Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-07-29
2001-01-16
Ng{circumflex over (o)}, Ng{circumflex over (a)}n V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S537000, C438S238000
Reexamination Certificate
active
06175137
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general to integrated circuits and, more specifically, resistive elements and a method of tailoring a resistance of a monolithic resistor.
BACKGROUND OF THE INVENTION
The need for higher precision resistors in integrated circuits is increasing due to the demand for higher precision measurement and instrumentation circuitry. Additionally, many integrated circuits that employ both analog and digital circuit components together require close matching of resistors. Some of these cases also require that the resistors to be matched have high resistance values. The two requirements of precision and high resistance values are especially difficult to achieve simultaneously when only a small area is available in the semiconductor wafer.
Resistors can be implemented in several ways in integrated circuits. They can be constructed using metal, polysilicon, n+ diffusions, p+ diffusions or tub regions. For metal and polysilicon, the sheet resistance is typically low and adequate overall resistance matching may be achieved for lower values of resistance. However, adequate matching of larger resistance requires the use of a large area in the semiconductor wafer. For tub regions, the sheet resistance is typically high. Resistance matching is difficult, however, due to the fact that the sheet resistance is dependent on the value of tub to substrate voltage. For n+ diffusions and p+ diffusions, matching properties are typically intermediate to the cases discussed above, with high precision and large resistance values still being difficult to achieve. An additional problem arises in that only some of the resistance formulation techniques may be practical in any given semiconductor production process.
Semiconductor wafer “real estate” is of prime concern in the design and production of semiconductor circuits. As CMOS technology continues to allow gate sizes to shrink, the potential for an increase in circuit density and therefore the total number of semiconductor devices allowed per semiconductor circuit chip has increased dramatically. In devices and circuits where high component precision is usually not required, such as digital circuits, resistor area size is more easily managed. However, in many analog devices where resistor precision, large resistance values and even resistance matching are required, the resistor area required may be the controlling factor in determining device density. This situation may significantly reduce the device density otherwise achievable, and even make a device design marginally economical from a market perspective.
Accordingly, what is needed in the art is a way to readily provide small area, precision resistors that allow adequate resistance matching even for large resistance values.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides a variable resistor, a method of manufacturing the same and a voltage bias circuit that incorporates at least one variable resistor. In one embodiment, the variable resistor includes: (1) a substrate including a doped region having an inherent resistance, (2) a controllable switch formed in the substrate, electrically coupled to the doped region and having a control terminal and (3) a controller, coupled to the control terminal, that toggles the controllable switch to modify a current flow through the doped region and thereby vary a resistance of the variable resistor.
The present invention therefore introduces the broad concept of intermittently introducing a monolithic resistor into a given circuit to vary the effective resistance of the resistor. A large monolithic resistor, which may vary in resistance from its inherent value, may be “tuned” in this manner to adjust the resistance to the intended value.
In one embodiment of the present invention, the controllable switch is coupled in electrical series with the doped region, the controller decreasing a duty cycle of the controllable switch to decrease the current flow through the doped region and thereby increase a resistance of the variable resistor. In this embodiment, the resistance can only be adjusted upward.
In an alternative embodiment, the controllable switch is coupled in electrical parallel with the doped region, the controller increasing a duty cycle of the controllable switch to increase the current flow bypassing the doped region and thereby decrease a resistance of the variable resistor. In this embodiment, the resistance can only be adjusted downward.
In yet another alternative embodiment, two controllable switches are employed, one placed in series and another placed in parallel with the doped region. In this embodiment, the resistance can be adjusted either upward or downward, depending upon which of the two controllable switches is toggled.
In one embodiment of the present invention, the doped region is selected from the group consisting of: (1) a metal region, (2) a polysilicon region, (3) an n+ doped region, (4) a p+ doped region and (5) a region of tub material. Those skilled in the pertinent art are familiar with these conventional monolithic, fixed-value resistor structures. The present invention, however, is not limited in scope to these structures, and may employ any conventional or later-discovered structure in the doped region.
In one embodiment of the present invention, the variable resistor may have a resistance value that is tailored or matched to a resistance value of a second resistor. The second resistor may be another variable resistor. Alternately, the second resistor may be a fixed-value resistor or any other type of resistor.
In one embodiment of the present invention, the variable resistor may be used in a mixed signal circuit having both analog and digital elements. The variable resistor may be a component in one of the analog elements or be matched to a resistor in the analog elements. Of course, the variable resistor may have control circuits that are part of the digital elements or be a component used in the digital elements.
In one embodiment of the present invention, the controllable switch is a complementary metal oxide semiconductor (CMOS) switch. However, those skilled in the pertinent art will understand that the controllable switch may be any conventional or later-discovered semiconductor switch. The controller may also be formed in the substrate. This allows the entire variable resistor, including the controller, to be monolithic. Of course, this need not be the case; the controller may be discrete from the substrate.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
REFERENCES:
patent: 4896243 (1990-01-01), Chatterjee et al.
patent: 5491357 (1996-02-01), Zambrano
Pe{acute over (o)}n Rogelio
Visee Maarten
Lucent Technologies - Inc.
Ng{circumflex over (o)} Ng{circumflex over (a)}n V.
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