Tunnel diode is an semiconductor diode characterized by a small thickness of the “p-n junction“, a very high concentration of dopants on both sides (“p” and “n”-type doped semiconductors) and a negative dynamic resistance for a certain range of polarizing voltages. It was invented in 1957 by the Japanese physicist Leo Esaki (hence sometimes it can also be called Esaki diode). During research on semiconductor junctions, he noticed their thus far unprecedented feature based on the tunneling phenomenon. This phenomenon causes charge carriers move through the narrow barrier layer at a very low voltage.
Tunnel diode operating principle
After supplying diode with a forward voltage (junction forward-biased), the rate which current “flows” through the diode increases faster than in a normal diode (herein, the tunnel effect has an essential role). Further voltage increase (from approx. 50 mV to approx. 350 mV) operating conditions in the forward bias become less favorable and current decreases. In this regard, tunnel diode acts like a negative resistance, whereas after reaching the “valley point” diode current increases once again and its characteristics cover with characteristics of normal semiconductor diode. It is also worth to be noted that in case of that diode there is no retaining action at reversing voltage.
The tunnel diode is usually made of Silicon (Si), Gallium Antimonide (GaSb), Gallium Arsenide (GaAs), while nowadays it is very rare to see diodes made from Germanium (Ge). Its main advantages may include its relatively low sensitivity to temperature changes and radioactive radiation and short switching time.
Crucial tunnel diode parameters
- peak point – determined by UP voltage and IP current,
- valley point – determined by UV voltage and IV current,
- dynamic resistance (for decreasing area of the diode characteristics) The UP and UV voltages that largely depend on the material that was used to manufacture a tunnel diode
Tunnel diode advantages and disadvantages
- high resistance to environmental factors,
- high operation speed,
- can handle high frequencies,
- low noise coefficient,
- low power dissipation,
- low cost,
- low tunneling current thus it’s classified as a low power component (not good parameter in case of oscillators),
- high manufacturing costs,
- no isolation between input and output.
Tunnel diode characteristics is shown below
Tunnel Diode manufacturing process
Esaki diodes are produced as P + N + connectors using alloying, alloying and epitaxial methods.
The first series of tunnel diodes were formed by the alloying method. The alloying method consists in blending spheres of strongly-doped metal to a degenerate semiconductor whose kind of dopants is opposite. Following the preceding step, a ball electrical connection is formed, after which a piece of semiconductor material is removed so that the diameter of the connector is reduced to the required value of the current in the connector. After the etching process, a cone is produced at the joint, whose diameter affects the values of the tunnel diode parameters. These parameters are: peak current, serial resistance, inductance, capacity. In addition, the dimensions of the diameter of the cone produced affect the structural strength of the whole element.
The planar design of the tunnel diodes made by Sony allows the possibility of reducing its size, increasing reliability, increasing the strength of the structure and reducing production costs.
The manufacturing process of the germanium planar diode, developed by Sylvania Electrio company, was as follows:
- Coating of germanium gold plate with chromium plating for ohm contact.
- Vapor deposition of the aresin alloys on the polished opposite surface of the germanium plate.
- Photolithographic removal of unnecessary fragments of the astonished alloy. Obtaining 5um diameter islands.
- Etching and melting islets.
- Apply SiO layer.
- Sanding the tile until a SiO layer with islets is obtained.
- Spread the gold layer to the islets. Establishment of mechanical contact.
Sony has also developed its manufacturing process for germanium diodes called bridging. It runs as follows:
- Execution of windows of 20 um width on the surface of the germanium plate with thin silicon.
- Evaporation by masking alloys with conductivity N. This creates a strip whose width is 10um and its position is positioned perpendicular to the window.
- Evaporation contact points at the end of the 50um x 500um strip.
- Fill the strip with the germanium layer, thus forming a P + N + connector.
- Etching the whole plate.
Another method is developed by Bell Telephone Labs. The company first made tunnel diodes with bar contact. This time the execution process was as follows:
- Evaporation on a germanium plate coated with SnO SiO2 layer with alloy Sn As.
- Melt the Sn As alloy to obtain a P + connector.
- Obtaining an ohmic contact through belt infusion and etching.
Bar connects are made when the gold layer is evaporated onto the chrome substrate. The gold layer is thickened in galvanic fashion to a value of 15um, and cavities of material from the substrate are removed under spots by etching. In this way of production, the diodes have increased the strength of the structure and improve the electrical performance.
Leo Esaki’s main finding is that the tunnel current is not a diffusion current, but a conduction current. Its flow rate is close to the speed of light, which translates into the ability to use the diode in very high-frequency systems.
Tunnel Diode Detector application
Tunnel diode detector is used to amplify and detect small high-frequency oscillations (in hundreds of GHz range). Tunnel diodes are also used in high-speed pulse systems (for example in electronic logic circuits for calculating machines), mobile microwave equipment, signal broadband amplifiers and frequency generation systems with frequencies above 300 MHz. They can be also applied in the aerospace hardware and radar devices.
Look on the example of this element in Tunnel diode pdf file. This is datasheet pdf for germanium tunnel diode:
Tunnel diode amplifiers
In present times, technology is advancing towards in the sensitivity of microwave receivers. Over the past decade, the parameters of microwave amplifiers that use tunnel diodes achieve unbelievable results. The characteristics of these amplifiers are different, but the main ones are: excellent noise parameters, sizes not exceeding 15 cm3, bandwidth of one octave, lightweight construction, the power consumption of 10mW, negligible cosmic rays, unevenness of the gain characteristic frequency with the value of 0.02db / MHz.
The above-mentioned parameters have made tunnel diode amplifiers used in portable devices, aviation electronic devices, electronic aerials devices, space electronics and radar electronics. Amplifiers with built-in tunnel diode are much cheaper than amplifiers using current wave tubes.
Competitive devices for amplifiers with tunnel diodes are transistor amplifiers. Compared to transistor amplifiers, microwave amplifiers have the following features: better noise parameters, higher upper limit of operating frequency, higher bandwidth value, lower power consumption.
Microwave amplifiers have higher S and C bandwidth and higher transmission bandwidth than transistor amplifiers. The advantage of transistor amplifiers is their ease of merging and getting better parameter values.
Tunnel diode oscillator
The project of an oscillator based on tunnel diode is possible due to the specific characteristics of the diode i.e. it can be used for generating oscillations in GigaHertz’s range.
A phenomenon so-called “negative resistance region”, which has been aforementioned, is allowing to design a working tunnel diode oscillator. Similar as in case of the unipolar transistor, on the basis of which one can also create an oscillator circuit. Below is the example of schematic of the oscillator built on the basis of a Tunnel diode. Tunnel diode is connected in series with the RLC circuit (so-called “tank circuit”). After turning the switch, the current that is determined by resistor R1 is applied to the resistor R2 and diode. Resistor R2 is used for further tuning of the current (1st Kirchhoff’s law) flowing to the diode and the tank circuit that will resonate with before designed and selected frequency. Values of the resistors must also be computed in a way that will bias Tunnel diode to the mid point of its negative resistance region to ensure its proper operation.