The one-dimensional Poisson equation for the surface space charge region at the drain is

• A. \(\frac{{{\partial ^2}{\rm{\Psi }}}}{{\partial \;{x^2}}} = - \frac{q}{{{\varepsilon _s}}}\left( {N_D^ + + N_A^ - } \right)\)
• B. \(\frac{{{\partial ^2}{\rm{\Psi }}}}{{\partial {x^2}}} = \frac{{ - q}}{{{\epsilon_S}}}\left( {N_D^ + + N_A^ - + p + n} \right)\)
• C. \(\frac{{{\partial ^2}{\rm{\Psi }}}}{{\partial {x^2}}} = \frac{{ - q}}{{{\epsilon_S}}}\left( {N_D^ + - N_A^ - + p - n} \right)$\)
• D. \(\frac{{{\partial ^2}{\rm{\Psi }}}}{{\partial {x^2}}} = \frac{{ - q}}{{{\epsilon_S}}}\left( {p - n} \right)\)