Diodes That Store Charge
These include PIN, Varactor, Step Recovery Diodes (SRD) and Noise diodes. In some cases a guard ring schottky that is improperly constructed can exhibit charge storage.
If there is charge storage then there is a frequency [Fcs = 1/(charge storage time)] and if the signal of interest is well above Fcs then the operating point of the diode does not follow the I-V curve. The RF signal sees an almost constant diode impedance. A PIN diode can be used to switch high powers for frequencies that are well above Fcs. The diode capacitance at low frequencies is different from the capacitance well above Fcs where it is the same as the depleted (large reverse bias) low frequency value (even with no bias).It is important to characterize a charge storage diode both above and below Fcs as well as knowing the storage time (or lifetime). If this is not done, then diodes that have some (but not all) parameters the same will NOT work the same in the final application.
Diodes That do Not Store Charge
These include Schottky (both simple and guard ring, Tunnel (aka Back) diodes. In this type of diode the RF signal follows the I-V curve. For example a Schottky diode can be used as a microwave detector but can not handle high power.Schottky
This type of diode also called a "hot carrier" diode is formed with a metal-semiconductor junction. They can be made using either P or N type semiconductors and with different metals resulting in different barrier heights. For diodes with about the same area the barrier height shows up as a difference in the forward voltage at 1 ma. All of these simple schottky diodes have a reverse break down voltage in the 3 to 9 volt range.Space Charge Limited Current
There is a limit to the current that can flow through a Schottky diode. It is defined in Sze's book "Physics of Semiconductor Devices". When Schottky diodes are used in mixers the mixer performance has limitations because of space charge current limiting. A measure of this effect can be had by looking at the diode resistance as measured by a 100 ma current pulse that is very narrow. I remember heavily funded government programs trying to get a handle on this effect but not knowing what the effect was. It may still be a mystery to many people.
WJ App Note: Predicting intermodulation (IM) suppressionin double-balanced (DB) mixers - did not understand this.
WJ App Note: Mixers in Microwave Sysems Part 1 - did not understand this.
WJ App Note: Mixers in Microwave Sysems Part 2 - did not understand this.Guard Ring Schottky
This is a Schottky diode with two additional diode junctions added. There is a "guard ring" junction surrounding the normal Schottky junction that also goes under the normal junction. If it is done correctly the guard ring increases the reverse breakdown voltage to the 30 - 45 volt range and the third junction never turns on in the forward direction. If the third junction does turn on then it can store charge and degrade the performance of the diode. To check for this the forward I-V curve is inspected to see if there is a "double break". The forward curve should have a single smooth exponential rise.Automated Testing
The DC behavior can be well modeled by assuming the diode follows the classical exponential I-V curve and in series has a fixed series resistance (Rs). By measuring a few points on the I-V curve and doing some curve fitting the Rs can be separated from the diode junction.
When measuring these low diode voltages a Kelvin connection is needed for good accuracy.
Tunnel & Back Diodes
Aertech, which 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. Metelics 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.
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. You can just put a resistor in parallel with the TD and this will stabilize a little of the IV curve just past the peak and valley points. As the value of the resistor gets lower and lower the area stabilized increases. A short will completely stabilize the diode, but then you can't see the I-V curve.
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 then had a gold screen bonded from 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. An improvement to that process involved gluing a couple of very small diameter glass rods (made by pulling a hot glass rod) on either side of the metal ball after sintering and before attaching the mesh. The rods tended to take some of the stress off of the tunnel junction and make the diode much more reliable.
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 proprieties. 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.
3626257 Semiconductor Device with SUperlattice Region Dec. 7 1971 by Leo Esaki,
classes: 257/15; 148/DIG65; 148/DIG67; 148/DIG72; 148/DIG97; 148/DIG169; 257/1; 257/28; 257/E29.073; 257/E29.078; 257/E47.004 is probably the first Tunnel Diode patent and is the oldest patent in the 257/15 class.
4198644:Tunnel diode by Esaki; Leo 1980
TRW had a trade secret method of making the planar back diode and Metelics developed their own process.
The GE Transistor manual had a good section on their through hole Tunnel Diodes with circuit examples.
Bob Mouw was the first person to make a double balanced mixer that worked at microwave frequencies. Prior to his invention double balanced mixers were made using a couple of ferrite balun transformers and a "ring" connected diode quad. Their upper frequency limit was around 2 GHz. Bob took the classical circuit and made a "dual" that used a "star" diode quad and hybrid coaxial transmission line transformers. I helped do the mathimatical analysis for this mixer. You can consider the diodes as switches that are turned on and off by the Local Oscillator. The two states either pass the input signal or invert the input signal. Doing an FFT on the waveform gives all the frequency domain outputs of a mixer, less those that are cancelled due to the mixer balance.
3512090 5 /1970 HYBRID JUNCTIONS AND MIXERS AND MODULATORS CONSTRUCTED THEREFROM
Issued/ Filed: May 12, 1970 / Oct. 18, 1967 455/326; 333/24R; 455/331 - Is the first version of the Mouw mixer pattern
3818385 06/18/1974 HYBRID JUNCTION AND MIXER OR MODULATOR 333/26; 333/35; 333/238; 333/243; 455/326
The diodes shown on the first page are in ceramic packages. Later there were much more advanced versions built. These typically were made in octave bandwidths.3638126 01/25/1972 HIGH-FREQUENCY CONVERTER - Bob later worked for Spacek
- Mouw-"Broad Band Hybrid Junction & Application to the Star Modulator" Nov. 68, IEEE-Transactions on Microwave Theory & Tech. Vol. MTT-16, No. 11.
- "Compensated-Balun;" Oltman, March 66; IEEE-Transactions on Microwave Theory & Techniques pp. 112-119.
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