Consider a transistor that has parasitic capacitance of the order of pico-Farads.
Then at low frequencies, consider 10Hz
$$X_C=1/(2\pi f C)$$
\$X_C\$ will be in the range of giga-ohms.
This means that the resistance of the device is so high, that it must not allow current to flow through it.
This says us that any device that has a parasitic capacitance must now allow dc through flow through it. But indeed devices with parasitic capacitances allow dc to flow through them. How is this true?
This also means that at dc, the reactance is infinity and no current must flow through the transistor, but indeed current flows the device and we do many dc analysis.
How does this become possible?
Best Answer
Parasitic capacitances do not allow any DC current to flow through them. These capacitances are generally in parallel (or a parallel-series combination) across a pair of terminals - for example between Source and Drain on a MOSFET.
Because they are in parallel there are two paths for the current to flow - through the parasitic element or directly through the device. At AC, some current will flow through the parasitic element, and some through the device (if the device conducts between those two terminals in the mode it is in).
At DC, no current will flow through parasitic capacitances, but some current can flow (in parallel) through the device. Again, this is only if the device conducts between those two terminals in the mode it is in.
If you post a diagram of the transistor model you are using with its parasitics labelled up I can better illustrate this for you.