFET structure and IV characteristics

+

+


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