Take one P channel Jfet and one N channel Jfet and connect them in series so the two sources are together, connect the gate of each transistor to the other one's drain. This is known as a lambda connection, and if you plot the voltage vs current from drain to drain you will see a negative resistance region, usually around 3v (depending on the transistors). The circuit will work as a tunnel diode oscillator up to 100-200mhz.