Non-Reciprocal Acoustoelectric Amplification in Germanium-Based Lamb Wave Delay Lines

Abstract

This paper reports, for the first time, on the use of acoustoelectric effect in single crystal germanium (Ge) for non-reciprocal amplification of Lamb waves in radio frequency (RF) delay lines. Such delay lines enable realization of integrated full-duplex wireless front-end modules for emerging 5G applications. A thin aluminum nitride (AlN) piezoelectric film, with single-phase unidirectional transducers, is used to excite fundamental anti-symmetric (A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) and symmetric (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) Lamb waves in Ge waveguide. The traveling waves are then amplified by application of a DC electric field across the Ge waveguide through the use of deformation-potential coupling between the traveling acoustic wave and DC-accelerated charge carriers. Mixed-domain analytical and numerical models are developed predicting 18 and 26 dB/mm amplification for S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> waves, respectively. Proof-of-concept delay lines are demonstrated operating at 72 MHz (A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) and 225 MHz (S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) frequencies showing transmission amplification of ~6 dB and ~7 dB upon application of 35 V and 45 V, respectively. Specifically, a non-reciprocal transmission ratio of 20 dB is measured for the S0 wave, highlighting the potential of the germanium-based RF acoustic delay-lines for adoption in the forthcoming full-duplex front-end modules.