Background for Leakage Current

1. Background for Leakage Current Sept. 18, 2006

2. Active power density increasing with device scaling and increased frequency • Leakage power density increasing due to lower Vt and gate leakage • Stressing packaging, cooling, battery life, etc. • Complicates IDDq testing as well Thinning gate oxides increase gate tunneling leakage Power Challenge Source from Bergamaschi

3. Problem Statement • Power Analysis on CMOS Inverter

4. Problem Statement • Dynamic Power • Average Short Circuit Current • Sub-threshold Leakage Current

5. Feature Size > 0.25um 0.18/0.13/0.09um… Performance(AP) < 200MHz 300/400/533MHz, 1GHz Core Voltage 5.0/3.3/2.5V 1.8/1.2/1.0V … VTH(Threshold) > +/- 0.6V +/- 0.5, 0.4, 0.3V … TR Leakage Negligible Exponential growing(SD/Gate) Stand-by Mode PLL-off(Clock-off) V/MTMOS, High VTH/High VDD Low Power Focus on Operating Power Focus on Operating/Stand-by Problem Statement • Domination of Leakage Current

6. As CMOS scales down the following stand-by leakage current rises rapidly. Source to drain leakage (diffusion+tunneling) as Lg scales down Gate leakage current (tunneling) as Tox scales down Body to drain leakage current (tunneling) as channel doping scales up Active and Leakage Power with CMOS Scaling

7. Vg=0V Sub-threshold Leakage Turn off Source to drain tunneling Gate oxide tunneling Vd=Vdd Drain to Body tunneling (BTB) Vg=Vdd Turn on Vd=0V Two cases of Leakage Mechanism

8. ) 1 10 2 0 10 -1 10 Drain leakage -2 10 High-K gate dielectric Current Density (A/cm -3 10 Gate leakage -4 10 -5 10 -6 10 20 25 30 35 40 Tox (A) Gate Leakage Current Reduction with High-K Gate Dielectric

9. Voltage Scaling for Low Power Low Power P VDD2 Low VDD I ds  (VDD- Vth)1~2 Low Speed Speed Up I ds  (VDD -Vth)1~2 Low Vth I leakage  e-C xVth High Leakage Leakage Suppression

10. 100m VTH control VDD control MTCMOS High speed High speed 10m VDD: 1.5V VDD control Dynamic power[W] VDD: 1.0V 1m VTH control Low speed Low speed VTH: 0.5V VTH: 0.25V 100n 1p 10p 100p 1n 10n 100n Leakage power[W] Low-Leakage Solution – Technology

11. Multi-Threshold CMOS Variable-Threshold CMOS Schematic Diagram VDD Vpb = VDD N-well VDD or V+ Low- Vth Vt Low Vt Control circuit Sleep Hi- Vth GND P-well Vnb = 0 or V- GND principle • On-off control of internal VDD or VSS • Special F/Fs, Two Vth’s • Threshold control with bulk-bias • Triple well is desirable Merit • Low leakage in stand-by mode. • Conventional design Env. • Low leakage in stand-by mode. • Conventional design Env. Demerit • Large serial MOSFET • ground bounce noise • Ultra-low voltage region?(1V) • Scalability? (junction leakage) • TR reliability under 0.1mm • Latch-up immunity, Vth controllability, Substrate noise, Gate oxide reliability • Gate leakage current VTCMOS & MTCMOS

12. MTCMOS : Reduce Stand-by Power with High Speed With High VTH switch (MTCMOS) Without High VTH switch Vdd • With High VTH switch, much lower leakage current flows between Vdd and Vss • High VTH MOSFET should have much lower ( >10X) leakage current compared to normal VTH MOSFET Vdd Normal or Low VTH MOSFET 0 1 0 1 Virtual Ground Vss 0 Vss High VTH switch

13. Active Sleep Active Multi-Threshold CMOS (MTCMOS) • Mobile Applications • Mostly in the idle state • Sub-threshold leakage Current • Power Gating • Low VTH Transistors for High Performance Logic Gates • High VTH Transistors for Low Leakage Current Gates Current Cutoff-Switch (High Vth) Logic Component (Low Vth) VDD Operating Mode Low Vth MOS Sleep Control (SC) SC High Vth MOS VGND VSS Time

14. High Vt CCS Sizing • The effect of CCS size • As the size decreases, logic performance also decreases. • As the size increases, leakage current and chip area also increase. • Proper sizing is very important. • CCS size should be decided within 2% performance degradation. VDD Low Vt Vop = VDD - V Switch Vmust be sizedwithin 2% performance degradation. Control GND

15. Leakage Current :Limiting Factor in VDSM Technology C.M.Kyung

16. ITRS roadmap • Scaling down allows the same performance with reduced voltage, leading to low power. • From 0.18 micron down, building a transistor with a good active current(Ion) and a low leakage current (Ioff) is difficult. • high-speed TR’s ; low channel doping • low-leakage TR’s ; high channel doping • Now three groups of TR’s; • High Performance (HP) ; high active current ; Thin Tox • Low Operating Power (LOP) ; low active current ; High Tox • Low Standby Power (LSTP) ; low static current ; High Tox

17. Device characteristics for HP, LOP, and LSTP Technologies • Table 2.1

18. Bulk CMOS vs. SOI • Buried oxide layer below active silicon layer -> electrical isolation of TR’s • Lower parasitic cap. • PD(Partially Depleted) • Floating body effect increases speed • Low threshold in dynamic mode • or FD(Fully Depl) • Ideal subthresold swing of 60 mV/decade

19. Reducing Subthreshold current in Bulk CMOS • VTCMOS (Variable Threshold) • Tune substrate bias to adjust Vth • Requires efficient DC-DC converter • For a given technology, there an optimum in VR , as decreasing subthreshold leakage is accompanied by an increase in drain junction leakage • When both High Vt and Low Vt TR’s are available, • MTCMOS (Multi-Threshold) ; Introduce high Vt power switch to limit leakage in stby mode • Use low Vt for critical path • This can be coupled with multiple VDD’s • Other tricks • Set up the logical internal states where the total leakage is minimal.

20. Five types of off-currents • Tunneling through gate oxide • Fowler-Nordheim tunneling -> direct tunneling • Subthreshold current • Gate-induced drain leakage (GIDL) • Thermal emission • Trap-assisted tunneling • BTBT • Reverse-biased pn junction current • -> band-to-band tunneling (BTBT) current • Bulk punch-through

21. Gate-induced drain leakage (GIDL) • Gate-induced drain leakage (GIDL) • Thermal emission • Trap-assisted tunneling • BTBT • Fig 3.12

22. Leakage current due to QM Tunneling • substrate and drain ; band-to-band tunneling ; • increases with E-field and dopant concentration due to scaling • source and drain ; • Surface punchthru due to DIBL • Punch-through at bulk • gate oxide ; • SiO2 has been used as it has so low trap and fixed charge density at the interface • Gate current is an exponential function of Tox and Vox • Hole tunneling is 10% of that of electron due to higher barrier height and heavier effective mass

23. Gate Leakage Current Reduction with High-K Gate Dielectric • As Tox scales gate leakage current increases exponentially due to exponential increase of tunneling probability with reduction of physical tunneling distance. • Physically thicker gate dielectric allows lower leakage current but lower oxide capacitance reducing on-current • Using high k (dielectric constant) material, both thicker physical thickness and higher oxide capacitance can be achieved. • Applying high-k gate dielectric, several orders of magnitude lower gate leakage current can be achieved with similar oxide capacitance

24. Approach 1 to reduce gate leakage ; High K materials • To suppress gate tunneling current, use materials with • High K -> increases thickness (t) • Higher barrier height (h) • Using high K • Increases short-channel effects due to thicker gate dielectric (This sets an upper limit on K, lower limit coming from I tunnel) • Mobility degradation due to poor interface quality

25. Approach 2 to reduce gate leakage ; stop scaling the thickness of gate oxide • Thicker gate oxide yields less control of gate on channel conduction, i.e., higher short-channel effects and DIBL effects.

26. Approach 3 to reduce gate leakage • Multiple gates allows better cotrol of channel by gate, and lets scaling continue without excessive short-channel effects • Double gate • FinFET • Triple gate • Quadruple or gate all-around