Explain in brief the following – (i) Monolithic capacitors (ii) Monolithic resistors.Ripunjay Tiwari
Ans. (i) Monolithic Capacitors – A capacitor is a structure in which a voltage induced separation of charge generates. Common capacitors are made by sandwiching a thin oxide layer among two conductive poly silicon layers in a MOS process. These capacitors do not depend on applied voltage and can be modeled as ideal capacitors. Large parasitic capacitor which is always formed between the lower plate and the substrate is the main drawback. Metal-diffusion, poly-diffusion or metal-poly capacitors with an SiO2dielectric are used, if the luxury of the double poly silicon layers is absent in MOS processes. In comparison of double poly capacitor these capacitors have a lower capacitance density and/or increased voltage dependence and/or less conductive lower plate. Metal-diffusion capacitors in the bipolar process. For the lower diffusion plate, heavily doped emitter diffusion is used.
(ii) Monolithic Resistors – Some monolithic resistors are passive devise and others are active devices. Generally ideal resistors are merely strips of polysilicon in standard MOS processes. Diffusion strips are also used for resistors although exhibit an nonlinear relationship between current and voltage. Diffusion strips or epitaxial strips are used for resistors in bipolar processes. These devices are quite linear. Mostly base diffusion is used due to its reasonably high sheet resistance. A contact is required to the epitaxial layer to stop forward biasing of the base collector junction. A base diffused resistor is shown in fig. 2.20.