An open-gate pseudo-conductive high-electron mobility transistor for amplifying signals produced by a body of a user in a sub-THz radiation range, comprising: a multilayer hetero-junction structure made of gallium nitride (GaN) and aluminium gallium nitride (AlGaN) single-crystalline or polycrystalline semi-conductor materials, deposited on a substrate layer, wherein: (a) 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 (b) 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; or (c) said structure comprises (i) one top AlGaN 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, and (ii) one bottom GaN buffer layer; said layers have N-face polarity, thus forming a two-dimensional hole gas conducting channel in the GaN buffer layer, close to the interface with said AlGaN barrier layer; source and drain contacts connected to said 2DEG or 2DHG conducting channel and to electrical metallizations for connecting said transistor to an electric circuit; and a Vivaldi antenna electrode placed on the top layer between said source and drain contact areas in an open gate area of the transistor and capable of detecting electrical signals in the sub-THz-frequency range of 200-800 GHz. a multilayer hetero-junction structure made of gallium nitride (GaN) and aluminium gallium nitride (AlGaN) single-crystalline or polycrystalline semi-conductor materials, deposited on a substrate layer, wherein: (a) 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 (b) 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; or (c) said structure comprises (i) one top AlGaN 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, and (ii) one bottom GaN buffer layer; said layers have N-face polarity, thus forming a two-dimensional hole gas conducting channel in the GaN buffer layer, close to the interface with said AlGaN barrier layer; (a) 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 (b) 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; or (c) said structure comprises (i) one top AlGaN 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, and (ii) one bottom GaN buffer layer; said layers have N-face polarity, thus forming a two-dimensional hole gas conducting channel in the GaN buffer layer, close to the interface with said AlGaN barrier layer; source and drain contacts connected to said 2DEG or 2DHG conducting channel and to electrical metallizations for connecting said transistor to an electric circuit; and a Vivaldi antenna electrode placed on the top layer between said source and drain contact areas in an open gate area of the transistor and capable of detecting electrical signals in the sub-THz-frequency range of 200-800 GHz.