Most of the experimental research on graphene focuses on the electronic
properties. The most notable feature about the early work on graphene transistors was
the ability to continuously tune the charge carriers from holes to electrons. An
example of the gate dependence in single layer graphene is shown in Fig. 3.4a. This
effect is most pronounced in the thinnest samples whereas samples from multiple
layers show much weaker gate dependence due to screening of the electric field by the
other layers.