Fig. 5.2 and Fig. 5.3 show that electron current through the source-sided barrier is both tunneling and thermionic emission. However, there is a parasitic current due to the thermionic emission of holes across the drain sided barrier. If the drain voltage becomes higher than the gate voltage, the thickness of the drain-sided SCHOTTKY barrier for holes is reduced. As a result, the parasitic band-to-band tunneling current of holes increases and ambipolar conduction occurs.
The current contributions of electrons and holes are represented by blue and red curves, respectively. Note that at transition points electrons and holes have the same contribution to the total current, whereas in other regions either the electron or hole contribution will dominate. The results indicate that the ambipolar conduction has a detrimental effect on the device operation in both the on- and off-state.