Leakage in MOS devices

Mohammad Sharifkhani

Reading

• Text book, Chapter III• K. Roy’s Proc. of IEEE paper

Introduction

• What is leakage?– IOFF (drain current when

transistor is supposed to be off)• Including gate leakage

• Why is it important?– Stand-by power; energy

consumption for no work

Introduction

• How bad is it?– 1nA/um @0.25um @30

degree C– 1uA/um @0.1um @80

degree C• Each generation for a

15mm2 chip– I off increase by 5x– Total Width increase by 50%– Total leakage current on a

chip 7.5x– Leakage power 5x

Introduction

MOS Leakage behavior

Leakage components

• 6 leakage components– I1: PN junction reversed

bias– I2: Subthreshold

leakage– I3: Gate tunneling

– I4: Hot carrier injection

– I5: GIDL

– I6: Punchthrough

PN junction reverse bias current

• Minority carrier drift/diffusion– Near the edge of depletion

region– The direct band-to-band

tunnelling model (BTBT)• Describes the carrier generation

in the high field region without any influence of local traps.

• Electron-hole generation in depletion region

• Band to band tunneling (BTBT) is dominant

PN junction reverse bias current• Tunneling current density

increases exponentially with doping:– Na, Nd– Vapp (drops too, minor effect)

• Doping increases with scaling• For typical devices it is

between 10pA – 500pA at room temperature; For a die with million devicesoperated at 5 V, this results in 0.5mW power consumption rather small

• For 0.25 μm CMOS: J = 10-100 pA/ μm2 at 25 deg C.

Subthreshold leakage

• Most important among all• Weak inversion

– Minority carriers in the channel is small but not zero

– Small Vds; drops across the reversed-bias pn; small field

– small field, carrier current is due to diffusion rather than drift (base in BJT)

• Wdm: maximum width of depletion layer; m<2

• When Vth is small Vgs = 0 does not turn ‘off’ the MOS

Subthreshold leakage

Subthreshold leakage

• Exponential relationship with Vgs and Vth– 255mV Vth variation 3 orders

of magnitude in leakage• St; milivolts/decade

– Threshold voltage variation effect on leakage

– About 70-120mV/dec– Smaller St: sharper slope

• Less voltage variation for 10x leakage increase

Subthreshold leakage (DIBL)• Drain Induced Barrier Lowering• Short channel devices• Depletion region of drain interacts

with source near channel surface• Voltage at the drain lowers the

potential barrier at the source– Lowers VTh– Increases subthreshold current

without any change onS• Causes source to inject carriers into

channel surface independent of the gate voltage

• More DIBL at higher VD and shorter Leff

• Moves curve up, to right, as VD increases

Subthreshold leakage (Body Effect)

• Vth roll off– Increase of Vth with

reduction of Channel Length• Reverse body bias

– Widens depletion region• Length ↓, Vth↑

• Bulk doping ↑ Vth substrate sensitivity ↑

• Reverse body bias ↑ Vth substrate sensitivity ↓

• Slope St remains the same

• Isolations– Local Oxide Isolation (LOCOS)– Trench isolation

• In LOCOS, the fringing field causes the gate-induced depletion region to spread outside the channel width and under the isolations– Gate has to work more to

create the channel (inversion)– More substantial (comparable)

as the channel width decreases

• Increase of Vth due to narrow-channel effect

• Kicks in for W<0.5um

Subthreshold leakage (Narrow Width Effect)

Subthreshold leakage (Narro Width Effect)

• Trench isolated technologies:– Vt decreases for effective channel widths W ≤ 0.5 μmNMOS

• For PMOS: A much more complex behavior – reduction of the width first

decreases the until the width is 0.4 m. The width reduction below 0.4 um causes a sharp increase

• Short-channel devices: source-to-drain distance comparable to depletion width in vertical direction

• Source and drain depletion regions penetrate more into channel length.

• Part of the channel being already depleted.

• Gate voltage has to invert less bulk charge to turn a transistor on.

Subthreshold leakage (Channel Length Effect)

• 23 fA/um to 8 pA/um– Factor of 356

• Smaller St:– Sharper transition (worse sensitivity)

• Two parameters increase the subthreshold leakage as temperature is raised: – 1) Vth linearly increases with

temperature– 2) the threshold voltage decreases.

• The temperature sensitivity of was measured to be about 0.8 mV C.

Subthreshold leakage (Temperature Effect)

Gate Leakage

• Tox ↓ Eox ↑• Two mechanisms of electron

tunneling– Fowler–Nordheim Tunneling:

electrons tunnel into conduction band of oxide layer

• Very high field strength; usually not present in products

– Direct Tunneling: electrons from the inverted silicon surface to the gate through the forbidden energy gap of the SiO2 layer

Hot Carrier Injection

• In a short-channel transistor, due to high electric field near the Si–SiO2 interface, electrons or holes can gain sufficient energy from the electric field to cross the interface potential barrier and enter into the oxide layer– Reliability risk! (Electrons can trap into or destroy

oxide)

• Increases as L drops (unless VDD drops accordingly)

Gate-Induced Drain Leakage (GIDL)

• GIDL is due to high field effect in the drain junction of an MOS transistor

• Vg<0 Thins out the depletion region between drain to well PN junction– Effect of new electric field on the old

PN depletion region holes tunnel to substrate from drain

– Since the substrate is at a lower potential for minority carriers, the minority carriers that have been accumulated or formed at the drain depletion region underneath the gate are swept laterally to the substrate, completing a path for the GIDL

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• The effect of GIDL is more visible at higher VDD and lower Vg

• Thinner oxide thickness and higher VDD (higher potential between gate and drain) enhance the electric field and therefore increase GIDL

• Increase from 4nA 36nA (for VD from 2.7V to 4V)

Gate-Induced Drain Leakage (GIDL)

• Increasing current for negative VG values• • Localized along channel width between gate and drain• • Major problem in Ioff current:• • Contributes to standby power, so must control this by• increasing oxide thickness, increasing drain doping, or• eliminating traps.• • For high performance device (low Vth), is not a major• issue.

Gate-Induced Drain Leakage (GIDL)

• When Source and Drain depletion region “touch” each other deep in the channel.

• Less gate influence on the current– Channel is created deeper in substrate– Higher St

• Varies quadratically with VD and with VS

Punchthrough

Leakage component contribution

• In each region, the last term dominates