Time-dependent tunnelling and the injection of coherent Zener oscillations

The Zener oscillator, a device based upon time-dependent tunnelling through the potential barrier of an n-i heterojunction under modulated bias, is investigated as a possible source of terahertz electromagnetic radiation. The authors show that coherent Zener oscillations can be injected from an initial thermal distribution of electrons. The electrons trapped in the confines of the tilted band structure oscillate, and the resulting circuit voltage is capable of maintaining the coherence of the oscillations. These oscillations release energy to the external circuit by drifting through the intrinsic semiconductor until collected at a metal contact. Scattered electrons which get out of phase are also eliminated by the same process. The device depends upon the existence of the Wannier ladder. In relation with the Zener oscillator, they report the results on a study of polar scattering by longitudinal optical phonons of localised or accelerated electrons. The degree of localisation of the electrons, or position uncertainty, is found not to affect the lifetime since the thermal broadening of electron Bloch states is smaller than the optical phonon energy. The scattering of Zener oscillations might be reduced in the steady state when the Zener frequency is much larger than the optical phonon frequency, but the turn-on transient remains problematic. An alternative implementation might be realised using a superheterojunction made of the junction of two superlattices. The Zener oscillator is opening up a new class of devices based on coherent electronic processes.