GATE 2020 EE – Question 62
A non-ideal Si-based pn junction diode is tested by sweeping the bias applied across its terminals from $-5$ V to $+5$ V. The effective thermal voltage, $V_T$, for the diode is measured to be $(29\pm2)$ mV. The resolution of the voltage source in the measurement range is 1 mV. The percentage uncertainty (rounded off to 2 decimal places) in the measured current at a bias voltage of 0.02 V is ________.
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Correct answer: 11.50 to 12.00
Explanation
With $I=I_s(e^{V/V_T}-1)$, the contribution from $V_T$ is $\frac{e^x}{e^x-1}\cdot\frac{V}{V_T}\cdot\frac{\delta V_T}{V_T}=2.007\times0.690\times0.069=9.55\%$ (with $x=\frac{0.02}{0.029}$). The contribution from the source resolution is $2.007\times\frac{1\text{ mV}}{29\text{ mV}}=6.9\%$. Combining them as independent errors, $\sqrt{9.55^2+6.9^2}\approx11.8\%$.