In a MOS capacitor with an oxide layer thickness of $10\: nm,$ the maximum depletion layer thickness is $100\: nm.$ The permittivities of the semiconductor and the oxide layer are $\varepsilon_{s}$ and $\varepsilon_{ox}$ respectively. Assuming $\varepsilon_{s}/\varepsilon_{ox} = 3,$ the ratio of the maximum capacitance to the minimum capacitance of this MOS capacitor is ________.