Leakage reduction techniques

2006. 10. 30

Three major leakage current components

1. Gate leakage ; ~ Vdd4

2. Subthreshold ; ~ Vdd3

3. P/N junction BTBT current

Circuit techniques to reduce leakage

• Design time techniques– DTCMOS(dual threshold CMOS)– Multiple power supply voltages (islands)

• Run time techniques– Reducing standby leakage

• Using transistor stacks• MTCMOS ; sleep transistor• VTCMOS

– Reducing active leakage• DVS ; dynamic Vdd scaling• DVTS ; dynamic Vth scaling

DTCMOS (Dual Threshold CMOS)

Increasing Vth followed by upsizing yields similar on current with much less leakage current at the cost

of switching power and chip area.

Vgs

Ids

Transistors on during precharge period is a non-critical path and is given high Vth.

If Vs is raised, there are three mechanisms leading to the reduction of drain current, i.e., Vds, Vgs and Vth

Input vector control, e.g., ABC=111 and 000 yields lowest leakage current for 3-in NOR and 3-in NAND. Explain why.

As gate leakage becomes dominant over subthreshold, ABC=100 can yield less leakage than ABC=000. Explain why.

MTCMOS ; reduces leakage during sleep mode only, at the cost of area and delay. Only NMOS may be used.

In Stby mode Clk=1, Sleep=1That prevents continuous supply of leakage current to GND through node 1, and leakage current to GND through node 2.

VTCMOS ; (a) Body (and N-well) biasing technique, Z(ero)BB or F(orward)BB for speed up & reducing SCE in active mode , while RBB in stby mode. Routing grid for body biasing adds area.103 reduction in 0.35 um technology. Effectiveness of RBB to lower Ioff

decreases as IBTBT increases exponentially due to HALO doping.

(b) Source biasing instead of p-substrate. Substrate is shared betweenTarget and control circuitry. VNEWLL is raised to conserve the stored charge if necessary. Reducing VDS thereby further decreases leakageCurrent thru less DIBL which raises Vth.

DVS (Dynamic Voltage Scaling) consists of 1. Processor operating in wide voltage/frequency range2. Regulation loop (F-V) generating min vtg needed for the given freq.3. Operating System that computes desired clock frequency

DVTS vs. DVS

Similar effect when leakage is dominant over switching power.

Merits & Issues of DVTS (Candidate topic for individual project)

• Simple hardware ; – charge pump is used as current demand

is low instead of buck converter which is used in DVS

• Transition energy overhead• Substrate noise ;

– due to absence of inductor, charge pumps can generate noise

• Process complexity

Continuous Feedback loop control

Vth hopping scheme

Leakage reduction is especially important in cache As each cell is in inactive state most of the time.

Five different Low-leakage SRAM cell techniques in Table

13.3

• Things for further study (research candidate)– Re-evaluation of each different technique in ter

ms of their effect on each different leakage component, i.e., subthreshold, direct tunneling gate oxide, BTBT leakage

– How the read/write delay is affected by each low leakage technique

– Transition latency/energy overhead– Impact on cell reliability incl. SER