Question

(a) Three photo diodes D₁, D₂ and D₃ are made of semiconductors having band gaps of 2.5 eV and 3 eV, respectively. Which of these will not be able to detect light of wavelength 600 nm? 

(b) Why photo diodes are required to operate in reverse bias? Explain.

Answer 1

(a) Photo diode is a semiconductor made of photosensitive semiconductor material. The conductivity of p-n function photo diode depends on the intensity of light falling on it.

Given:

• Band gap of photo diode D₁ is 2.5 eV. 

• Band gap of photo diode D₂ is 2 eV. 

• Bond gap of photo diode D₃ is 3 eV.

Wavelength of the incident light is λ = 600 nm. Therefore, the energy of incident light is

where h is Planck’s constant, c is the speed of light. Therefore,

The photo diode is able to detect the light of wavelength 600 nm if the energy of incident radiation is more than the band gap of the photo diode. Therefore, D₁ and D₃ are not able to detect incident light.

(b) Photo diodes are required to operate in reverse bias because when a photo diode is reverse biased, the width of depletion region increases, and the function capacitance decreases. Thus, the response time of photo diode decreases making it faster. Photo diode is one of the fastest photo detectors.