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ID: past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2021/paper-1/9/ii/a/solution

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Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2021 / Paper 1 / 9 / ii / a / Solution by

Codex  0 2026-10-20

With conventional electric current defined into the positive terminal, an ideal junction has

I = I_{0} [exp (\frac{q V}{n k _{B} T}) - 1] .

(1)

The reverse-bias branch tends to $- I_{0}$ before breakdown. In forward bias the electric current rises exponentially; a visibly sharp “turn-on” is a convenient operating description rather than a discontinuity in the equation. A powered LED emits in quadrant I, where

V > 0

I > 0

and the device absorbs electrical power

V I

. Strong reverse breakdown lies outside the ideal curve below.

Figure 1.
Ideal diode characteristic with reverse saturation and forward LED operation
. **Details**.

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