الفهرس 
DESIGN AND charACTERIZATION OF VERY LOW CURRENT TO FREQUENCY CONVERTEROnly 14 pages are availabe for public view
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Abstract
very low level current measurement
(1 2 -13A)
requires a high input impeadance amplifier,generally using an
unprotected MOSFET and a high value resistor (=lE12 ohm ). An
irnl!roved
integrated
current
to frequency converter
(CFC)
fully
in NMOS technology is presented in this
thesis.
This converter uses the switched capacitor
technique.
This
version is characterized by better topology of devices, wider
uynamic range of frequency and the incooperation of circuitry
that detects the saturation of the 1fF characteristics and
then enforces a range change which effectively moves the
current measurement points towards the linear portion of the
characteristics.
In CHAPTER 1, we recall the MaS processing
(ma ter ia l,
oxidation,
diffusion, masking, etching,
scrihing and
fabrication), structure
and characteristics.
We also
intoduce a
formulation of
the MOSFET capacitor in its
different modes of operation.
We present also in this chapter the methods of very
low level current measurements, the different sources of
leakage currents in MaS devices and the precautions
to be
considered
for the compensation or the minimization of this
leakage which enables the very low level current measurements
at a very good resolution accuracy and linearity.
In CHAPTER 2, we explain a new accurate, sensitive and
linear current to frequency converter (CFC) and present a
detailed study of the analysis and design considerations for
the different blocks constructing it. We have proposed models
for predicting the performance and characterizing these
building blocks.
These models take into account alJ
restrictions and limitations associated with the MOSFET VLSI circuits.
These models are very important for the study and
the treatment of some nonlinearity problems revealed from
comparing the theory and experimental results. However, our
simulation results show that the CFC which we have presented
properly functions at very good linearity and accuracy.
In CHAPTER 3,
we introduce the different precautions to
be considered when preparing the test samples,to guarantee
the lowest possible level of leakage current during
discussmeasurements. We present our results of simulation,
and compare it with experimental results performed on an old
version of CFC. Our results revealed that:
a)
Lower
limit of current measurement is related
to the
uncompensated leakage current created at the input of the
current controlled oscillator,
b) The suplementary current is related
to the proximity
effect,
c) The
related
frequency jump in the CFC transfer function is not
to the OPAMP oscillation due to instability
as was beleived to be,
but is related to the
threshold
voltage
instability of
the MOSFET’s used
in the l/F
resistance and,
d) Frequency
related to
exponential
saturation of the CFC characteristlcs is
the deviation from the exponential c/c’s of
current generator, but is related to
relatively long delay of the double bistable multivibrator
used in the current controlled oscillator.
We finally propose
a) Using low leakage MOSFET techniques,
b) Using deposied and diffused guard rings
channel of the first MOSFET and the source of
MOSFET used in the R(F) resistance,
not
the
the
between the
the second ~) New techniques to compensate Eor the carrier heating in
the R(F) resistance and,
J)L1w use of more advanced technology (CMOS,
CMOS/SOS) .
MOS/SOS or