Question

Phot diodes are preferable used in reverse bias conditions for measuring light intensity because
A. Current in reverse biase is less which is easier to measure compared to higher currents in forward bias
B. High fractional change due to photo effects in minority carrier dominated reverse bias current is more easily measurable than the high ractional change in the forward bias current
C. Low fractional change due to photo-effect in minority carries dominated reverse bias current is more easily measurable than the high fractional change in the forward bias current
D. Current in zero in reverse bias set-up

Answer 1

Correct Answer - B
Consider a ase of an n type semi-conductor te majority carries density (n) is considerably larger than the minority hole density `rho` (i.e., n gt gt p). On illumination `n_(f)=n+Deltan` & `p_(f)=p+Deltap`
since `Deltan=Deltap` {equal hole electron pair generation in bond breaking]
`(Deltan)/(n)lt lt (Deltap)/(p)`
`therefore` it is difficult to measure small `(Deltan)/(n)`
Compared to `Deltap//p`
`therefore` Photodiode is used in Reverse bios mode since it is easier to measure the higher fractional change of minority carriers.