I think the spike in the middle of the graphene ambipolar curve is due to the unique electronic properties of graphene, specifically the transition through the Dirac point where the density of states is minimal, and the material exhibits its lowest conductiv

The spike in the middle of the graphene ambipolar curve, known as the Dirac peak or charge neutrality point, occurs due to graphene's unique electronic properties. At the Dirac point, the Fermi level intersects the conduction and valence bands, resulting in minimal density of states and charge neutrality. However, the conductivity doesn't drop to zero because of Thermal excitation, Disorder and defects and Quantum capacitance. Additionally, electron-hole puddles from inhomogeneities can cause asymmetry in the spike