How do you have any transistor parasitics ?

How do you have any device are transistor parasitics?

Real device have parasitic that are necessary artifact's of the device structure.The transistor itself introduces significant gate capacitance(Cg)This capacitance which comes from the parallel plate formed by the poly gate and the substrate form the majority of the capacitive load in small logic circuit.Ch=0.9fF/,micrometer square for both n-type and p-type transistor in a typical 2micrometer process.The total gate capacitance for a transistor is computed by measuring the are of the active region (or W × L) and multiplying the area by the unit Cg.

      During fabrication the dopants in the source/drain region diffuse in all directions including under the gate.The source/drain overlap region is independent of the transistor length so it is usually given in units of farada per unit gate width Then the total source overlap capacitance for a transistor would be

                      Cha = ColW

There is also a gate/bulk overlap capacitance due to the overhang of the gate past the channel and onto the bulk.

          The source and drain region also have a non-trivial capacitance to the substrate and a very large resistance.

                                  Google searches 

                                    Atul Raja Kujur