▁so ▁that , ▁according ▁to ▁such ▁equations , ▁we ▁also ▁verify ▁the ▁energy ▁balance ▁ ▁by ▁means ▁of ▁the ▁electro kin emat ics ▁theorem . ▁With ▁the ▁above ▁relationships ▁the ▁balance ▁can ▁be ▁extended ▁also ▁to ▁the ▁open ▁devices ▁by ▁taking ▁into ▁account ▁the ▁disp lacement ▁current ▁across ▁. ▁ ▁Fl uct u ations ▁ ▁A ▁meaning ful ▁application ▁of ▁the ▁above ▁results ▁is ▁also ▁the ▁computation ▁of ▁the ▁fl uct u ations ▁of ▁the ▁current ▁ ▁when ▁the ▁electro de ▁volt ages ▁is ▁constant , ▁because ▁this ▁is ▁useful ▁for ▁the ▁evaluation ▁of ▁the ▁device ▁noise . ▁To ▁this ▁end , ▁we ▁can ▁explo it ▁the ▁first ▁equation ▁of ▁section ▁Open ▁devices , ▁because ▁it ▁concerns ▁the ▁more ▁general ▁case ▁of ▁an ▁open ▁device ▁and ▁it ▁can ▁be ▁reduced ▁to ▁a ▁more ▁simply ▁relationship . ▁This ▁happens ▁for ▁frequencies ▁, ▁( ▁being ▁the ▁trans it ▁time ▁of ▁the ▁j th ▁car rier ▁across ▁the ▁device ) ▁because ▁the ▁in ▁time ▁integral ▁of ▁the ▁above ▁equation ▁of ▁the ▁Fourier ▁transform ▁to ▁be ▁performed ▁to ▁compute ▁the ▁power ▁spectral ▁density ▁( PS D ) ▁of ▁the ▁noise , ▁the ▁time ▁derivatives ▁provides ▁no ▁contribution . ▁Indeed , ▁according ▁to ▁the ▁Fourier ▁transform , ▁this ▁result ▁der ives ▁from ▁integr als ▁such ▁as ▁ ▁, ▁in ▁which ▁. ▁Therefore , ▁for ▁the ▁P SD ▁computation ▁we ▁can ▁explo it ▁the ▁relationships ▁ ▁Moreover , ▁as ▁it ▁can ▁be ▁shown , ▁this ▁happens ▁also ▁for ▁, ▁for ▁instance ▁when ▁the ▁j th ▁car rier ▁is ▁stored