A capsule, being a pure reactance (capacitance) has no noise at all.
Indeed, in itself, it has no
electrical noise. However, that is not all.
Due to the acoustic radiation impedance, the capsule picks up noise. Another way of looking at it is that the air molecules, agitated by brownian movement, hit the diaphragm. The resulting noise level depends on the diaphragm area (and other physical conditions, such as temperature).
Because this noise is erratic, it increases according to the square root of the area, when the signal increases linearly with the area, resulting in an increase of 3dB S/N with each doubling of the area. So LDC's have intrinsically a better S/N than SDC's.
Now there is another very significant factor, the diaphragm damping. Damping is necessary to linearize the frequency response, which would be a huge midrange peak without it. This damping reduces the sensitivity.
Another technique has been developed, which consists in applying less damping and compensating with electronic EQ. This allows reducing the intrinsic noise to sub-zero values.