Tunnel diodes

Tunnel diodes are used in various timing circuits in Tek gear made from the early 1960's until the 1980's. Tunnel diode characteristics (peak and valley voltages and currents) tend to drift. Usually this can be handled by adjusting the surrounding circuit. Sometimes tunnel diodes completely fail. Replacement usually involves scavenging a similar tunnel diode from some other device. There are some people in the Tek community who may have some tunnel diodes they can sell.

Before concluding that a tunnel diode is bad, it is important to be sure that it has been measured correctly. A high resistance reading on a DMM indicates that the diode is bad. A low resistance on a DMM and a low voltage on a diode tester are both normal when measuring a tunnel diode. A more thorough test of a tunnel diode is to drive it through a resistor with a ramp voltage source while observing the voltage across the tunnel diode. The resistor should be calculated so that the peak current just exceeds the peak current that the tunnel diode is rated for. Of course if a curve tracer is available, it is great for measuring the I-V curve of the diode.

The fast switching action of the tunnel diode can be understood by modeling it as a nonlinear voltage controlled current source (VCCS) in parallel with a small parasitic capacitor. The nonlinear VCCS is controlled by the voltage at the terminals of the diode and is responsible for the S-shaped I-V curve. (Alternatively and equivalently, it can be modeled as nonlinear resistance. However, the nonlinear VCCS model might be preferable because it avoids the confusing notion of negative resistance.) Consider a tunnel diode biased by a DC current source that is slowly brought up from zero to a current just a few microamperes less than the diode's peak current. The quiescent voltage will be just less than the peak voltage. Note that the I-V curve is nearly horizontal at this point, and therefore the incremental resistance of the diode is very high at this point. For simplicity, we can assume that the incremental resistance is infinite at this quiescent point.

(The following section will be clarified in the coming edits.)

Now that we have established the initial bias conditions, let's look at the event when the tunnel diode switches state. Assume that the triggering signal is coupled to the tunnel diode through a resistor. The current through the resistor adds to the current from the DC current source. Since we are assuming that the incremental resistance of the diode is infinite at the initial bias point, all of the current due to the trigger signal flows into and out of the diode's capacitance. If enough charge is added, the instantaneous voltage across the diode will be in the second region, where the slope of the VCCS function is negative.

Once the diode enters the second region, increases in diode voltage cause decreases in diode current. Applying Kirchhoff's current law at the node where the diode meets the DC current source, we can see that the current entering the parasitic capacitor at any instant is the difference between the DC current source and the nonlinear VCCS current at the this instantaneous voltage. We can use this fact to estimate the switching time of the tunnel diode. (The shape of the transition can also be estimated.) As an example, let's take the case of a tunnel diode with 10mA peak current and 5pF capacitance. A first-order estimate of the switching time can be made by assuming that to make the transition from V1 to V2, a certain amount of charge needs to be added to the parasitic capacitance of the diode. From Q=CV, we know that delta_Q=C*delta_V, or delta_Q=C*(V2-V1). We can assume that V1 is 65mV and V2 is 465mV. So delta_Q=(5*10^-12)*0.4, which is 2 picocoulombs. Now we bravely assume that the charging current during the transition is constant, and is half of the peak current. 5mA is 5 millicoulombs per second. (2*10^-12)/(5*10^-3)=0.4ns.

<math>frac{2 * 10 ^ {-12}}{5*10^{-3}} = 0.4 nanoseconds.</math>

Tunnel diodes

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