theory attributes the OMFE to the change of electron hopping coefficient in a field. Thus, it does not have all the troubles as those spin-dynamics related theories in- volve concepts of excitons and bipolarons [7–11, 18, 19]. 

The strong OMFE in nonmagnetic organic materi- als is the consequences of combined effects of the Van der Waals bonding between organic molecules, a large molecule-molecule separation and localization length. In the usual inorganic semiconductor, all these conditions are not satisfied. Firstly, the bonding between atoms are covalent so that the field-independent hopping co- efficient is order of several eV, much bigger than the field-related contribution. Secondly, in the hopping con- duction of the conventional doped semiconductors, the localization length of a localized state and the hopping distance are order of angstroms so that ξd/l 2 B is many orders magnitude smaller than that in the organic semi- conductors in the weak field. In a strong field l$_{B}$ < d , the opposite regime of OMFE phenomenon, magnetic con- finement dominates electron hopping, and the resistance increases exponentially with the magnetic field. This is why the similar magnetic-field effects had not been ob- served in inorganic semiconductors. Whether the mech- anism presented here is genuine or not is subject to the experimental tests. Thus, we are proposing to design and manufacture devices with various ξd/l 2 B by using different materials and molecule structures. It would be a definite proof of present theory if all devices show OMFE when d/l$_{B}$ < 1 and ξd/l 2 B is order of 1. 

In conclusion, we present a novel mechanism for the OMFE for nonmagnetic organic semiconductors. The mechanism is very general and robust for organic semi- conductors, but is normally not important for usual cova- lently bonded inorganic semiconductors. The mechanism can not only explain all experimentally observed OMFE, but also provide a solid theoretical bases for the empir- ical OMFE formulas. New experiments are needed to firmly establish this mechanism as the genuine cause of the OMFE. 

This work is supported by Hong Kong UGC grants (#604109, HKUST17/CRF/08, and RPC07/08.SC03). S.J. Xie is supported by the National Basic Re- search Program of China (Grant No.2009CB929204 and 

No.2010CB923402) and the National Natural Science Foundation of the People’s Republic of China (Grant No. 10874100) 

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