A Class presentation for VLSI course by : Maryam Homayouni Based on the work presentation “ANALYSIS & MITIGATION OF CMOS GATE LEAKAGE” Rahul M.Rao, Richard B. Brown University of Michigan Kevin Nowka, Jeffry L. Burns IBM Austin Research Lab

This Work focuses on: understanding gate leakage current developing circuit techniques for total leakage minimization Input vector control circuit reconfiguration techniques for total leakage minimization of static and dynamic circuits design guidelines for optimal device size selection for stacked sleep devices in an enhanced MTCMOS configuration

Introduction Leakage component increase Device dimensions and Threshold voltage Scaling sub-threshold leakage gate oxide thickness scaling gate leakage current Leakage minimization techniques which consider Gate Leakage: Boosted-gate MOS PMOS dominated[4] Gate leakage effect on circuit performance and dynamic beh- avior of the floating body in SOI devices was examined in [5,6]

Gate Leakage Analysis Gate leakage = exponential function of the electric field across the gate oxide gate leakage current shows exponential dependence on VGS high gate bias: drain-to-source bias gate leakage current Low gate bias: Gate leakage current is insensitive to body node VGS & VGD determine its gate leakage

Gate leakage estimation bias conditions seen by a device in a circuit depend on its position in the circuit and the applied input vector a device exhibits gate leakage only when there exists a potential difference between the gate & drain/source terminals of the device The gate leakage of any device in the circuit can be estimated from its bias state if there exists a conducting path from its device terminals to the supply rails The total gate leakage of the circuit can be computed as the sum of the gate leakage of the individual devices

Leakage estimation in NAND3 Assumption: the internal nodes attain full logic levels (i.e., are either at VDD or VSS) gate leakage is negligible if no conducting path exists from the device terminals to the supply-rails As seen, the leakage estimates obtained by this method are very accurate, with an average error of less than 1% and a maximum error of less than 2%.

Leakage minimization techniques Transistor Stacks The sub-threshold leakage through the transistor stack is minimized when all of the devices in the stack are turned ‘OFF’(<000>) All of the PMOS devices have high Vgd and Vgs High field across the gate oxide causing gate leakage and increase due to greater width of PMOS devises To reduce gate leakage, it is necessary to maintain the terminals of most of the devices at the same potential<110>

Transistor Stack The total leakage for some of these vectors is clearly dominated by the gate leakage component <110> is the minimum total leakage state for Nand3 cell it is necessary to reevaluate conventional leakage minimization schemes and input vector assignments to account for the effect of gate leakage

Enhanced MTCMOS Scheme design guidelines for optimal device size selection for total leakage minimization using stacked sleep devices in an MTCMOS configuration by taking into consideration the effect of gate leakage in active and standby modes In an enhanced MTCMOS configuration ,sub-threshold leakage is reduced in sleep mode due to the added effect of stacking of high threshold voltage transistors

..Enhanced MTCMOS Scheme maximum savings in subthreshold leakage are obtained if the devices in the stack are sized as: the lower device in the stack is bigger than the upper device an increased negative gate-to-source bias and a reduced drain-to-source bias (i.e. reduced DIBL) for the upper device resulting in reduced sub-threshold leakage. The average gate leakage is always greater than that for the MTCMOS configuration

..Enhanced MTCMOS Scheme The stacked sleep devices can also be sized to maximize he savings in gate leakage factor as Thus the lower device is required to be bigger than the upper ones a lower gate leakage in the off state optimizing for gate leakage is nearly identical to optimizing for total leakage

Test circuits test circuits have been implemented in a sub 0.1µm advanced SOI process. An 8- bit Brent Khung adder has been used as a design vehicle and has been implemented in various dynamic circuit configurations, with MTCMOS and various enhanced MTCMOS configurations having sleep device stack optimized for sub-threshold and gate leakage.

CONCLUSION growing importance of gate leakage current efficient technique for gate leakage estimation that is accurate to within 5% of SPICE simulation results Optimal leakage reduction vectors for transistor stacks design guidelines for optimal device size selection for stacked sleep devices in an enhanced MTCMOS configuration optimizing for gate leakage is nearly identical to optimizing for total leakage.

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VLSI POSTER December 2004