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.
