Abstract: An insulating barrier extending between a first conductive region and a second conductive region is disclosed. The insulating barrier is provided for tunnelling charge carriers from the first to the second region, the insulating barrier comprising a first portion contacting the first region and a second portion contacting the first portion and extending towards the second region, the first portion being substantially thinner than the second portion, the first portion being constructed in a first dielectric and the second portion being constructed in a second dielectric different from the first dielectric, the first dielectric having a lower dielectric constant than the second dielectric.