Consider a situation in which reverse biased current of a particular $P-N$ junction increases when it is exposed to light of wavelength $\lambda \le 621 \, nm$. During this process,enhancement in carrier concentration takes place due to the generation of hole-electron pairs. The value of the band gap is nearly (in $eV$):

• A
  $1$
• B
  $4$
• C
  $2$
• D
  $0.5$