EE143 F2010 Lecture 22 Electrical Characteristics of MOS ...

[Pages:36]EE143 F2010

Lecture 22

Electrical Characteristics of MOS Devices

? The MOS Capacitor

? Voltage components

xox

? Accumulation, Depletion, Inversion Modes

? Effect of channel bias and substrate bias

? Effect of gate oxide charges

? Threshold-voltage adjustment by implantation

? Capacitance vs. voltage characteristics

? MOS Field-Effect Transistor

? I-V characteristics

? Parameter extraction

VG +

"metal" oxide

semiconductor

Professor N Cheung, U.C. Berkeley

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EE143 F2010

1) Reading Assignment

Lecture 22

Streetman: Section of Streetman Chap 8 on MOS

2) Visit the Device Visualization Website and run the visualization experiments of

1) Charge carriers and Fermi level, 2) pn junctions 3) MOS capacitors 4) MOSFETs

Professor N Cheung, U.C. Berkeley

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EE143 F2010

Work Function of Materials

Lecture 22

METAL

SEMICONDUCTOR

E Vacuum

o energy level

Eo

Work function

= q q

Ef

EC

Ef EV

qM is determined by the metal material

qS is determined by the semiconductor material,

the dopant type,

Professor N Cheung, U.C. Berkeley

and doping concentration

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EE143 F2010

Work Function (qM) of MOS Gate Materials

Eo = vacuum energy level EC = bottom of conduction band band

q = 4.15eV (electron affinity)

Ef = Fermi level EV = top of conduction

Lecture 22

Eo

qM Ef

Examples: Al = 4.1 eV TiSi2 = 4.6 eV

Professor N Cheung, U.C. Berkeley

Eo qM

Ef

Ei

Eo q = 4.15eV

EC 0.56eV

qM

0.56eV

n+ poly-Si

EV Ef

(Ef = EC)

q = 4.15eV

EC 0.56eV

Ei 0.56eV

EV p+ poly-Si (Ef = EV) 4

EE143 F2010

Work Function of doped Si substrate

Lecture 22

* Depends on substrate concentration NB

Eo qs

Ef Ei

Eo

q = 4.15eV

EC

0.56eV

qs

|qF|

0.56eV

Ef

EV

n-type Si

F

kT q

ln

NB ni

q = 4.15eV

EC 0.56eV

Ei

|qF|

0.56eV

EV

p-type Si

s (volts) = 4.15 +0.56 - |F|

Professor N Cheung, U.C. Berkeley

s (volts) = 4.15 +0.56 + |F|

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EE143 F2010

Lecture 22

The MOS Capacitor

VG +

"metal"

xox

oxide

+_ VFB

+

_ Vox (depends on VG)

+

_ Vsi (depends on VG)

semiconductor

VG VFB Vox VSi

Cox

ox

xox

[in Farads /cm2]

Professor N Cheung, U.C. Berkeley

Oxide capacitance/unit area

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EE143 F2010

Flat Band Voltage

? VFB is the "built-in" voltage of the MOS:

VFB M S

? Gate work function M:

Al: 4.1 V; n+ poly-Si: 4.15 V; p+ poly-Si: 5.27 V

? Semiconductor work function S :

s (volts) = 4.15 +0.56 - |F| for n-Si s (volts) = 4.15 +0.56 + |F| for p-Si

? Vox = voltage drop across oxide (depends on VG) ? VSi = voltage drop in the silicon (depends on VG)

Lecture 22

Professor N Cheung, U.C. Berkeley

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EE143 F2010

MOS Operation Modes

Lecture 22

A) Accumulation: VG < VFB for p-type substrate

M

Charge Distribution

O

Si (p-Si)

holes

Thickness of accumulation layer ~0

VSi 0, so Vox = VG - VFB

QSi' = charge/unit area in Si

=Cox (VG - VFB )

Professor N Cheung, U.C. Berkeley

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