A physically-based transient model for low frequency noise of both microplasma and impact ionization in PN diodes is discussed and implemented in the SPICE simulator. The simulation indicates that the model correctly describes the non-monotonic behavior of both the DC and the noise characteristics of diode at the onset of avalanche breakdown. The model is based on a new microplasma switching theory, and the results of simulation confirm the findings of this theory. The microplasma switching threshold is the condition of equality of free- to space charge concentration in the depletion layer. The microplasma turn-on is initialized by the charge generation due to few recombination centers in the microplasma region at high avalanche multiplication due to impact ionization. The microplasma on-current is approximately twice the threshold current and the on-current sustains until the low, but larger than 1, avalanche multiplication compensates for the carrier diffusion from microplasma region into the depletion layer. When the multiplication becomes lower than the diffusion, the microplasma switches off.