The diffused portion of the semiconductor region can be used for the construction of resistors. There are four fabrication methods-diffused, epitaxial, pinched and thin film. The diffused resistor is obtained during diffusion processes of either base or emitter. This resistor is formed in isolated regions of epitaxial layer. This is the most economical approach of fabrication of integrated resistor. The main limitation here is that higher value resistors are difficult to fabricate. The sheet resistance of the base and emitter diffusion is 200 ohms per square and 2.2 ohms per square respectively. The epitaxial resistor can be obtained in the epitaxial layer between two metal contacts and it is possible to realize large resistance values. A pinched resistor can be formed by narrowing down the cross section area of diffusion regions. This increases the sheet resistance up to mega ohms range. Thin film resistors can be produced by various thin film deposition techniques. Commonly used materials are silicon dioxide and nichrome. Sheet resistances in the range 40-400 ohms per square can be obtained by this method. Its smaller parasitic components results in a better frequency response. It is very much stable and its resistance can be controlled by laser trimming. But all these advantages in the expense of some additional fabrication processes.
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