One type of RTDs is formed as a single quantum well structure surrounded by very thin layer barriers. This structure is called a double barrier structure. Carriers such as electrons and holes can only have discrete energy values inside the quantum well. When a voltage is placed across an RTD, a terahertz wave is emitted, which is why the energy value inside the quantum well is equal to that of the emitter side. As voltage is increased, the terahertz wave dies out because the energy value in the quantum well is outside the emitter side energy.