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Hexagonal boron nitride is shown to exhibit very significant persistent photoconductivity after UV illumination. This behavior can be initiated by sub-bandgap or close to bandgap illumination. Neither temperature nor pressure affects the buildup of photoinduced carriers. The effect persists at least for months at room temperature and is maintained significantly after heating up to 300 °C. Up to six orders of magnitude increased conductivity has been durably established in the devices, and the effect is reproducible. Double exponential fitting gives time constants up to 4600 days. Irradiation after having saturated the devices is shown to drastically reduce the decay rate. The bulk origin of such effect has been demonstrated. p-hBN/n-AlGaN diodes based on magnesium doped h-BN have been used to determine the type of conductivity through studies of junction capacitance variations under UV irradiation. Depending on illumination wavelength, both n- and p-type durable photoinduced carriers can be produced. These results are of interest for UVC LEDs in which the usual conductive AlGaN layers are still a hurdle toward efficient UV emitters. [ABSTRACT FROM AUTHOR]