  Organic Zener diodes with a precisely adjustable reverse breakdown from -3 V
to -15 V without any influence on the forward current-voltage curve are
realized. This is accomplished by controlling the width of the charge depletion
zone in a pin-diode with an accuracy of one nanometer independently of the
doping concentration and the thickness of the intrinsic layer. The breakdown
effect with its exponential current voltage behavior and a weak temperature
dependence is explained by a tunneling mechanism across the HOMO-LUMO gap of
neigh- boring molecules. The experimental data are confirmed by a minimal
Hamiltonian model approach, including coherent tunneling and incoherent hopping
processes as possible charge transport pathways through the effective device
region.
