Integrated Capacitors – The common parallel plate capacitor structures are as shown in the Fig. 1.24.
In most widely used type shown in Fig. 1.24 (a), the two polysilicon plates are seperated by silicon dioxide (Si02). Here the lower plate rests on the top of the substrate.
The capacitor shown in Fig. 1.24 (b) is MOS capacitor. It consists a implanted or diffused heavily doped layer within substrate while a polusilicon or metal plate on the top of a thin oxide layer. For MOS capacitors, generally gate oxide is used with no extra processing step.
In ICs, indlictors, transformers and chockes are not integrated because of bulkiness. The inductors can be fabricated on a chip in the form of a thin film spirals by successively depositing the conducting patterns. But using fabricated inductor, a very small value of inductor is possible which is of the order of few nano-henries. But •the practical approach is not to integrate inductors. The inductors are simulated on a chip with the help of R-C networks or some other type of a network. In case of RF and IF circuits, the use of inductor is unavoidable. Under such conditions, inductors are used externally with the integrated circuit.