CBSE Class 12 · 2019 · SemiconductorsShort Answer

(a) Three photodiodes

D_{1}

D_{2}

and

D_{3}

are made of semiconductors having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which of them will not be able to detect light of wavelength 600 nm?
(b) Why are photodiodes required to operate in reverse bias? Explain.

Answer: $D_{1}$ and $D_{3}$ cannot detect (band gap $> 2.07$ eV); reverse bias gives sensitive detection.

Solution

(a) Energy of a 600 nm photon:

E = \frac{1240 eV nm}{600 nm} \approx 2.07 eV .

A photodiode detects light only if the photon energy exceeds its band gap (

E \geq E_{g}

). Here

E = 2.07 eV

exceeds only

E_{g} = 2 eV

(diode

D_{2}

); diodes

D_{1}

(

2.5 eV

) and

D_{3}

(

3 eV

) have larger band gaps and therefore cannot detect the 600 nm light.
(b) In reverse bias the depletion region is wide and the field is strong, so the electron–hole pairs generated by the incident photons are swept apart quickly. The fractional change in the (small) reverse current caused by illumination is large, making the device a sensitive and linear detector of light intensity.

D_{1} and D_{3} cannot detect; reverse bias gives sensitive detection

CBSE Class 12 Board · 2019 · Subjective

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CBSE Class 12 · 2019 · SemiconductorsShort Answer

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