26-02-2013, 11:51 AM
Use of nano-scale double-gate MOSFETs in low-power tunable current mode analog circuits
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Abstract
Use of independently-driven nano-scale double
gate (DG) MOSFETs for low-power analog circuits is
emphasized and illustrated. In independent drive configuration,
the top gate response of DG-MOSFETs can be
altered by application of a control voltage on the bottom
gate. We show that this could be a powerful method to
conveniently tune the response of conventional CMOS
analog circuits especially for current-mode design. Several
examples of such circuits, including current mirrors, a
differential current amplifier and differential integrators are
illustrated and their performance gauged using TCAD
simulations. The topologies and biasing schemes explored
here show how the nano-scale DG-MOSFETs may pave
way for efficient, mismatch-tolerant and smaller circuits
with tunable characteristics.
Introduction
In low-power analog systems, current-mode signal processing
has been usually considered an attractive strategy
due to its potential for high-speed operation and
low-voltage compatibility [1, 2]. These features can be
especially rewarding in the context of mixed-signal system
design in sub-100 nm CMOS era, where SOI substrates
provide a viable platform for active and passive RF device
integration while also hosting ultra-small CMOS devices
for the digital system blocks. However, in most currentmode
circuits, the tuning of circuit response is achieved by
use of extra transistors, leading to losses in area and
performance. Moreover, device mismatch can lead to significant
reduction in the circuit performance. In the present
work, we explore novel low-voltage analog circuits using
double-gate (DG) MOSFETs, which provide means to
alleviate above concerns by utilizing the new architectural
features and operational modes of these nano-scale
transistors.
Device structure and modeling
DG-MOSFETs considered in this work are chosen to
facilitate the mixed-mode circuit design methodology,
which seeks to integrate analog circuits on the same substrate
as digital building blocks with minimal overhead to the fabrication sequence. This implies using DG-MOSFETs
with a minimal body thickness (tSi B 30 nm), oxide
insulator thickness (tox B 5 nm) and gate length
(L B 100 nm), and maximum ION/IOFF ratio optimized
normally for minimum switching delay power product
[11]. It is also assumed that both gates have been optimized
for symmetrical threshold VT = ±0.25 V using a dualmetal
process. A generic DG-MOSFET structure based on
these design guidelines, and in agreement with the experimentally
demonstrated devices [12] is given in Fig. 1(a).
2D simulations of this structure are accomplished using
DESSIS [13] in drift-diffusion approximation for carrier
transport, which is sufficient for low-power circuit-configurations
explored here.
Tunable current mirrors
The simple current mirror (CM) (see Fig. 3(a)) constitutes
one of the simplest yet most important design blocks for
analog circuit engineering. It can be used to copy reference
currents or set operating points across the integrated analog
circuit blocks. Normally, depending on the ratio of transistor
width between the input (reference) and output branch, the mirror characteristics can be set, which is
constant once the circuit is built. In our case, however, a
similar gain factor can be easily obtained, and dynamically
changed, by appropriate bottom biases of DG-MOSFETs
used in the mirror block, as shown in Fig. 3(b). Tunability
not only greatly enhances the variety of applications for
this otherwise simple circuit, but could also lead to area
and/or power savings over similar circuits built using bulk
MOSFETs.
Conculsions
Unique and novel examples of low-power current mode
analog circuit blocks based on DG-MOSFETs have been
investigated. Using mixed-mode (device + circuit) TCAD
simulations, we have shown how the bottom-gate of
independently driven DG-MOSFETs may be used to
design and test current mode analog circuits with tunable
performance metrics. In particular, we have provided
examples for a tunable simple current mirror, a tunable low
voltage cascade current mirror.