GATE 2015 EC – Question 43
The built-in potential of an abrupt p-n junction is 0.75 V. If its junction capacitance ($C_J$) at a reverse bias ($V_R$) of 1.25 V is 5 pF, the value of $C_J$ (in pF) when $V_R = 7.25$ V is ________.
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Correct answer: 2.4 to 2.6
Explanation
For an abrupt junction, $C_J \propto \frac{1}{\sqrt{V_{bi} + V_R}}$. The ratio is $\frac{C_2}{C_1} = \sqrt{\frac{0.75 + 1.25}{0.75 + 7.25}} = \sqrt{\frac{2}{8}} = \frac{1}{2}$. So $C_J = 2.5$ pF.