The simplest model of an RTD consists of an electron incident on a double barrier potential (Figure \ref{fig:barrier}). The two potential barrier regions are achieved by fabricating a heterostructure. This is a well known setup, and can been solved in detail by solving for the transmission coefficient \(T(E)\). It is seen that unlike a single barrier setup, for which the transmission coefficient is always less than one, a double barrier leads to a fully transmitting setup at certain electron energies (or barrier separation). This means there is no scattering of electrons and the electrons effectively do not see any barrier \cite{Mizuta1995}. These energies are the resonant energy modes of the potential configuration.