Lambda diodes are devices composed of two complementary FETs, or a N channel FET and a PNP transistor.
When configured correctly they exhibit a current versus voltage curve that looks like the Greek Lambda letter, hence that is where they got it’s name.
This curve consist of two portions the first part where current increases with increasing voltage and the second half where the current decreases with increasing voltage thereby exhibiting a negative resistance. If biased to operate on the negative resistance portion of the curve the device can be used as an amplifier or an as an oscillator.
See this article for further details.
A Gate Dip Oscillator using a Lambda diode.
Another article on Lambda Diodes.