Building an amplifier out of diodes
Gunn diodes have a region of negative resistance, i.e., at certain input voltage the current flowing through the diode is increased when the voltage is decreased. These diodes can be used to amplify RF, but they only become practical at microwave frequencies.

Wikipedia: Gunn diode

https://www.electronics-notes.com/articles/electronic_components/diode/gunn-microwave-diode.php