Introduction to FinFet

Haiying Zhao

What does FinFet look like

Source Drain

Gate

Source Drain

Gate

Source Drain

Gate

3D view of FinFET

3D view of multi-fin FinFET

What does FinFet look like

Moore’s law and scaling theory

Ideal scaling:

Reduce W,L by a factor of a

Reduce the threshold voltage and supply voltage by a factor of a

Increasing all of the doping levels by a

(W,L,tox,VDD,VTH, etc, are scaled down by a factor a)

For a ideal square-law device, Id is reduced by a, but gm and intrinsic gain

Gm* ro remain the same.

As scaling into submicron region, Short Channel effects prevent further scaling.

Barrier lowering increases as channel length is reduced, even at zero applied drain bias, because the source and drain form pn junction with the body, and so have associated built-in depletion layers associated with them that become significant partners in charge balance at

short channel lengths, even with no reverse bias applied to increase depletion width

Short Channel Effects:DIBL

DIBL: drain induced barrier lowering. DIBL = d(Vth)/d(Vds)

Short Channel Effects: Subthreshold swing

Biasing a nmos in subthreshold resgion, Vgs < Vth, Vds is large enough.

Id = u Cd W/L (KT/q)^2 (exp(Vgs – Vth)/M)) Cd is capacitance of the depletion layer under gate.

M = (1 + Cd/Cox)*KT/q

Id = f(Vgs-Vth)

To turn off the transistor, How much reduction of (Vgs-Vth) could lead to a small enough Id.

Subthreshold Swing S = d(Vgs)/d(log(Id)) = 2.3 KT/q( 1+ Cd/Cox)

The smaller S is , the better it is.

Bad Subthreshold Swing will result in higher off-state current if the Vgs applied to turn off the transistor is the same.

V = u E ( E is small enough)

V = Vsat ( E is strong enough)

As Vgs increases , the drain current saturates well before pinch-off occurs.

Short Channel Effects: Velocity Saturation

Barrier lowering increases as channel length is reduced, even at zero applied drain bias, because the source and drain form pn junction with the body, and so have associated built-in depletion layers associated with them that become significant partners in charge balance at

short channel lengths, even with no reverse bias applied to increase depletion width

Short Channel Effects:DIBL

DIBL: drain induced barrier lowering. DIBL = d(Vth)/d(Vds)

Conclusion

To reduce short channel effects, we need to reduce Xd(channel depletion layer thickness), Xj( Junction depletion width),Xox (oxide layer thickness under gate).

The scale length of bulk MOSFET is an indication of Lg. Lg>

Dealing with Short Channel Effects in bulk MOSFET

1.Increasing body doping concentration

2.Using halo implant

High doping density results in:Lower carrier mobility; high tunneling effect which increases off-state currents;Larger depletion capacitors leading to high subthreshold swing which increases off-state currents;Larger parasitic capacitance, Cgd, Cds.

Dealing with Short Channel Effects in Fully depleted Silicon on Insulator (SOI)

Use ultra-thin film (tsi is small) as the conducting body, depletion layer is confined in the film.( Xd<= tsi).

Eliminate the junction parasitic capacitors. Cuff off the leakage current path from drain to substrate.

From FD/SOI to FinFET

Bend up the gate and narrow the gate. Fin width = 2* film thickness

The effect body thickness is reduced by 2. Xd can be regarded as

Fin width /2. To obtain good control of SCE, Leff > 1.5*Wfin ( Fin width).

Finfet can operate at two mode, single gate and double gate.

FinFet characteristics

Lg = 15nm

Lg = 30nm

Threshold Voltage = 0.196 V

Subthreshold Slope = 72 mV/decade

Off Current = 70 A/m

DIBL = 64.67 mV/V

Some values:

Approximate dc I-V equations?

Square law?

One way is using nth power law to computer the FinFet current.

FinFet: Challenges or Opportunities

Carrier mobility:

Lightly doped or undoped fin body increases carrier mobility.

Short channel length enables velocity overshoot, which increases mobility.

Low Vth decreases the vertical electric field ,which increases carrier mobility.

Tunneling effects:

Gate to channel tunneling,

Band to band tunneling at

PN junction

FinFet: Challenges or Opportunities

Parasitic resistance: a raised source/drain structure can be used to reduce the parasitic resistance.

However, the overlap capacitance is increased.

Prasitic resistance is the main adverse factor which prevents finfets’ application, which leads to lower speed and high noise.

FinFet: Challenges or Opportunities

Cds

Source

DrainGate

rds

rdCgd

Cgs

ri

rg

rs

gm*vgs’

vgs’

+ Cds

Source

DrainGate

rds

rdCgd

Cgs

ri

rg

rs

gm*vgs’

vgs’

+

FinFet: big advantages

1. Having excellent control of short channel effects in submicron regime and making transistors still scalable. Due to this reason, the small- length transistor can have a larger intrinsic gain compared to the bulk counterpart.

2. Much Lower off-state current compared to bulk counterpart.

3. Promising matching behavior.

Applications

1. Low power design in digital circuit, such as RAM, because of its low off-state current.

2. Power amplifier or other application in analog area which requires good linearity.