http://www.mathworks.com/help/toolbox/physmod/elec/ref/diode.html

http://www.ittc.ku.edu/~jstiles/312/handouts/Zener%20Diode%20Models.pdf

http://cc.ee.ntu.edu.tw/~lhlu/eecourses/Electronics1/Electronics_Ch3.pdf

http://web.engr.oregonstate.edu/~traylor/ece112/lectures/diodes_new.pdf