IV.3 - Tunnel diodes, backward diodes?

Tunnel diodes exhibit a current "dip" in their forward I-V characteristics. That is, for a certain range of forward voltages the current actually falls, instead of increasing. This creates a negative differential resistance, making it useful in oscillators and switching circuits. The underlying quantum-mechanical tunneling effect is extremely fast. Leo Esaki, who developed the tunnel diode was awarded a Nobel Prize in Physics for his efforts.

A backward diode is a diode with an extremely low breakdown voltage, causing it to conduct better in the reverse direction than in the forward direction. Backward diodes are similar in structure to tunnel diodes and may show negative resistance, in which case they are usually called tunnel diodes. Backward diodes are also known as "Uni Tunnel Diodes".

Here's what Gabriel Paubert told me about backward diodes:

• Custom Components backward diodes give excellent detectors, we use them for broadband detectors in radioastronomy and I can give a schematic of the one I have designed. We get an excellent dynamic range even on astronomical signal which is white gaussian noise and exceptional stability, which was our main goal. We easily measure the random fluctuation of 1GHz bandwidth gaussian noise, which is one part in 100000 in 10 seconds. By the way, with our circuit we get a much better tangential sensitivity than specified in the catalog, and we measure it at 10 Hz with a 1/f corner frequency close to 2 Hz. Most detectors are specified over wide video bandwidth but do not include the low frequencies because of flicker noise. Altogether we obtain a detector with 1.2 VSWR, +/- 0.1 dB frequency response, good linearity (accurate square-law), large dynamic range, in fact simultaneously all desirable characteristics over almost 5 octaves. We have been using these diodes since 1989, albeit in small quantities, about 150 in 6 years, and are very satisfied with the performance we get.

Brooke Clarke (brooke@pacific.net) offered this following historical insight:

• Aertech started in Mtn. View, CA then moved to Sunnyvale, CA. Along the way they were bought by TRW and later by FEI, and finally went out of business when the cold war ended. Metelecs was founded by someone who worked at Aertech.

  Aertech had two key product lines:

  Back Diode detectors and Tunnel diode amplifiers. I worked mainly on the TDAs, but knew most of what went on with the detectors.

  A Back Diode is a tunnel that has a peak current less than 1 mA and is designed for use in detectors.

  I built test equipment for characterizing Tunnel (and back) diodes. See: http://www.pacificsites.com/~brooke/RASS_PP.html#TD

  The impedance that the diode "sees" looking back into the test system MUST be less than the -R value of the diode AT ALL FREQUENCIES less than the self resonate frequency of the diode. For tunnel diodes, like used in amplifiers, this was in the tens of GHz. Making a circuit that has this characteristic requires a lossy coax or lossy radial line.

  Early tunnel diodes were made by hand one at a time. A small sphere of metal was alloyed to a chip of semiconductor material (Germanium, Gallium Arsenide, or Gallium Antimide) The chip was die attached into a metal ceramic package and had a gold screen bonded fro one side of the package flange to the metal sphere and to the other side of the package, like a bridge. Then the diode was etched in a caustic solution forming a mushroom shaped cross section. The stem of the mushroom was extremely narrow and the actual junction was in this stem.

  Later a "planar process" was developed that allowed back diodes to be made at the wafer level. This is used at Metelics.

  Note that when you let the smoke out of a semiconductor it fails to proceed. During the manufacturing processing, where the smoke is put into the semiconductor it is very important to know the lowest temperature that has an effect on the semiconductor properties. In the case of tunnel/back diodes that temperature is much lower than for other diodes like PINs. This means that the maximum operational temperature for tunnel/back diodes is much lower than for all other types of diode.

  TRW bought Aertech because we were making amplifiers for classified space programs and they wanted more control. Note that a TD amplifier draws a few mA of current at just over a volt. We once got a return after maybe 5 years and the problem was that the Mercury "D" battery had died. Tunnel and back diodes are heavily doped semiconductors. This means that they are not effected by radiation. Diodes like PINs that have light doping are very sensitive to an extra carrier.

  Back Diode detectors have wide video bandwidths because the source impedance is low. That's why the radio astronomer got such good results. They also have low flicker and 1/f noise. We built a lot of detectors for military crystal video ECM receivers. They also made good mixers, just use a coupler to combine the RF and LO and feed it into a BD detector then into an IF strip for a quick noise figure measurement.

Some manufacturers:

• M-pulse Microwave, 576 Charcot Avenue, San Jose, CA 95131
  Phone: (408)432-1480 FAX : (408)432-3440
  e-mail: techsupport@mpulsemw.com
  http://www.mpulsemw.com/
   
• Germanium Power Devices, 300 Brickstone Square York street, Box 3065 Shawsheen Village Station, Andover, MA 0180-3065
  Phone: 978-475-5982 FAX 978-470-1512
  http://www.gpd-ir.com/
   
• Metelics Corp, 975 Steward Drive, Sunnyvale CA 94086
  Phone: 408-737-8181
  FAX: 408-733-7645
  http://www.metelics.com/
   
• (maybe) Custom Components, Box 334, Lebanon, NJ 08833
  phone (908) 534-6151
  fax (908) 534-5625
   
  Custom Components makes Germanium and GaAs tunnel diodes. (Update: GaAs has been discontinued, Jan 1996. They took six weeks to reply to my request for a quotation, so I'm not sure how enthusiastically they make tunnel diodes.) They may be out of business, I'm not sure.