The work develops an analytical method for determining the parameters of microplasmas. This phenomenon of localized avalanche breakdown plays an important role in avalanche-transit diodes and other semiconductor devices operating in an avalanche mode, while the switch-on voltage and differential resistance are the two main electrophysical parameters of the microplasma. The latter is determined by the sum of series resistances - the current cord itself and the spreading re-sistance. The paper proposes a variant of using the diode impedance measurement in the breakdown mode and its analyti-cal interpretation.