when i was young, there were a lot of tunnel diodes being used in a lot of different applications. but through the years, tunnel diodes have just about disappeared, except for a few microwave applications. tunnel diodes have a negative resistance region in their forward conduction curve but usually only over a span of about 0.1V. Gunn diodes also have a negative resistance region, but most Gunn diodes are made specifically for microwave applications. however there's a device that can be made with a pair of JFETs called a "Lambda" diode. the device works well at frequencies up to the limits of the JFETs used.
this allows some very simple oscillators to be built. shown here is a pair of JFETs (one N type, and one P type) and the forward bias curve. notice the very large negative resistance region, and that unlike tunnel diodes, the device goes into cutoff at voltages above the negative resistance region instead of the junction current rising. the shape and span of the negative resistance region can be changed by using different JFETs, the particular pair shown (2N5484 and 2N5114) gives a negative resistance region that begins at 1.9V and goes to about 9.5V.
this allows some very simple oscillators to be built. shown here is a pair of JFETs (one N type, and one P type) and the forward bias curve. notice the very large negative resistance region, and that unlike tunnel diodes, the device goes into cutoff at voltages above the negative resistance region instead of the junction current rising. the shape and span of the negative resistance region can be changed by using different JFETs, the particular pair shown (2N5484 and 2N5114) gives a negative resistance region that begins at 1.9V and goes to about 9.5V.