What is the charge neutrality point in solid state physics? I came across the term in solid state physics recently, usually in the context of a gate voltage applied to some solid in order to tune the charge carrier density.
What is the definition of the charge neutrality point?
My understanding is that via the gate voltage, empty states can be filled by electrons or electrons can be drained from filled states. Naively, without gate voltage every solid should be charge neutral, i.e. the charge neutrality point should correspond to no gate voltage. Is this correct?
 A: Gate voltages are used in solid state devices such as field-effect transistors (FETs) to change the electron (or hole) density via the field effect. The same is possible in 2D layered materials such as graphene (e.g. encapsulated in boron nitride gate insulators) or transition-metal dichalcogenides.
The field effect can be understood in the simplest geometry of a planar 2D electron gas separated by a thin insulator from a planar gate electrode on the basis of a parallel plate capacitor. The change in density of the 2D electron gas is proportional to the change in the voltage between the electron gas and the gate. Changing the density via the gate voltage will also change the Fermi energy of the 2D electron gas.
For each material there is a position of the Fermi energy where the material is charge neutral. This is the charge neutrality point. In current research literature this term occurs frequently in connection with single-layer graphene or bilayer graphene, because there, the gate voltage can shift the Fermi energy from the valence band via the charge neutrality point to the conduction band.
When the conductance of such a 2D system is measured as a function of the applied gate voltage, it exhibits a minimum at the charge neutrality point. The reason is that the density of states at the charge neutrality point is very small.
At which specific gate voltage the charge neutrality point is reached depends on specific details of the material, such as its doping, built-in potentials between materials etc. In high-quality single- or bilayer graphene the charge neutrality point is usually very close to zero volts.
In other materials, such as InAs/GaSb double quantum wells, in which electrons and holes can coexist in the two neighboring layers, the charge neutrality point can be at voltages that are very different from zero.
