Tunnel Diode Theory – Tunneling effect

Tunnel diode 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 effect. This tunneling effect causes charge carriers move through the narrow barrier layer at a very low voltage. 

Principle of operation

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 noting, that in case of that diode there is no retaining action at negative 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.

• peak point – determined by peak voltage V_P and peak current I_P,
• valley point – determined by valley voltage V_V and valley current I_V,
• dynamic resistance (for decreasing area of the diode characteristics) The V_P and V_V voltages that largely depend on the material that was used to manufacture a tunnel diode.

Tunneling effect in tunnel diode

The operation of a tunnel diode is based on the tunneling effect, i.e., the capacity of microparticles to pass through a potential barrier having energy less than the minimal energy needed to overcome the barrier. The opportunity of such an effect is described by the wave properties of the microparticles.
The mechanism of operation of tunnel diodes is associated with the tunneling of electrons through the potential barrier.

This phenomenon is the basis for the operation of tunnel diodes, appropriate for amplification and generation of microwave oscillations and for the construction of ultrafast pulsed devices.
This phenomenon is the basis for the operation of tunnel diodes, ideal for amplifying as well as generating microwave oscillations as well as for building ultrafast pulse devices.
Currently now experimental studies have actually practically verified the possibility of tunnel diode procedure at frequencies c ~ 2 GHz, and in the future – as much as 100 GHz.

In this instance, the effect can be seen in areas of the order of 10e V/ cm. The tunneling effect at the semiconductor-metal user interface is even more most likely if the barrier width does not enhance by the space of the charging area. The tunneling effect is the basis for the operation of tunneling diodes.
This paper reviews the current state of development and also manufacturing of tunneling semiconductor devices. It quickly discusses the concept of operation of a tunnel diode and presents an introduction of the concept of its current-voltage characteristics. The dependancy of tunnel diode parameters on the properties of the result semiconductor product is explained.

A qualitative difference in the operation of tunnel diodes is the signal transmission mechanism. In vacuum tubes as well as transistors, this transfer takes place by moving the emitted charge carriers from one electrode to another, which takes a substantial quantity of time, proportional to the carrier course length. The tunneling effect provides a signal transmission rate close to the propagation speed of light with extremely small carrier variations. This allows the tunneling diode to attain very high frequencies. In addition, the tunneling diode is less vulnerable to the damaging effects of nuclear radiation, is much less dependent on structural perturbations, and most notably, its temperature limitation has to do with 50% more than that of transistors.

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 allows the possibility of reducing its size, increasing reliability, increasing durability of the structure and cutting down production costs.

The manufacturing process of the germanium planar diode, developed by Sylvania Electrio company is done in the following order:

1. Coating of germanium gold plate with chromium plating for ohm contact.
2. Vapor deposition of the aresin alloys on the polished opposite surface of the germanium plate.
3. Photolithographic removal of unnecessary fragments of the astonished alloy. Obtaining 5um diameter islands.
4. Etching and melting islets.
5. Apply SiO layer.
6. Sanding the tile until a SiO layer with islets is obtained.
7. Spread the gold layer to the islets. Establishment of mechanical contact.

Sony has also developed its manufacturing process for germanium diodes called bridging, which is done in the following order presented below:

1. Execution of windows of 20 um width on the surface of the germanium plate with thin silicon.
2. Evaporation by masking alloys with conductivity N. This creates a strip whose width is 10um and its position is positioned perpendicular to the window.
3. Evaporation contact points at the end of the 50um x 500um strip.
4. Fill the strip with the germanium layer, thus forming a P + N + connector.
5. Etching the whole plate.

Another method is developed by Bell Telephone Labs. The company first made tunnel diodes with bar contact. This process is written down below:

1. Evaporation on a germanium plate coated with SnO SiO2 layer with alloy Sn As.
2. Melt the Sn As alloy to obtain a P + connector.
3. Obtaining an ohmic contact through belt infusion and etching.

Bar connections 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 Current-voltage characteristics

Tunnel Diode Advantages and disadvantages

Advantages:

• high resistance to environmental factors,
• high operation speed,
• can handle high frequencies,
• low noise coefficient,
• low power dissipation,
• low cost,
• lifespan.

Disadvantages:

• 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 applications

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 as an examples of tunnel diode applications.

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,
• power consumption of 10mW,
• negligible cosmic rays,
• unevenness of the gain characteristic frequency with the value of 0.02db / MHz.

The aforementioned parameters of tunnel diode amplifiers are sought after 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 advantages:

• 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 compared to 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 (FET), 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.

Resonant tunnel diode

Resonant tunnel diode operation is based on the phenomenon of quantum tunneling of electrons (similar to a tunnel diode).
Unlike the tunnel diode, electron tunneling happens with quasi-bounded quantum states in a potential well developed between two potential barriers.
The primary component of the RTD diode is the dual potential barrier region. The quasi-bound states are produced by the quantum dimension effect (spatial quantization).

An RTD diode is characterized by the following parameters:

– E0 = energy barrier height (conduction band discontinuity),

– LB = width (thickness) of the barrier layer,

– LW = width (thickness) of the quantum well layer.

Typical values of these parameters for the heterostructure GaAs/AlxGa1−xAs/GaAs/AlxGa1−xAs/GaAs: E0 ∈ (∼100,∼1000) meV LB, LW ∈ (∼10,∼100) nm

Allow us consider the phenomena happening in the transmission band of a heterostructure with a double barrier.

If the width LW of the potential well is completely little and also the energy barrier is completely high, localized quantum states with energies En = E1, E2, … These are quasi-bound states because the En energy degrees exist over the minimum electron energy outside the quantum well, and the matching wave features handle nonzero worths outside the barrier regions.

If the LB thickness of the barrier is very small, resonant tunneling can take place. It consists in the truth that the electron with energy E = En case on the left barrier, for instance, tunnels through the heterostructure with likelihood equal to 1 (the transmission coefficient amounts to 1).

If the energy E differs from the energy of the quasi-bound level En, the transmission coefficient decreases sharply. For example, for an electron with energy E differing from E1 by 10 meV, the transmission coefficient is reduced 10⁵ times.

It follows that for E = En there will be distinct electron current tops that stem from resonant tunneling. The energies E1 and also E2, corresponding to the positions of the very first two transmission coefficient peaks, rely on the LW width of the potential well, while they (virtually) do not rely on the LB width of the potential barrier.

Cross-section of a mesa-type RTD structure.

The structure of the mesa-type RTD is as follows:

– GaAs and AlAs layers are applied by MBE method alternately on n+GaAs substrate,

– barrier thickness LB = 1.7 nm, well thickness LW = 4.5 nm,

– voltage is applied through ohmic contacts,

Principle of operation of a resonant tunnel diode

V = 0 Equilibrium state = no current flow.

0< V< VP Electrons with energies near the Fermi degree tunnel from the left electrode region through the left barrier right into the potential well and then tunnel through the best barrier right into unoccupied states in the appropriate electrode area. A current flows with the diode with the current I rapidly raising as the voltage V increases.

Resonance: V = V1 = VP Current flow with maximum current I = IP. The energy of the electron infused from the left electrode is equal to the energy E1 of the first quasi-bound state. For E = E1, the first maximum of the transmission coefficient occurs. ⇒ The probability of an electron tunneling from the left electrode to the quantum well region P ~= 1. In the potential well region, electrons form quasi-bound states with short lifetimes.

After their decay, electrons tunnel through the right barrier to unoccupied states in the right electrode region.

The voltage VP corresponding to the current peak satisfies the condition:

Resonant tunnel diode characteristics

• Similarity of current-voltage characteristics of a tunnel diode as well as a resonant tunnel diode,
• The RTD is also identified by the value of P/V = IP/IV ratio. In standard RTDs, P/V ≤ 10, while for RTDs in quantum wire, P/V reaches 100,
• In order to lower the thermionic current Ith, the barrier height E0 must be increased and also relatively low voltages V ought to be utilized,
• The RTD diode can operate at really high signal frequencies due to the low capacitance of the device (the cutoff frequency can get to 1 THz),.
• Applications of RTD diode: ultrafast pulse creating, radiation detectors in THz variety, oscillators for THz signal generation.

Tunneling effect in tunnel diode

The operation of a tunnel diode is based on the tunneling effect, i.e., the capacity of microparticles to pass through a potential barrier having energy less than the minimal energy needed to overcome the barrier. The opportunity of such an effect is described by the wave properties of the microparticles.
The mechanism of operation of tunnel diodes is associated with the tunneling of electrons through the potential barrier.

This phenomenon is the basis for the operation of tunnel diodes, appropriate for amplification and generation of microwave oscillations and for the construction of ultrafast pulsed devices.
This phenomenon is the basis for the operation of tunnel diodes, ideal for amplifying as well as generating microwave oscillations as well as for building ultrafast pulse devices.
Currently now experimental studies have actually practically verified the possibility of tunnel diode procedure at frequencies c ~ 2 GHz, and in the future – as much as 100 GHz.

In this instance, the effect can be seen in areas of the order of 10e V/ cm. The tunneling effect at the semiconductor-metal user interface is even more most likely if the barrier width does not enhance by the space of the charging area. The tunneling effect is the basis for the operation of tunneling diodes.
This paper reviews the current state of development and also manufacturing of tunneling semiconductor devices. It quickly discusses the concept of operation of a tunnel diode and presents an introduction of the concept of its current-voltage characteristics. The dependancy of tunnel diode parameters on the properties of the result semiconductor product is explained.

A qualitative difference in the operation of tunnel diodes is the signal transmission mechanism. In vacuum tubes as well as transistors, this transfer takes place by moving the emitted charge carriers from one electrode to another, which takes a substantial quantity of time, proportional to the carrier course length. The tunneling effect provides a signal transmission rate close to the propagation speed of light with extremely small carrier variations. This allows the tunneling diode to attain very high frequencies. In addition, the tunneling diode is less vulnerable to the damaging effects of nuclear radiation, is much less dependent on structural perturbations, and most notably, its temperature limitation has to do with 50% more than that of transistors.

Tunnel diode detector

Microwave low-level detection is completely dependent on the very uncertain quartz point-contact crystal rectifier (Reference 1)for for many years, has advanced significantly by the invention of tunnel diode (or back diode) detectors as well as hot carrier diode detectors. Both of these devices have shown advantages such as less noise (particularly the 1/f noise) as well as more sensitive rectification current sensitivity low temperature variations as well as a high burn-out capacity and a excellent resistance to noise generation during vibration and shock as well as stability during aging, broad range of bandwidth, and precise analysis of performance based on measurements of static parameters.

The remarkable performances of the high-temperature carrier diode under bias conditions has been documented in the literature along with an appropriate analysis. For the tunnel diode, however, even though there are numerous sources that provide specific examples of sensitivity performance the studies are shown to be limited and insufficient which means that key aspects that affect the overall performance of the diode are left unnoticed. In particular, it’s not widely recognized how tunnel diodes, when biased in their negative resistance region tunnel diode, when biased to its negative resistance region is capable of at the very least, an order of magnitude higher the sensitivity of any other diode detector at frequencies below the cutoff for resistivity. It is also capable of bi-polar modulation of the video output. This capability is significant in the case of highly sensitive TRF (tuned radio frequency) receivers like those used in small solid-state transponders that are used in spacecraft, missiles, and even aircraft.

If the tunnel diode detector is biased by its negative resistance and operates below the the frequency of cutoff for resistivity it is an F amplifier, and it must be able to determine an impedance which ensures table operation i.e. it needs an isolator or circulator that is non-reciprocal to guarantee stability from irregular oscillations.

Low-level microwave detectors typically comprise a mix of circuits for transmission lines as well as lumped reactive circuits, and the diode. The full equivalent circuit may be complex, especially for broadband designs. However, for the purposes of this analysis, it is recommended to simplify it down to a simpler equivalent circuit.

This circuit is referred to as it’s diode junction, and it is commonly employed to perform diode analysis . The circuit elements R g and L are both functions of frequency as they are what is the equivalent series of an impedance, which comprises all the diode series inductance and diode package capacitance, diode package capacitance and the whole external the R F circuit input.

Reference:

https://docplayer.pl/46215296-Ix-diody-polprzewodnikowe-janusz-adamowski.html

https://wened.ru/pl/management/an-amazing-semiconductor-device-is-a-tunnel-diode-tunnel-diodes.html

https://ntrs.nasa.gov/api/citations/19670018284/downloads/19670018284.pdf