A Non-Reciprocal Lamb-Wave Delay Line Exploiting Acoustoelectric Effect in Single Crystal Germanium

The paper reports on the use of acoustoelectric amplification in single crystal germanium (Ge) to achieve non-reciprocity in radio frequency (RF) delay lines operating based on Lamb waves. Such delay lines are important in attaining integrated full-duplex wireless communication systems, an integral part of forthcoming 5G applications. Fundamental anti-symmetric (A0) and symmetric (S0) Lamb waves are excited in Ge waveguide using a 200nm-thik aluminum nitride (AlN) piezoelectric transducer with unidirectional IDTs. The generated traveling waves are then amplified through deformation potential coupling between the wave and DC accelerated electrons in Ge. Finite element model (FEM) is used to identify A0 and S0 modes at 72 MHz and 267 MHz respectively. Analytical models, based on acoustoelectric interaction in single crystal Ge, is used to predict strong amplification of 18 dB/mm (A0) and 26 dB/mm (S0). Fabricated delay lines operating at 70 MHz (A0) and 226 MHz (S0) demonstrate forward transmission amplification of ∼6 dB and ∼7 dB upon application of 35 V and 45 V, respectively. In particular, a non-reciprocal transmission ratio of 20 dB is measured for the S0 mode, showcasing the prospects of Ge-based RF delay lines in realization of full-duplex front-end modules.