A thermal-to-electric conversion apparatus, comprising a hot radiating surface (S H) juxtaposed to a cold surface (S C) separated by a small gas-filled or vacuum gap (g): characterised by the hot radiating surface (S H) comprising single charge carriers supplied from a hot-side reservoir (r H) that are excited from a lower energy level (state a) to an upper energy level (state b) by the hot-side temperature for electrostatically coupling excitation energy to the cold surface (S C); the cold surface (S C) comprising an array of converter elements, each converter element comprising: single charge carriers supplied from an output of a first cold-side reservoir (r 1) maintained at ground potential to a first cold side quantum well (W C1) being excited from a lower energy level (state 1) to a higher energy level (state 2) by the electrostatically coupled energy from the hot radiating surface (S H); the excited single charge carriers at higher energy level (state 2) from the first quantum well (W C1) that tunnel through a potential barrier being received by a second quantum well (W C2) connected to an input of a second cold-side reservoir (r 2) maintained at an elevated voltage (+); and an electrical load connected between an input of the first cold-side reservoir (r 1) and an output of the second cold-side reservoir (r 2). characterised by the hot radiating surface (S H) comprising single charge carriers supplied from a hot-side reservoir (r H) that are excited from a lower energy level (state a) to an upper energy level (state b) by the hot-side temperature for electrostatically coupling excitation energy to the cold surface (S C); the cold surface (S C) comprising an array of converter elements, each converter element comprising: single charge carriers supplied from an output of a first cold-side reservoir (r 1) maintained at ground potential to a first cold side quantum well (W C1) being excited from a lower energy level (state 1) to a higher energy level (state 2) by the electrostatically coupled energy from the hot radiating surface (S H); the excited single charge carriers at higher energy level (state 2) from the first quantum well (W C1) that tunnel through a potential barrier being received by a second quantum well (W C2) connected to an input of a second cold-side reservoir (r 2) maintained at an elevated voltage (+); and an electrical load connected between an input of the first cold-side reservoir (r 1) and an output of the second cold-side reservoir (r 2). single charge carriers supplied from an output of a first cold-side reservoir (r 1) maintained at ground potential to a first cold side quantum well (W C1) being excited from a lower energy level (state 1) to a higher energy level (state 2) by the electrostatically coupled energy from the hot radiating surface (S H); the excited single charge carriers at higher energy level (state 2) from the first quantum well (W C1) that tunnel through a potential barrier being received by a second quantum well (W C2) connected to an input of a second cold-side reservoir (r 2) maintained at an elevated voltage (+); and an electrical load connected between an input of the first cold-side reservoir (r 1) and an output of the second cold-side reservoir (r 2).