Diodes, one-way current conductors, are used for voltage regulation, AC/DC conversion, and circuit protection. Forward-biased at 0.6V, conduct current, reverse bias blocks it.

1N3716 tunnel diode header image: Caliston [Public domain]. Posted in Engineering , Hackaday Columns , Parts , Slider Tagged Gunn diode , negative resistance , Poulsen arc , Tunnel diode Search ...

The tunnel diodes we mentioned above are probably the best known devices that exhibit negative resistance, and there was a time in the early 1960s before transistors gained extra performance that ...

For example, improving the switching efficiency (the fraction of contacted molecules at forward bias) would potentially lead to orders-of-magnitude gain in the rectification ratio.

In forward mode, approximately 150 µA flows through the diode, causing D4 to be reverse-biased and holding comparator U1’s inverting input low. This turns off the red LED.