2 ReadingText book, Chapter IIIK. Roy’s Proc. of IEEE paper
3 Introduction What is leakage? Why is it important? IOFF (drain current when transistor is supposed to be off)Including gate leakageWhy is it important?Stand-by power; energy consumption for no work
4 Introduction How bad is it? Each generation for a 15mm2 chip 1nA/um degree C1uA/um degree CEach generation for a 15mm2 chipI off increase by 5xTotal Width increase by 50% Total leakage current on a chip 7.5x Leakage power 5x
8 PN junction reverse bias current Minority carrier drift/diffusionNear the edge of depletion regionThe 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 regionBand to band tunneling (BTBT) is dominant
9 PN junction reverse bias current Tunneling current density increases exponentially with doping:Na, NdVapp (drops too, minor effect)Doping increases with scalingFor 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 smallFor 0.25 μm CMOS: J = pA/ μm2 at 25 deg C.Eg: energy bandgapE>10^6 V/cm
10 Subthreshold leakage Most important among all Weak inversion Minority carriers in the channel is small but not zeroSmall Vds; drops across the reversed-bias pn; small fieldsmall field, carrier current is due to diffusion rather than drift (base in BJT)Wdm: maximum width of depletion layer; m<2
11 Subthreshold leakageWhen Vth is small Vgs = 0 does not turn ‘off’ the MOS
12 Subthreshold leakage Exponential relationship with Vgs and Vth 255mV Vth variation 3 orders of magnitude in leakageSt; milivolts/decadeThreshold voltage variation effect on leakageAbout mV/decSmaller St: sharper slopeLess voltage variation for 10x leakage increase
13 Subthreshold leakage (DIBL) Drain Induced Barrier LoweringShort channel devicesDepletion region of drain interacts with source near channel surfaceVoltage at the drain lowers the potential barrier at the sourceLowers VThIncreases subthreshold current without any change onSCauses source to inject carriers into channel surface independent of the gate voltageMore DIBL at higher VD and shorter LeffMoves curve up, to right, as VD increasesFix DIBL:– Higher surface & channel doping– Shallow source/drain junction depths
14 Subthreshold leakage (Body Effect) Vth roll offIncrease of Vth with reduction of Channel LengthReverse body biasWidens depletion regionLength ↓, Vth↑Bulk doping ↑ Vth substrate sensitivity ↑Reverse body bias ↑ Vth substrate sensitivity ↓Slope St remains the same
15 Subthreshold leakage (Narrow Width Effect) IsolationsLocal Oxide Isolation (LOCOS)Trench isolationIn LOCOS, the fringing field causes the gate-induced depletion region to spread outside the channel width and under the isolationsGate has to work more to create the channel (inversion)More substantial (comparable) as the channel width decreases Increase of Vth due to narrow-channel effectKicks in for W<0.5um
16 Subthreshold leakage (Narro Width Effect) Trench isolated technologies:– Vt decreases for effective channel widths W ≤ 0.5 μmNMOSFor PMOS: A much more complex behaviorreduction of the width first decreases the until the width is 0.4 m. The width reduction below 0.4 um causes a sharp increase
17 Subthreshold leakage (Channel Length Effect) Short-channel devices: source-to-drain distance comparable to depletion width in vertical directionSource 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.
18 Subthreshold leakage (Temperature Effect) 23 fA/um to 8 pA/umFactor of 356Smaller St:Sharper transition (worse sensitivity)Two parameters increase the subthreshold leakage as temperature is raised:1) Vth linearly increases with temperature2) the threshold voltage decreases.The temperature sensitivity of was measured to be about 0.8 mV C.
19 Gate Leakage Tox ↓ Eox ↑ Two mechanisms of electron tunneling Fowler–Nordheim Tunneling: electrons tunnel into conduction band of oxide layerVery high field strength; usually not present in productsDirect Tunneling: electrons from the inverted silicon surface to the gate through the forbidden energy gap of the SiO2 layer
20 Hot Carrier InjectionIn 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 layerReliability risk! (Electrons can trap into or destroy oxide)Increases as L drops (unless VDD drops accordingly)
21 Gate-Induced Drain Leakage (GIDL) GIDL is due to high field effect in the drain junction of an MOS transistorVg<0 Thins out the depletion region between drain to well PN junctionEffect of new electric field on the old PN depletion region holes tunnel to substrate from drainSince 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-----+-++++
22 Gate-Induced Drain Leakage (GIDL) The effect of GIDL is more visible at higher VDD and lower VgThinner oxide thickness and higher VDD (higher potential between gate and drain) enhance the electric field and therefore increase GIDLIncrease from 4nA 36nA (for VD from 2.7V to 4V)
23 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 byincreasing oxide thickness, increasing drain doping, oreliminating traps.• For high performance device (low Vth), is not a majorissue.
24 PunchthroughWhen Source and Drain depletion region “touch” each other deep in the channel. Less gate influence on the currentChannel is created deeper in substrateHigher StVaries quadratically with VD and with VS
25 Leakage component contribution In each region, the last term dominates