Tunnel diodes

Back in the 1960s these were all the rage.  I remember making a simple tunnel diode oscillator at university in about 1969.

I am not sure if you can buy them still. They are 2 pin negative resistance diodes that work by quantum mechanical tunnelling. 

Very simple uW TXs are possible and there was one published that was copied about 200km away in New Zealand on 80m.

See https://en.wikipedia.org/wiki/Tunnel_diode .

See https://www.rfwireless-world.com/Vendors/Tunnel-diode-manufacturers.html .