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

The $I-V$ characteristics of an $LED$ are correctly shown by which graph?

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

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 $\Omega$.

$A$ family uses $8 \; kW$ of power.
$(a)$ Direct solar energy is incident on the horizontal surface at an average rate of $200 \; W$ per square meter. If $20 \%$ of this energy can be converted to useful electrical energy, how large an area is needed to supply $8 \; kW$?
$(b)$ Compare this area to that of the roof of a typical house.

$A$ $p-n$ photodiode is fabricated from a semiconductor with a band gap of $1.4 \ eV$. It can detect a signal of wavelength: