Also almost forgotten - the lambda diode. Like the tunnel diode, it can also be used as a negative resistor.
Small error in the video. The currents in the recorded characteristic curve are shown too high by a factor of 10. The peak is around 4.7 mA and not 47 mA!

0:00 - Introduction
0:52 - What is a lambda diode?
1:43 - Recording the characteristic curve
3:02 - Principle of operation
3:52 - Oscillators with lambda diode
5:19 - Lambda diode as a voltage monitor
6:23 - Epilogue