A standard NP junction diode will maintain a forward voltage drop of about .2 volts for a germanium diode and about .6 volts for a silicone diode. If you have a voltage source that is connected in series through a potentiometer a fixed resistor and a standard diode when you measured the voltage across the forwarded biased diode you would see the constant .2 or .6 volt drop as you change the potentiometer’s resistance up and down. As the potentiometer’s resistance decreased the voltage across the fixed resistor will raise because there will be an increase in current.
Now if you take the same circuit described above but replace the diode with a tunnel diode you will get a very different action. When you first start to decrease the resistance of the potentiometer the current will start to rise and thus the voltage across the fixed resistor will rise. As you continue to decrease the resistance, at a certain point, the voltage across the fixed resistor will start to drop because the current is dropping. So now with the same applied voltage but with a decrease in resistance the current decreases instead of increasing. This phenomenon is know as negative resistance.
Using this phenomenon a Dynatron oscillator can be constructed. If an LC (coil and capacitor) tank circuit could be constructed that had no resistance then when voltage was applied and removed it would start oscillating and would continue to oscillate as the energy transferred back and forth between the capacitor and the inductor. We know that such a circuit does not exist that is why there needs to be some type of compensation for the energy that is lost. In the case of the dynatron oscillator that energy is replaced by the negative resistance possessed by the tunnel diode placed in parallel with the LC circuit. When the negative resistance and the positive resistance become equal they will cancel each other out and the circuit will continue to oscillate.
Not only can the negative resistance be used as an oscillator but it can also be used as a very low noise amplifier and thus make excellent preamplifiers for VHF, UHF, and microwave receivers.
The tunnel diode was very popular in the early to mid 1960s but the newness wore off and so did the interest but they can still make some interesting and useful projects for the ham shack. They also provide some very simple projects.