  This paper reviews the experimental and theoretical state of the art in
ballistic hot electron transistors that utilize two-dimensional base contacts
made from graphene, i.e. graphene base transistors (GBTs). Early performance
predictions that indicated potential for THz operation still hold true today,
even with improved models that take non-idealities into account. Experimental
results clearly demonstrate the basic functionality, with on/off current
switching over several orders of magnitude, but further developments are
required to exploit the full potential of the GBT device family. In particular,
interfaces between graphene and semiconductors or dielectrics are far from
perfect and thus limit experimental device integrity, reliability and
performance.
