Lambda diodes are devices composed of two complementary FETs, or a N channel FET and a PNP transistor.
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.
A variant of the Lambda Diode uses a PNP transistor in place of the P-Channel FET, it has a slightly different curve but it is fully functional. This is covered on the link below the figures to Ramon Vargas Patron's web site.
See this article by Ramon Vargas Patron for further details, I first built the oscillator on figure 7 of Vargas web page, then I built the BC Band receiver on figure 8 of the same page using a 2N5486 FET and a 2N3906 PNP transistor and I added a TL431 shunt regulator as a audio amplifier, it was pretty loud and worked quite well. At night you could pick up a lot of DX signals.
Although very sensitive it would be useless for use in a SDR receiver as the bandwidth is too narrow, and the regeneration has to be adjusted as one goes across the band. The same principle could be applied to using a varactor as parametric regenerative amplifier so it too would be too selective for use as the basis of a SDR receiver.
For use as a simple receiver it worked quite well so I'm wondering if building a regenerative receiver based on a Lambda diode would work as effectively in the 80M or 40M bands, my thoughts tend to think so as oscillators using that circuit can go as high as 100MHz. A regenerative receiver is an oscillator just at the edge of oscillating.
Below is an article of the use of a lamda diode use as the front end of a Gate Dipped Oscillator (GDO) a useful device in finding resonat frequency.
A Gate Dip Oscillator using a Lambda diode.
Another article on Lambda Diodes.