Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
gate leakage current is antoher concern in choosing MOSCAP as decoupling capacitor. IO MOSCAP can provide smaller leakage current , but you will pay more area to complete the specific capacitance, CORE MOSCAP has opposite relationship. It is due to the gate oxide thickness. MiM has very small leakage current (~pA), but extra mask is necessary.
1. The MOSCAP has a larger capacitance for the same area.
2. While it is true that the capacitance of the MOSCAP varies with applied voltage, if it is connected between supplies there is no such problem (variation is small because VDD is sufficiently larger than Vt).
3. The (gate) leakage current of a MOSCAP is usually negligible compared to your circuit's current consumption.
4. The channel resistance is very useful to damp the oscillations (the decoupling capacitance and the inductance of the bondwires make a resonator circuit). You don't have this in MiMs.
So, the MOSCAP is preferable for decoupling. Finally:
5. You do need an extra masking to make the MiM Capacitors. MiMs are preferable to Me-Me capacitors because their capacitance per unit of area is larger (about 4x-10x depending on the structure of Me-Me caps). Moreover the capacitance value of MiMs is much better controlled than the capacitance of Me-Me capacitors (corner capacitance variations are smaller). Most times there is even no characterization of Me-Me capacitors. Finally the matching of MiM caps is usually better. This is why, in some designs you end up paying for the additional mask.