In a $PN$ junction photocell,the value of the photo-electromotive force produced by monochromatic light is proportional to

What kind of $p-n$ junction diode does not require any kind of bias?

The figure shows $2$ $LEDs$ that can be used as a polarity detector. Applying a positive source voltage results in a green light,while negative supplies result in a red light. Packages of such combinations are commercially available. Find the resistor $R$ to ensure a current of $20 \ mA$ through the $ON$ diode for the configuration. Both diodes have a reverse breakdown voltage of $3 \ V$ and an average turn-on voltage of $2 \ V$. The source voltage is $8 \ V$. Find $R$ in $\Omega$.

The semiconductor used for the fabrication of visible LEDs must at least have a band gap of (in $eV$)

For detecting light intensity,we use:

$A$ $p-n$ photodiode is manufactured from a semiconductor with a band gap of $3.1 \,eV$. Which of the following wavelengths can be detected by it?