Tunnel junctions (TJs) offer a unique approach to utilizing nonequilibrium tunneling injection of holes and have demonstrated potential applications in ultraviolet (UV) emitters. However, high operating voltage caused by the wide bandgap of the III-nitrides has impeded the further promotion of TJ. Here, 275 nm n⁺-Al_0.45Ga_0.55N/p⁺-Al_0.5Ga_0.5N ultrathin tunnel junction (UTJ) deep-UV light-emitting diodes (LEDs) are developed for minimizing the electrical losses and achieve, to the current knowledge, the lowest operating voltage (5.7 V) at 300 mA reported to date. This study discovers that Mg-Si co-doping occurs in the n⁺-Al_0.45Ga_0.55N layer during the growth of n⁺-Al_0.45Ga_0.55N/p⁺-Al_0.5Ga_0.5N UTJ due to the memory and diffusion effect of Mg, which leads to the formation of Ohmic contact between high-work-function Ni/Au and Mg-Si co-doped n⁺- Al_0.45Ga_0.55N layer in UTJ. The Zener diode, fluorine resin, and optimally designed glass lens are incorporated into the chip encapsulation to enhance the reliability and optical performance of the device. Furthermore, a high-efficiency sterilization deep-UV light source is developed integrated with 120 UTJ deep-UV LED chips. Complete surface inactivation at a long irradiation distance (20 cm) is achieved within only seconds using the high-power sterilization deep-UV light source. These results indicate that UTJ is promising in developing deep UV LEDs and their integrated light sources.