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A Self-adjusting Scheme to
Determine Optimum RBB
by Monitoring Leakage Currents
Nikhil Jayakumar*
Sandeep Dhar$
Sunil P. Khatri*
$National
Semiconductor, Longmont,CO.
*Texas A&M University, College Station, TX.
Introduction


Leakage power is expected to exceed dynamic power
consumption in the near future.
Existing techniques to reduce leakage

Static
 High
VT power gating switches (MTCMOS, HL).
 Dual VT assignment (DUET).

Dynamic
 DTMOS,
SCCMOS.
 Reverse Body Biasing (RBB).

RBB reduces leakage (body effect) but if RBB is
too high, leakage can actually increase due to
other leakage components.
Leakage Components

Sub-threshold leakage


Gate leakage


Drain -> Source
Drain -> Gate / Source -> Gate
Drain -> bulk leakage
Bulk Band to Band Tunneling (BTBT)
 Surface BTBT (Gate Induced Drain Leakage - GIDL)
 Reverse biased PN junction current

Effect of RBB on Leakage






Sub-threshold decreases exponentially with Vt,
which increases with RBB.
Gate leakage (drain-gate) does not change
appreciably with RBB.
At RBB BTBT dominates GIDL.
Mainly, sub-threshold leakage & BTBT change with
RBB.
Sub-threshold leakage decreases with RBB,
while BTBT increases with RBB.
Hence there exists an optimum RBB point for
minimum leakage.
Variation in Leakage Components with RBB
Plot of leakage current components with RBB as measured on a large
NMOS device on a test-chip at 25oC
Optimal RBB Determination –
Previous work

In “Optimal Body Bias Selection for Leakage
Improvement and Process Compensation over Different
Technology Generations” – C. Neau and K. Roy
(ISLPED03), a circuit is presented to help find the
optimal RBB.
Based on the assumption that sub-threshold leakage is
negligible compared to BTBT for stacked devices.
 Claims that optimal RBB is at point at which leakage
through 2 stacked devices is equal to half the leakage
through a single leaking device.


This assumption underestimates the sub-threshold
leakage component.
Optimum RBB
Leakage current measured for a large
NMOS device and 2 large NMOS devices in
series on a 0.13μ test-chip at 25oC

Point A marks the
optimum RBB point as
would be suggested
by the previous
scheme.

Point B marks the
actual optimum RBB.

This work proposes a
circuit which
dynamically finds
point B.
Leakage Monitoring and
Self-adjusting Scheme

The Leakage Monitoring Scheme consists
of 3 components

A Leakage Current Monitor (LCM).
 This
leakage monitor is designed to work over a wide
range of leakage currents.



A Digital Block to interface with the LCM and control
the body-bias voltage.
A programmable body bias voltage generator
controlled by the Digital Block.
We discuss the first 2 blocks.
Leakage Monitoring and
Self-adjusting Scheme

The Leakage-Monitoring scheme is based on measuring the
time taken for a leaking device to discharge (or charge) a
capacitive load.

At the heart of this is the LCM.
The Leakage Current Monitor (LCM)
Shown here: LCM for NMOS
leakage.
 Node Nchk is initially
precharged.

Leakage through a
representative device ML is
measured by sampling
node Nchk at regular
intervals and seeing when
(number of sampling
periods after which) the
node Nchk is discharged.
 A capacitor bank and a
small gate bias are
provided to increase or
decrease rate of discharge
of the node.


Allows the LCM to work
over a wide range.
Operation of the Scheme
We start at a point on the
curve where leakage will
decrease with RBB
We then move along the
curve till …
we hit the point at which
leakage starts increasing

The Digital Block measures the time taken for the leaking
device in the LCM to discharge (charge) the monitored node
in the LCM.
Salient features of our Scheme

Low power



Only about 11.4μA at 1.2V and 125oC for
0.13 μm TSMC process
Leakage current measurement based on time taken
to discharge a node.
Uses same leakage monitoring cell to handle large
variations of leakage currents

Capacitor bank and switch-able gate bias used to
adjust range.
Area required
 Layout
(standard cell based) done for the leakage
monitors for PMOS and NMOS.
 Height of standard cell = 3.285μ.
 Width of cell for
Pulse generator = 38.22μ (126 μ2)
 LCM nmos = 77.87μ (256 μ2)
 LCM pmos = 86.41μ (284 μ2)

 Total
area approx = 665 μ2
Conclusion





Reverse body biasing is a useful technique to
reduce leakage.
However, if the RBB is too high, the leakage
current may inadvertently increase.
The optimum RBB point can vary with process
and temperature variations.
Hence a scheme such as ours that can dynamically
find the optimum RBB point can help greatly.
Also the scheme itself does not consume very
high power and it has a very modest silicon area
requirement.
Questions ?