A microelectronic sensor for non-invasive monitoring of at least one physiological parameter of a patient, comprising at least one open-gate pseudo-conductive high-electron mobility transistor (PC-HEMT), printed on a flexible printed circuit board (PCB) , and connected to its dedicated electrical contact line printed on said PCB ; characterised in that said transistor comprises: (a) a multilayer hetero-junction structure made of gallium nitride (GaN) and aluminium gallium nitride (AlGaN) single-crystalline or polycrystalline semiconductor materials, and deposited on a substrate layer or placed on free-standing membranes ; ▪ said structure comprises (i) one top GaN layer recessed in an open gate area of the transistor to the thickness of 5-9 nm and having the surface roughness of 0.2 nm or less, (ii) one bottom GaN buffer layer , and (iii) one AlGaN barrier layer in between; said layers have Ga-face polarity, thus forming a two-dimensional hole gas conducting channel in the top GaN layer, close to the interface with said AlGaN barrier layer ; or ▪ said structure comprises (i) one top GaN layer recessed in an open gate area of the transistor to the thickness of 5-9 nm and having the surface roughness of 0.2 nm or less, (ii) one bottom GaN buffer layer , and (iii) one AlGaN barrier layer in between; said layers have N-face polarity, thus forming a two-dimensional electron gas conducting channel in the top GaN layer, close to the interface with said AlGaN barrier layer ; and (b) source and drain contacts connected to said 2DEG or 2DHG conducting channel and to electrical metallizations for connecting said transistor to an electric circuit . (a) a multilayer hetero-junction structure made of gallium nitride (GaN) and aluminium gallium nitride (AlGaN) single-crystalline or polycrystalline semiconductor materials, and deposited on a substrate layer or placed on free-standing membranes ; ▪ said structure comprises (i) one top GaN layer recessed in an open gate area of the transistor to the thickness of 5-9 nm and having the surface roughness of 0.2 nm or less, (ii) one bottom GaN buffer layer , and (iii) one AlGaN barrier layer in between; said layers have Ga-face polarity, thus forming a two-dimensional hole gas conducting channel in the top GaN layer, close to the interface with said AlGaN barrier layer ; or ▪ said structure comprises (i) one top GaN layer recessed in an open gate area of the transistor to the thickness of 5-9 nm and having the surface roughness of 0.2 nm or less, (ii) one bottom GaN buffer layer , and (iii) one AlGaN barrier layer in between; said layers have N-face polarity, thus forming a two-dimensional electron gas conducting channel in the top GaN layer, close to the interface with said AlGaN barrier layer ; and ▪ said structure comprises (i) one top GaN layer recessed in an open gate area of the transistor to the thickness of 5-9 nm and having the surface roughness of 0.2 nm or less, (ii) one bottom GaN buffer layer , and (iii) one AlGaN barrier layer in between; said layers have Ga-face polarity, thus forming a two-dimensional hole gas conducting channel in the top GaN layer, close to the interface with said AlGaN barrier layer ; or ▪ said structure comprises (i) one top GaN layer recessed in an open gate area of the transistor to the thickness of 5-9 nm and having the surface roughness of 0.2 nm or less, (ii) one bottom GaN buffer layer , and (iii) one AlGaN barrier layer in between; said layers have N-face polarity, thus forming a two-dimensional electron gas conducting channel in the top GaN layer, close to the interface with said AlGaN barrier layer ; and (b) source and drain contacts connected to said 2DEG or 2DHG conducting channel and to electrical metallizations for connecting said transistor to an electric circuit . <EOS>