Tunnel diodes

Tunnel diodes (Esaki diodes) are used in various circuits in Tektronix gear made from the early 1960s until the 1980s.

Applications

Tunnel diodes were used where it was desirable to have fast and clean switching between two states. They were used in

trigger circuits as Schmitt triggers,
sweep and timing circuits as flip-flops,
pulse generators for converting a slow-rise signals to fast-rise pulses,
countdown/sync circuits

Issues of Drift and Failure

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. Germanium tunnel diodes are extremely sensitive to overheat, especially at soldering work. Be aware and use low melting solder and appropriate tool to protect the body from overheating!

Relevant Distinguishing Parameters

Many different types of tunnel diodes were made. The primary parameter that describes one is the peak current, which is the current at the top of the hill in the I-V curve. The other two relevant electrical parameters are the capacitance of the diode and whether it is made of GaAs or Ge. In some circuits, another model of tunnel diode can be substituted with only minor modifications to the surrounding circuit. Stan Griffiths describes such a modification here:

Tunnel Diode Replacement and Modification

Emulation using Common Parts

A common question is whether an electrical equivalent to a tunnel diode can be made out of modern, available parts. It is not hard to emulate the I-V curve, but there is no known circuit that can be made from available parts that has right I-V curve and the high switching speed of the real diode.

Testing a Tunnel Diode

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 (note the negative-resistance part of the curve may not show due to the fast transit).

A short article in Service Scope 49, April 1968 describes a quick checking setup that approximates a curve tracer using the scope's X sawtooth output (see in photo section below).

Tektronix 571 curve tracer run of 10 mA tunnel diode.

10mA tunnel diode mounted in Tektronix 571 curve tracer fixture.

Modeling

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.

Estimating Switching Speed

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 10 mA peak current and 5 pF 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 = C * V, we know that ∆ Q = C * ∆ V, which is ∆ Q = C * (V₂ - V₁)

With V₁ = 65 mV and V₂ = 465 mV, ∆ Q = 5 * 10^-12 F * 0.4 V = 2 picocoulombs.

Now we bravely assume that the charging current during the transition is constant, and is half of the peak current. 5 mA is 5 millicoulombs per second.

t = (2 * 10^-12 C) / (5 * 10^-3 A) = 0.4 ns

Tunnel Diodes Used in Tektronix Instruments

STD615 (152-01-02-00) - Ge, 10 mA, 28 pF
TD1081 (152-0099-00) - Ge, 50 mA, 6 pF
TD253 (152-0154-00) - Ge, 10 mA, 9 pF
TD3A (152-0125-xx) - Ge, 4.7 mA, 18 pF
1N3129 - Ge, 20 mA, 20 pF
1N3130 - Ge, 50 mA, 6 pF
1N3712 - Ge, 1 mA ±10%, 10 pF
1N3713 - Ge, 1 mA ±2.5%, 5 pF
1N3714 - Ge, 2.2 mA ±10%, 25 pF
1N3715 - Ge, 2.2 mA ±2.5%, 10 pF
1N3716 - Ge, 4.7 mA ±5%, 50 pF
1N3717 - Ge, 4.7 mA ±2.5%, 25 pF
1N3718/TD4 - Ge, 10 mA ±10%, 90 pF
1N3719 - Ge, 10 mA ±2.5%, 25 pF
1N3720 - Ge, 22 mA ±10%, 150 pF
1N3721 - Ge, 22 mA ±2.5%, 100 pF
152-0140-01 - Ge(?), 10 mA, 8 pF
152-0177-00/-01/-02 - Ge, 10 mA, 4.7 pF
152-0181-00 - Ge(?), 1 mA, 5 pF
152-0182-00 - Ge(?), 10 mA, 50 pF
152-0254-01 - Ge, 100 mA, 6 pF
152-0329-00 - Ge(?), 19 mA, 1.5 pF
152-0379-00 - Ge(?), 20 mA, 10 pF
152-0383-00 - 50 mA, t_r 31 ps
152-0386-00 - Ge(?), 10 mA, 25 pF
152-0402-00 - 2.2 mA 25 pF
152-0489-00 - Ge(?), 21 mA, 1.5 pF
153-0040-00 - 50 mA low-capacitance
153-0400-00 - 50 mA low-capacitance

Reading

Textbooks and references

Tunnel diode / Backward diode @ Wikipedia

Documents Referencing Tunnel diodes

Document	Class	Title	Authors	Year	Links
GenRad Experimenter V.34 No.7-8, July-Aug 1960.pdf	Article	Measurements of the Equivalent-Circuit Parameters of Tunnel Diodes		1960	Tunnel diodes
GenRad Experimenter V.34 No.7-8, July-Aug 1960.pdf	Article	The Use of the General Radio Immittance Bridge in Tunnel-Diode Measurements	E.Adler • R.C.Wonson	1960	Tunnel diodes
Sylvester P. Gentile, Basic Theory and Application of Tunnel Diodes, 1962.pdf	Book	Basic Theory and Application of Tunnel Diodes	Sylvester P. Gentile	1962	Tunnel diodes
Rufus P. Turner, Diode Circuits Handbook, 1963.pdf	Book	Diode Circuits Handbook	Rufus P. Turner	1963	Tunnel diodes
R.V.L'Archeveque, High Speed Quantization of Voltage Signals, (PhD Thesis), 1964.pdf	Book	High Speed Quantization of Voltage Signals (PhD Thesis)	R.V.L'Archeveque	1964	Tunnel diodes
Millman taub chapters 12 and 13.pdf	Book	Pulse, Digital, and Switching Waveforms: Devices and Circuits for Their Generation and Processing	Herbert Taub • Jacob Millman	1965	Tunnel diodes
Service Scope 38 Jun 1966.pdf	Article	Tunnel Diode Switching Circuits and the Back Diode		1966	Tunnel diodes
Service Scope 39 Aug 1966.pdf	Article	Tunnel Diode Switching Circuits and the Back Diode, Part II		1966	Tunnel diodes
Service Scope 49 Apr 1968.pdf	Article	Quick Check for Tunnel Diodes	Tony Bryan	1968	Tunnel diodes • 454
062-1009-00.pdf	Book	Measurement Concepts: Semiconductor Device Measurements	John Mulvey	1969	Curve tracers • Tunnel diodes
Andrews improved td pulse bias.pdf	Article	Improved Bias Supply for Tunnel-Diode Picosecond Pulse Generators	James R. Andrews	1970	Tunnel diodes
Tekscope 1970 V2 N2 Apr 1970.pdf	Article	Troubleshooting Sampling Systems	Charles Phillips	1970	Sampling • Tunnel diodes
Tekscope 1970 V2 N3 Jun 1970.pdf	Article	Troubleshooting Sampling Systems, Part 2	Charles Phillips	1970	Tunnel diodes
Tekscope 1972 V4 N4 Jul 1972.pdf	Article	Tunnel Diodes: In-Circuit Testing Using the 7D13 Digital Multimeter Plug-In		1972	Tunnel diodes • 7D13
Andrews directional-coupler td trig.pdf	Article	Directional-Coupler Technique for Triggering a Tunnel Diode	James R. Andrews	1975	Tunnel diodes
Andrews nahman flat pulse gen.pdf	Article	Reference Waveform Flat Pulse Generator	James R. Andrews • Barry A. Bell • Norris S. Nahman • Eugene E. Baldwin	1983	Tunnel diodes
Tektronix Curve Tracers - Device Testing Techniques.pdf	Book	Tektronix Curve Tracers - Device Testing Techniques		1985	Curve tracers • Tunnel diodes

Cross-reference

General Electric

General Electric Tunnel Diode Manual, 1st ed. 1961 / alternate copy
General Electric Transistor Manual (1964)
- Chapter 14: Tunnel Diode Circuits
- Chapter 19: Tunnel Diode Specifications
GE TD26x/TD27x Datasheet
General Electric Tunnel Diodes in 1966 Transistor Catalog (OCR)
1959 General Electric Research Information Services Report on Tunnel Diodes
General Electric 1N3712 to 1N3721 Data

RCA

RCA 1963 Tunnel Diode Manual (OCR)

Other Manufacturers

Images

Testing a tunnel diode on a 575
Testing a tunnel diode with an audio oscillator and a 7D20 in X-Y mode
Tunnel diode quick check method, from Service Scope 49, April 1968
Tunnel diode quick check example using the method from Service Scope 49, April 1968. Right beam is zoomed with delayed timebase to show step speed.
Tektronix 571 curve tracer I-V run of a 10 mA germanium tunnel diode
TD 253 Tunnel Diode from a Tek 547
Tunnel Diodes TD253 and TD3A in a Tek 547 Trigger and Sweep Section)
1D2 Tunnel Diode (2.2 mA) in a Tek 547 Delay Pickoff