3. Basic Electrical Quantities As you learned in lecture, a photodetector conver

Question

3. Basic Electrical Quantities As you learned in lecture, a photodetector converts light (photons) into charge (electrons) To get an idea of how we might choose the photodetector's size, imagine a photodetector is like a bucket vou leave out in the rain Falling Drops (Photons) Water Height (Voltage) Drops in Bucket Electrons) Photodetector (Bucket) Raindrops (representing photons) are collected as water (representing electrons). The height of the water represents the voltage you would measure in the photodetector. (a) Assume that the rain falls evenly over the entire area. if you were to double the area of the bucket, how would it change the height of the water'? (b) Now imagine that we can't measure the water level in the photodetector directly - instead we have to connect it to a separate measurement bucket, representing a circuit

Explanation / Answer

Raindrops (representing photons) are collected as water (representing electrons). The height of the water represents the voltage we would measure in the photodetector.

(a) Assume that the rain falls evenly over the entire area. If we were to double the area of bucket, then changing the surface area exposed to the rain wouldn't change the height of the water.

An increased area woul allow a proportionality larger volume of water to collect in the bucket, therefore maintaining the same height.

No rain falls on the measurement bucket & all its water comes from the photodetector bucket :

(c) An expression for height of the water in either bucket which is given as :

h = R Ap / (Ap + Ac)

where, Ap = area of the photodetector bucket

Ac = area of the circuit (meaurement) bucket

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