  We present a quantitative theory of the suppression of the optical linewidth
due to charge fluctuation noise in a ${p}$$-$${n}$ diode, recently observed in
Anderson et al., Science 366, 1225 (2019). We connect the local electric field
with the voltage across the diode, allowing for the identification of the
defect depth from the experimental threshold voltage. Furthermore, we show that
an accurate description of the decoherence of such spin centers requires a
complete spin$-$1 formalism that yields a bi-exponential decoherence process,
and predict how reduced charge fluctuation noise suppresses the spin center's
decoherence rate.
