A surface acoustic wave radio-frequency identification (RFID) sensor chip comprising: (a) a piezoelectric substrate, said substrate comprising: a piezoelectric layer, and a multilayer heterojunction structure, said structure being made of III-V single-crystalline or polycrystalline semiconductor layers, deposited on said piezoelectric layer and comprising at least one buffer layer and at least one barrier layer, said layers being stacked alternately; (b) at least one pair of metal interdigitated transducers mounted on said piezoelectric substrate, for receiving a radio frequency input signal, transducing said input signal into a surface acoustic wave, propagating said surface acoustic wave along a surface of said piezoelectric substrate and transducing said propagated surface acoustic wave into an output RF signal; (c) at least one pseudo-conducting two-dimensional electron gas or two-dimensional hole gas structure deposited on said piezoelectric substrate for forming a pseudo-conducting 2DEG or 2DHG channel in said heterojunction structure at the interface between said buffer layer and said barrier layer; and (d) electrical metallizations capacitively-coupled to said IDTs and to said pseudo-conducting 2DEG or 2DHG structures for inducing displacement currents, thereby creating non-ohmic source and drain contacts, for connecting said sensor chip to an electric circuit; characterised in that: (i) said III-V single-crystalline or polycrystalline semiconductor materials are GaN/AlGaN; and (ii) said pseudo-conducting 2DEG or 2DHG structure is a semiconducting structure formed on said piezoelectric substrate by recessing or growing a top layer of said multilayer heterojunction structure to a thickness of 5-9 nm with a surface roughness of the recessed or grown top layer being 0.2 nm or less, thereby forming the pseudo-conducting 2DEG or 2DHG channel capable of conducting electric current in the current range between normally-on and normally-off operation modes of the channel. (a) a piezoelectric substrate, said substrate comprising: a piezoelectric layer, and a multilayer heterojunction structure, said structure being made of III-V single-crystalline or polycrystalline semiconductor layers, deposited on said piezoelectric layer and comprising at least one buffer layer and at least one barrier layer, said layers being stacked alternately; a piezoelectric layer, and a multilayer heterojunction structure, said structure being made of III-V single-crystalline or polycrystalline semiconductor layers, deposited on said piezoelectric layer and comprising at least one buffer layer and at least one barrier layer, said layers being stacked alternately; (b) at least one pair of metal interdigitated transducers mounted on said piezoelectric substrate, for receiving a radio frequency input signal, transducing said input signal into a surface acoustic wave, propagating said surface acoustic wave along a surface of said piezoelectric substrate and transducing said propagated surface acoustic wave into an output RF signal; (c) at least one pseudo-conducting two-dimensional electron gas or two-dimensional hole gas structure deposited on said piezoelectric substrate for forming a pseudo-conducting 2DEG or 2DHG channel in said heterojunction structure at the interface between said buffer layer and said barrier layer; and (d) electrical metallizations capacitively-coupled to said IDTs and to said pseudo-conducting 2DEG or 2DHG structures for inducing displacement currents, thereby creating non-ohmic source and drain contacts, for connecting said sensor chip to an electric circuit; (i) said III-V single-crystalline or polycrystalline semiconductor materials are GaN/AlGaN; and (ii) said pseudo-conducting 2DEG or 2DHG structure is a semiconducting structure formed on said piezoelectric substrate by recessing or growing a top layer of said multilayer heterojunction structure to a thickness of 5-9 nm with a surface roughness of the recessed or grown top layer being 0.2 nm or less, thereby forming the pseudo-conducting 2DEG or 2DHG channel capable of conducting electric current in the current range between normally-on and normally-off operation modes of the channel.