EE130 Lecture 33, Slide 1Spring 2007

Lecture #33

OUTLINE

The MOS Capacitor:

• C-V examples

• Impact of oxide charges

Reading: Chapter 18.1, 18.2

EE130 Lecture 33, Slide 2Spring 2007

Does the QS or the HF-capacitor C-V characteristic apply?

(1) MOS capacitor, f=10kHz.

(2) MOS transistor, f=1MHz.

(3) MOS capacitor, slow VG ramp.

(4) MOS transistor, slow VG ramp.

Examples: C-V Characteristics

VG

VFBVT

C

Cox

QS

HF-Capacitor

EE130 Lecture 33, Slide 3Spring 2007

Example: Effect of Doping

• How would C-V characteristic change if substrate doping NA were increased?

– VFB

– VT

– Cmin

VG

VFBVT

C/Cox

1

EE130 Lecture 33, Slide 4Spring 2007

Example: Effect of Oxide Thickness

• How would C-V characteristic change if oxide thickness xo were decreased?

– VFB

– VT

– Cmin

VG

VFBVT

1

C/Cox

EE130 Lecture 33, Slide 5Spring 2007

Oxide Charges

• In the oxide:– Trapped charge Qot

• High-energy electrons and/or holes injected into oxide

– Mobile charge QM

• Alkali-metal ions, which have sufficient mobility to drift in oxide under an applied electric field

• At the interface:– Fixed charge QF

• Excess Si (?)

– Trapped charge QIT

• Dangling bonds

In real MOS devices, there is always some charge in the oxide and at the Si/oxide interface.

EE130 Lecture 33, Slide 6Spring 2007

Effect of Oxide Charges

• In general, charges in the oxide cause a shift in the gate voltage required to reach the threshold condition:

(x defined to be 0 at metal-oxide interface)

• In addition, they may alter the field-effect mobility of mobile carriers (in a MOSFET) due to Coulombic scattering

ox

oxSiO

T dxxxV0

)(1

2

EE130 Lecture 33, Slide 7Spring 2007

ox

FMSFB C

QV

Fixed Oxide Charge QF

Ec

EFS

Ev

Ec= EFM

Ev

M O S

3.1 eV

4.8 eV

|qVFB |

qQF / Cox

EE130 Lecture 33, Slide 8Spring 2007

Parameter Extraction from C-V

From a single C-V measurement, we can extract muchinformation about the MOS device.• Suppose we know that the gate-electrode material is

heavily doped n-type poly-Si (M=4.05eV), and that the gate dielectric is SiO2 (r=3.9):

– From Cmax = Cox we determine the oxide thickness xo

– From Cmin and Cox we determine substrate doping (by iteration)

– From substrate doping and Cox we calculate the flat-band capacitance CFB

– From the C-V curve, we can find

– From M, S, Cox, and VFB we can determine Qf

FBCCGFB VV

EE130 Lecture 33, Slide 9Spring 2007

FSiO

oMSFB Q

xV

2

0

–0.15V

–0.3V

xo

VFB

10nm 20nm 30nm

Determination of M and QF

Measure C-V characteristics of capacitors with different oxide thicknesses. Plot VFB as a function of xo:

EE130 Lecture 33, Slide 10Spring 2007

Mobile Ions• Odd shifts in C-V characteristics were once a mystery:

• Source of problem: Mobile charge moving to/away from interface, changing charge centroid

ox

MFB C

QV

EE130 Lecture 33, Slide 11Spring 2007

Interface Traps

Traps cause “sloppy” C-V and also greatly degrade mobility in channel

ox

SITG C

QV

)(