A p-n junction diode has a depletion layer of thickness 400 nm and an electric field of 106 V/m. The value of the barrier potential that the conduction electrons must have so that they can diffuse from n-side onto p-side, junction diode is forward biased at 0.2 V, is given by:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 0.2 V
Across the depletion layer there is a built-in electric field, and the potential difference across the layer is the barrier potential. Electrons from the n-side must overcome it to diffuse to the p-side. For a uniform field,
\( \displaystyle V_0 = E\,d \)
\( \displaystyle V_0 = (10^{6}\,\mathrm{V\,m^{-1}}) \times (400\times10^{-9}\,\mathrm{m}) \)
\( \displaystyle V_0 = 0.4\,\mathrm{V} \)
A forward bias opposes the built-in potential and lowers the barrier by the applied voltage:
\( \displaystyle V_{\mathrm{barrier}} = V_0 - V_{\mathrm{forward}} \)
\( \displaystyle V_{\mathrm{barrier}} = 0.4\,\mathrm{V} - 0.2\,\mathrm{V} = 0.2\,\mathrm{V} \)
Note: It seems that there is a mistake in the original NTA paper, which prints “… onto p-side, junction diode is forward biased at 0.2 V, is given by” when it should read “… onto p-side, when the junction diode is forward biased at 0.2 V, is given by”. The words “when the” are missing, but the intended meaning is clear, so the question can still be solved.