Passive Components:Capacitors and Junction Capacitors.

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Capacitors

As in the inductor case, the limitation on integrated capacitors is die size due to the limited capacitance/ per unit area available on a die. These limitations are imposed by the dielectrics used with their dielectric constants and breakdown voltages. Most integrated capacitors are either junction capacitors or MOS capacitors [13,14,15,16,17,18].

Junction Capacitors

A junction capacitor is formed when a p–n junction is reversed-biased. This can be formed using the base–emitter, base–collector, or collector–substrate junctions of an npn structure in bipolar ICs. Of course, the particular junction must be maintained in reverse bias to provide the desired capacitance. Since the capacitance arises from the parallel plate effect across the depletion region, whose thickness in turn is voltage-dependent, the capacitance is also voltage-dependent decreasing with increased reverse bias. The capacitance depends on the reverse voltage in the following form:

Passive Components-0081

clip_image003The built-in potential, y0, depends on the impurity concentrations of the junction being used. For example, y0= 0.7 V for a typical bipolar base–emitter junction. The exponent n depends on the doping profile of the junction. The approximations n = 1 for a step junction and n = 1 for a linearly graded junction are commonly used. The resultant capacitance depends on C0, the capacitance per unit area with zero bias applied. This depends on the doping level and profile. The base–emitter junction provides the highest capacitance per unit around 1000 pF/mm2 with a low breakdown voltage (~5 V). The base–collector junction provides ~100 pF/mm2 with a higher breakdown voltage (~40 V).
MOS Capacitors

MOS capacitors are usually formed as parallel plate devices with a top metallization and the high conductivity n+ emitter diffusion as the two plates with a thin oxide dielectric sandwiched in between. The oxide is usually a thin layer of SiO2 with a relative dielectric constant er of 3–4 or Si3N4 with er of 5–8. Since the capacitance obtained is e0erA/toxide, the oxide thickness, toxide is critical. The lower limit on the oxide thickness depends on the process yields and tolerances as well as the desired breakdown voltage and reliability. MOS capacitors can provide around 1000 pF/mm2 with breakdown voltages up to 100 V. Unlike junction capacitors, MOS capacitors are voltage-independent and can be biased either positively or negatively. Their breakdown, however, is destructive since the oxide fails permanently. Care should be taken to prevent overvoltage conditions.

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