A lambda diode is an electronic circuit that combines a complementary pair of junction gated field effect transistors into a two-terminal device that exhibits an area of differential negative resistance much like a tunnel diode. The term refers to the shape of the V–I curve of the device, which resembles the Greek letter λ (lambda).
Lambda diodes work at higher voltage than tunnel diodes. Whereas a typical tunnel diode[1] may exhibit negative differential resistance approximately between 70 mV and 350 mV, this region occurs approximately between 1.5 V and 6 V in a lambda diode due to the higher pinch-off voltages of typical JFET devices. A lambda diode therefore cannot replace a tunnel diode directly.
Moreover, in a tunnel diode the current reaches a minimum of about 20% of the peak current before rising again towards higher voltages. The lambda diode current approaches zero as voltage increases, before rising quickly again at a voltage high enough to cause gate–source Zener breakdown in the FETs.
It is also possible to construct a device similar to a lambda diode by combining an n-channel JFET with a PNP bipolar transistor.[2] A suggested modulatable variant but is a bit more difficult to build uses a PNP based optocoupler and can be tweaked by using its IR diode. This has the advantage that its properties can be fine tuned with a simple bias driver and used for high sensitivity radio applications. Sometimes, a modified open can PNP transistor with IR LED can be used instead.
