Which one of the following graph represents forward bias...

Conceptual problem. Theory based. Graph D has exponential characteristic, which is similar to ideal diode. The current-voltage function (also called the " $i-v$ characteristic") for an ideal diode is $i(v)=I_S\left[\exp \left(\frac{v}{\eta V_T}\right)-1\right], v>V_Z$ where $I_S$ is the reverse saturation current, $v$ is the applied voltage (reverse bias is negative), $V_T$ is constant dependent on temperature, and $\eta$ is the emission coefficient, which is 1 for germanium devices and 2 for silicon devices. $V_Z$ is the reverse bias break done voltage.