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Until now, a common feature of many wide band gap heterojunction diodes is an unexplained large ideality factor n>2. In this context we investigate the diode characteristics of heterojunction diodes consisting of a crystalline semiconductor material such as ZnO covered with a thin semiconducting film of amorphous or disordered material. As thin disordered film we use sp2-bonded turbostratic boron nitride. These heterojunctions exhibit a pronounced rectifying behavior, low saturation current, and low parasitic currents. Moreover, we observe an apparently giant ideality factor reaching values of n>100. As a consequence, the turn-on voltage is around 5–10 V and the I-V curves can be measured for bias voltages between ±80 V without reaching saturation or electrical breakdown. We present a quantitative model for the unusual diode characteristics of these metal-amorphous semiconductor-semiconductor diodes. We demonstrate that the I-V characteristics of the heterojunctions are well described by a serial arrangement of an ideal Schottky diode, a Frenkel–Poole type resistance, and an Ohmic contact resistance, emulating a p-n- or Schottky diode characteristic with giant ideality factor. We propose that heterojunctions exhibiting apparently large ideality factors n>2 may possess an interfacial disordered or amorphous layer with Frenkel–Poole conduction properties. [ABSTRACT FROM AUTHOR]