Strained Silicon MOSFET

R91943037 Jie-Ying Wei

Department of Electrical Engineering and

Graduate Institute of Electronics Engineering

National Taiwan University, Taipei, Taiwan, R.O.C.

Cubic Lattice at Equilibrium

Lattice constant for a Si1-xGex alloy as a function of x

Critical thickness of Si1-xGex layers as a function of Ge fraction

The size change of each valley in a constant energy surface diagram indicates a

shift up(smaller) or down(larger) in energy

LH： light hole band HH： heavy hole band SO： spin-orbit band

Sub-bands in an MOS inversion layer. Additional energy separation reduces inter-valley scattering

Band Alignment

Surface Channel MOSFET Structure

Extraction

• Mobility

• Band Offsets

Mobilityeff

L

WgD

Qinv;

Eeff 1

sQb Qinv;

gD : fromdID

dVDSat small VDS;

Qb,Qinv : from split C V;

1

2for electron;

1

3for hole;

Split C-V measurement configuration

Measured split C-V capacitance from a surface strained-Si n-MOSFET

grown on a relaxed-Si0.7Ge0.3

VT :the intersection of the CGC and CGB curves

Gate-channel capacitance curve CGC

Gate-bulk capacitance curve CGB

When VGS < V FB , holes begin to accumulate at the Si/SiGe interface, confined by the valence band offset. The hole confinement causes the observed plateau at C’

OX in CGB curve.

Effective mobility of surface-channel, strained-Si n-MOSFET at room temperature (Na=2E16)

Peak mobility enhancement ratio at room temperature as a function of apparent Ge fractions in the buffer layer

Transconductance for W*L = 5*10 µm strained-Si n-MOSFETs Performance saturation with Ge fractions x > 0.2

Extraction

• Mobility

• Band Offsets

Full C-V characteristics of a surface strained-Si n-MOSFET (on relaxed Si0.7Ge0.3)

compared to a CZ Si control

Some parameters

• Qf : match the flatband voltages between the measured data and the theoretical curves

• ΔEC = ΔVT since the thickness of the Si channel(10nm) is less than the Debye length of the material(20nm)

• ΔEV : the difference between Va and V’a is not straight-forward, so simulation of the theoretical curve is required

Threshold voltage shift (ΔVT )as a function of Ge fraction x

Two major assumptions in band offset extraction using SEDAN simulation

• All material properties, other than the bandgap, in strained-Si and relaxed SiGe are identical to bulk Si.

The results may be affected by 1. the material dielectric constant 2. the electron affinity 3. the density-of-state (DOS) effective mass

• Data of Braunstein, at al. is accurate for the bandgap of relaxed SiGe.

The results were identical, except for a shift in the flatband voltage

Strained-Si band parameters and channel thickness extracted from C-V measurments

Bandgap of strained-Si grown on a relaxed SiGe buffer layer

IEDM 2002

1. Strained Silicon MOSFET Technology

2. Low Field Mobility Characteristics of Sub-100nm Unstrained and Strained Si MOSFETs

Strained Silicon MOSFET TechnologySchematic illustration a surface-channel str

ained-Si n-MOSFET

Effective mobility enhancement ratios

Mobility behavior in strained Si(20% Ge) and unstrained Si n-MOSFETs as a function of doping

Comparison of hole mobility enhancement ratios in strained Si p-MOSFETs as a function of

vertical effective field, Eeff

Low field Mobility Characteristics of Sub-100nm Unstrained and Strained Si MOSFETs

eff Leff2

d IDd VDQinv

Leff2

gD

Qinv

Rtotal RFET Rext Leff

Qinv Rext

1

Qinv d Rtotald Leff

The slopes of the lines were used to calculate mobility

Comparison of mobility extracted on long channel and shor

t channel devices using the conventional and dR/dL method

Mobility enhancement factor as

a function of temperature

Reference

1. Jeffrey John Welser “ The application of strained-silicon/relaxed-silicon germanium heterostructures to metal-oxide-semiconductor field-effect transistors”

2. Kern Rim “Application of silicon-based heterostructures to enhanced mobility metal-oxide-semiconductor field-effect transistors”

3. J.L. Hoyt, IEDM 2002

4. K. Rim, IEDM 2002