Conventional light-emitting diodes (LEDs) face an efficiency droop at low current due to non-radiative recombination overtaking radiative recombination at low carrier density. To overcome this universal problem, we develop LEDs with high efficiency at ultralow current and voltage, using a novel quantum well design and high-quality interfaces to suppress non-radiative recombination and enhance radiative recombination. The device exhibits close to unity internal quantum efficiency at a low current density of <1 × 10−4 A cm−2, more than three orders of magnitude lower than conventional LEDs. The LED bias voltage is reduced to ~30% below the photon voltage (hν/q). Wireless communication is demonstrated at these low-power conditions, which enables new applications in smart dust and sensor networks1–6, low-cost block chain and authentication7–9, medical applications10,11 and wherever high efficiency at low power is needed. New phenomena such as high-efficiency electroluminescent cooling becomes possible as the LED unity internal quantum efficiency extends to smaller voltage and current.