Characteristic curves and spectral response curves.

The idea

A photodiode is a reverse-biased p-n junction. Its characteristic curve plots current against voltage for several fixed irradiances; in reverse bias the current is almost independent of voltage, so each irradiance gives a near-horizontal line whose magnitude is the photocurrent. Increasing irradiance shifts the curve downwards (larger reverse current). The spectral response curve plots responsivity, or photocurrent at fixed irradiance, against wavelength. It peaks where photon energy is just above the band gap, then falls at long wavelength once photons cannot free electrons, and falls at short wavelength as absorption occurs too near the surface. Comparing the two families of curves shows that bias voltage sets the operating point while wavelength and irradiance set the size of the response.

What earns the marks

• States that a photodiode is operated in reverse bias, so the p-n junction has a wide depletion region and a strong internal field.
• Describes the characteristic curve as current against voltage for a set of constant irradiance values, with the reverse-bias region nearly flat.
• Explains that in reverse bias the photocurrent is limited by the rate at which photons generate electron-hole pairs, not by the applied voltage.
• States that increasing irradiance increases the number of electron-hole pairs per second and so increases the reverse photocurrent.
• Describes the spectral response curve as responsivity or photocurrent against wavelength at fixed irradiance and fixed reverse bias.
• Explains the long-wavelength cut-off in terms of photon energy falling below the energy needed to excite an electron across the band gap.
• Explains the short-wavelength fall in terms of strong absorption close to the surface, where carriers recombine before reaching the junction.
• Identifies the peak of the spectral response as the wavelength giving the best compromise between photon energy and penetration depth.

Common mistakes and how to put them right

• Treating the photodiode as forward biased.

  Put it right by stating that the detector is reverse biased, so the dark current is small and the photocurrent is the dominant reverse current.

• Reading the flat reverse-bias region as a constant current for all irradiances.

  Put it right by labelling each curve with its own irradiance and noting that higher irradiance gives a larger reverse current.

• Assuming the spectral response rises without limit as wavelength decreases.

  Put it right by explaining that very short wavelengths are absorbed near the surface, so fewer carriers reach the junction.

Exam tips

• Sketch both axes with quantity and unit, and label each characteristic curve with its irradiance value rather than leaving unlabelled lines.
• When explaining a cut-off, name the band gap and compare photon energy with it, using E = hf or E = hc/λ as appropriate.
• Link each feature of a curve to a physical process: generation rate for irradiance, photon energy and absorption depth for wavelength.