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A MOS capacitor is fabricated on $p$-type Si (Silicon) where the metal work function is $4.1$ eV and electron affinity of Si is $4.0$ eV. $E_C-E_F=0.9$ eV, where $E_C$ and $E_F$ are the conduction band minimum and the Fermi energy levels of Si, respectively. Oxide $\epsilon _r=3.9$, $\epsilon _o=8.85\times 10^{-14}$ F/cm, oxide thickness $t_{ox}=0.1\mu m$ and electronic charge $q=1.6\times 10^{-19}$ C. If the measured flat band voltage of this capacitor is $-1$ V, then the magnitude of the fixed charge at the oxide-semiconductor interface, in $nC/cm^2$, is ___________.