All who is involved in electronic design knows that one of the critical issues
in today’s electronic is the power consumption. Designers are always looking for
new approaches in order to reduce currents while still retain performance.
Floating-gate (FGMOS) circuits have previously been shown to be a promising
technique to improve speed and still keep the power consumption low when
power supply is reduced below subthreshold voltage for the transistors.
In this thesis, the goal is to determine how good floating-gate circuits can be
compared to conventional static CMOS when the circuits are working in
subthreshold. The most interesting performance parameters are speed and power
consumption and specifically the Energy-Delay Product (EDP) that is a
combination of those two. To get a view over how the performance varies and how
good the FGMOS circuits are at their best case, the circuits have been designed and
simulated for best case performance.
The investigation also includes trade-offs with speed and power
consumption for better performance, how to select floating-gate capacitances, how
a large circuit fan-in will affect performance and also the influence of different
kinds of refresh circuits.
The first simulations of the FGMOS circuits in a 0.13 μm process have
several interesting results. First of all, in the best case it is shown that FGMOS has
potential to achieve up to 260 times in better EDP-performance compared to CMOS
at 150 mV power supply. Continuing with simulations of FGMOS capacitances
shows that minimum floating-gate capacitance can be as small as 400 fF and more
realistic performance shows that EDP is 37 times better for FGMOS (with parasitic
capacitances included). Other aspects of FGMOS design have been to look at how
refresh circuits will affect performance (semi-floating-gate circuits) and how a
larger fan-in will change noise margin and EDP. It turns out that refresh circuits
with the same transistor size does not give a noticeable change in performance
while an increase of 8 times in size will give between 5 and 10 times wors EDP.
When it comes to fan-in the simulations shows that a maximum fan-in of 5 is
possible at 250 mV supply and it decrease to 3 when supply voltage is reduced to
150 mV.
Finally, it should be kept in mind that tuning the performance of FGMOS
circuits with trade-offs and by changing the floating-gate voltages to achieve
results like the ones stated above will also always affect the noise margins, NM, of
the circuits. As a consequence of this, the NM will sometimes be so close to 1 that a
fabricated circuit with that NM may not be as functional as simulations suggests.
The probability to design functional FGMOS circuits in subthreshold does not
seem to be a problem though.