| 摘要 |
There is disclosed an improved technique for reducing or eliminating parasitic capacitance associated with diffused resistors in which the normal buried layer is eliminated and in which each resistor is either fully or partially surrounded by an isolation ring. The epitaxial layer between the resistive element and the isolation ring is provided with a back bias either at one end of the resistor or at the point along the resistor at which no signal exists. In one case this may be accomplished by shorting the epitaxial layer to the low impedance end of the resistor. In the embodiment in which one end of the resistor is shorted to the epitaxial layer, the resistor is polarized having a low impedance end and a high impedance end, such that the end to which the epitaxial layer is shorted is the end which is coupled to the low impedance node of a circuit. In the case where the resistor is to be coupled across nodes having equal impedance, the epitaxial layer is provided with back bias adjacent that point along the length of the resistor at which no signal appears. There is also disclosed a guard circuit, which minimizes parasitic capacitance in cases where a low resistivity epitaxial layer or a buried layer is provided. The guard circuit is in the form of an impedance convertor having a high impedance output applied to the high impedance end of the resistor while the low impedance of the convertor is coupled to an epitaxial layer contact adjacent this high impedance end. In the guard circuit embodiment, an additional epitaxial layer contact is provided adjacent the low impedance end of the resistor and is shorted thereto so as to provide the aforementioned back bias.
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