Leakage Components and Their Measurement

1. Leakage Components and Their Measurement Puneet Sharma ECE Department, UCSD

2. Leakage Components • Subthreshold leakage • Gate direct tunneling leakage • Junction tunneling current (including gate induced drain leakage (GIDL) and band-to-band tunneling) • Others: • Hot carrier injection current • Punchthrough current

3. Subthreshold Leakage (Isub) • Most significant component upwards of 25oC • Dominates gate direct tunneling until 65nm • High-K at 45nm and beyond will reduce gate direct tunneling • Occurs in “off” state (VGS < VTh) between source and drain • Two stacked devices (i.e., off devices in series) will dramatically reduce Isub for both (leakage current is the same for all devices in the stack but leakage power is primarily in the devices for which VS≠ 0 (body effect)) • Increases exponentially as VTh decreases • Increases exponentially with temperature • Depending on VTH roll-off curve, Isub can increase or decrease with LGate. For most processes Isub will increase significantly as LGate is reduced from nominal SPICE Measurement For NMOS D=1, G=0, S=0, B=0 Measure b/w D and S For PMOS D=0, G=1, S=1, B=1 Measure b/w D and S Isub

4. Channel in “on” state IGDO IGSO IGCS IGCD Isub IGB Gate Direct Tunneling Leakage (IGate) SPICE Measurement For NMOS IGCS D=0, G=1, S=0, B=0 Measure b/w G & S IGCD D=0, G=1, S=0, B=0 Measure b/w G & D IGSO D=X, G=0, S=1, B=0 Measure b/w G & S IGDO D=1, G=0, S=X, B=0 Measure b/w G and D IGB D=1, G=1, S=1, B=0 Measure b/w G and B For PMOS IGCS D=1, G=0, S=1, B=1 Measure b/w G & S IGCD D=1, G=0, S=1, G=1 Measure b/w G and D IGSO D=X, G=1, S=0, B=1 Measure b/w G and S IGDO D=0, G=1, S=X, B=1 Measure b/w G and D IGB D=0, G=0, S=0, B=1 Measure b/w G and B In SPICE models IGCMOD must be 1 for IGC and IGBOD 1 for IGB • Due to quantum tunneling of electrons/holes through gate oxide • Increases as gate oxide thickness decreases and potential difference across gate oxide increases • Components: • Gate to channel (IGC) • Main contributor for NMOS • Occurs in “on” state and when source and drain are in opposite state from gate • Comprises of gate to source (IGCS) and gate to drain (IGCD) • Gate to source/drain overlap regions (a.k.a. edge direct tunneling (EDT)) • Occurs in “off” state and when source and/or drain are in opposite state from gate • Comprise of gate to source (IGSO) and gate to drain (IGDO) • Gate to body (IGB) • Occurs in “on” state and source and drain are in same state as gate • Negligible for NMOS • Comparable to or larger than IGC for PMOS • Practically unaffected by threshold voltage and temperature

5. ISB IDB Junction Tunneling Current • Occurs between drain/source to substrate due to band-to-band tunneling (BTBT) in “on” state and due to GIDL in “off” state • Occurs due to potential difference between body and source/drain  much more common to have drain-to-body leakage than source-to-body • GIDL occurs from drain to body when D=1, G=0, S=X, B=0 SPICE Measurement For NMOS IDB D=1, G=0, S=X, B=0 D=1, G=1, S=1, B=0 Measure b/w D and B ISB D=1, G=1, S=1, B=0 Measure b/w S and B For PMOS IDB D=0, G=1, S=X, B=1 D=0, G=0, S=0, B=1 Measure b/w D and B ISB D=0, G=0, S=0, B=1 Measure b/w S and B

6. Other Leakage Components • Hot carrier injection • Generally not considered leakage but more of a reliability issue • Occurs from gate to substrate, increases as LGate is reduced • Over time, HCI causes VTh shift which changes Isub • Punchthrough current • Occurs from drain to source • Of concern only when physical gate length under 10nm

7. RSCE SCE [YuNNH96] [YuNNH96] Reverse Short Channel Effect • Reverse short channel effect arises due to halo implants • Due to DIBL, short channel effect causes VTh to increase as LGate increases • To combat DIBL, halo implant is used for non-uniform doping (higher doping at S/D terminals, lower at channel center) • Halo induces reverse SCE (VTh decreases as LGate increases) • Affects subthreshold leakage only Halo doping (pocket implant) profile

8. Narrow Channel (Width) Effects • Narrow channel effects cause a shift in threshold voltage as channel width is reduced • depend primarily on isolation technique (LOCOS or STI) • For STI, reverse narrow channel effect (RNCE) is observed which cause VTh to decrease as width is decreased • For LOCOS, narrow channel effect (NCE) is observed which causes VTh to increase as width decreased Reverse Narrow Channel Effect [HsuehSDA88]

9. Variation of different leakage components with (a) oxide thickness, (b) doping profile. “Doping1” has a stronger halo profile than “Doping2”. For NMOS device 25nm and 50nm of effective channel length. [AgarwalKMR04] Simulation result for variation of different leakage components with temperature for NMOS device (Leff=25nm) [AgarwalKMR04] General Rules of Thumb • Every 3A change in Tox gives ~10X change in IGate. High-K will, however, will reduce IGate by 100x. • Subthreshold swing is usually ~85mV/dec at room temp. and a bit higher at operating temp (i.e., 90-100mV shift in VTh (or Vgs in subthreshold) will lead to 10X change in Isub.) • In most 65nm technologies, we found SCE to dominate RSCE up to 10% biasing • Gate biasing increases gate tunneling leakage due to increase in tunneling area. However, not enough to mitigate subthreshold leakage reductions over 250C • RNCE becomes noticeable under 300nm for 90nm process • Some results from AgarwalKMR04 (may not be general):

10. Sample Scripts and Results • All leakage components described in these slides (except for IGSO which is uncommon) can be mesured through the following three SPICE files in ~projects/LEAKAGE-TUT/example_scripts/ • d1g0s0b0.sp (NMOS terminals set as d=1, g=0, s=0, b=0) • d0g1s0b0.sp • d1g1s1b0.sp • Results for NMOS, 90nm TT process, 1.2V, 80oC • Isub = 131.5pA • IDB (with GIDL) = 1.240pA • IDB = 1.202pA • ISB = 1.202pA • IGCD = 0.3428pA • IGCS = 0.3422pA • IGDO = 0.01043pA • IGB = 2.277e-06pA

11. References • K. Roy, S. Mukhopadhyay, H. Mahmoodi-Meimand, “Leakage Current Mechanisms and Leakage Reduction Techniques in Deep-Submicrometer CMOS Circuits”, IEEE 2003. • A. Keshavarzi, K. Roy and C. F. Hawkins, “Intrinsic Leakage in Low Power Deep Submicron CMOS ICs”, ITC 1997. • A. Agarwal, C. H. Kim, S. Mukhopadhyay and K. Roy, “Leakage in Nano-Scale Technologies: Mechanisms, Impact and Design Considerations”, DAC 2004. • D. J. Frank, “Power-Constrained CMOS Scaling Limits”, Vol. 46, No. 2/3, IBM Journal of R&D, 2002. • B. Yu, E. Nowak, K. Noda and C. Hu, “Reverse Short-Channel Effects & Channel-Engineering in Deep-Submicron MOSFET’s: Modeling and Optimization”, 1996. • K. Hsueh, J. Sanchez, T. Demassa and L. Akers, Inverse-Narrow-Width Effects and Small-Geometry MOSFET Threshold Voltage Model, 1988.