FIELD OF INVENTION
This invention relates to a method of varying threshold voltage in
closely located MOSFETs.
PRIOR ART
Many of the novel application with electronics are possible because
of integrated circuits where devices in a circuit are fabricated or. a single
silicon wafer. One of the key advantages of this is that the characteristics
of the devices are very close to each other which make the functionality
of the circuit predictable.
However, for some selected circuits, transistors are needed to have
different threshold voltage. This may be needed to achieve different on
currents (for higher speed) or off currents (for lower sub-threshold
leakage) in selected transistors. This is often accomplished in the
industry by allowing different gate oxide thickness and/or threshold
adjust implant. Due to lithographic and implantation limits, it
becomes difficult to place transistor devices with different threshold
voltages next to each other sufficiently close to achieve compactness of
circuits. This results in extra cost by way of chip real-estate. This real
estate cost can become particularly significant in replicated circuits,
such as SRAMS, where each unit may be replicated many tens of
thousands of times.
The present invention suggests a method of varying the 'hrcshok!
of at least two transistors which can be located next to each otherlocated
as close as the technology will allow any two transistors lo be
located with no extra processing cost.
OBJECTS OF THE INVENTION
The primary object of the present invention is to propose a method
of varying threshold voltage in MOSFETs located on the same chip.
Another object of the present invention is to propose a method of
varying threshold voltage in closely located MOSFETs.
Still another object of the present invention is to propose a method
of varying threshold voltage in closely located MOSFETs which is cost
effective as compared to the prior art.
Another object of the present invention is to propose a method of
varying threshold voltage in closely located MOSFETs which involves less
number of steps as compared to the known art.
Further object of the present invention is to propose a method of
varying threshold voltage in closely located MOSFETs which allows the
transistors to be located close to each other.
Still further object of the present invention is to propose a method
of varying threshold voltage in closely located MOSFETs which allows
compactness of circuits.
STATEMENT OF THE INVNETION
According to this invention there is provided a mot hod of varying
threshold voltage in closely located MOSFETs comprising the steps of
varying the spacing between the selective buried insulating layer
disposed under the source and drain of the MOSFET, where the
insulating layer is such as silicon dioxide or any other appropriate
insulator.
Further, according to this invention there is provided a method of
varying threshold voltage in closely located MOSFETs comprising steps of
positioning of the MOSFETs on a chip with different spacing between ?.heburied
insulating layer disposed under the source and drain of *ho
MOSFET; and increasing or decreasing the difference in the threshold
voltages between the different devices by biasing the well ve/Jve V.MM
the source of the MOSFET to obtain larger/smaller threshold volumes
differences between the devices.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Further objects and advantages of this invention will be more
apparent from the ensuing description when read in conjunction with the
accompanying drawing and wherein:
Figure . 1 shows the SELective Buried OXide (SELBOX) structure
with buried oxide under the source and drain of the MOSFET.
Figure .2 shows the plot showing threshold (Vt) variation in
NMOSFET with different substrate bias.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO
ACCOMPANYING DRAWING.
The proposed application provides a method of varying threshold
voltage in closely located MOSFETs, which may be explained with the
help of the following embodiments b}