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Basic Characteristics/Operating Principle and Application Circuit of Tunnel Diode

Basic Characteristics/Operating Principle and Application Circuit of Tunnel Diode

September 21 , 2023 0

In electronics, tunnelling refers to the direct flow of electrons from the n-type conduction band through the depletion region to the p-type valence band. A tunnel diode is a highly doped p-n junction diode in which the current decreases with increasing voltage. The current in a tunnel diode is caused by the tunneling effect. These diodes are widely used as high-speed switching devices in computers and in high-frequency oscillators and amplifiers.

Tunnel Diode Concept

Also known as Ezaki diode, it is a crystal diode with tunnel effect current as the main current component. Tunnel diode is a semiconductor diode made of a mixture of gallium arsenide (GaAs) and gallium antimonide (GaSb) and other materials, the advantages of which are good switching characteristics, fast speed, high operating frequency; the disadvantage is poor thermal stability. Generally used in certain switching circuits or high-frequency oscillation and other circuits.After disappearing from the world of semiconductors for a long time, the tunnel diode has actually been rebooted as it enables the conversion of heat into electricity. The tunnel diode is also known as the Ezaki diode, after its Japanese inventor.
During the 1850s and 1860s, tunnel diodes were implemented in many applications, mainly in radio frequency circuits, where their extraordinary qualities were used to produce very fast level sensors, oscillators, mixers, etc.

Tunnel Diode Principle Of Operation

Compared to standard diodes, tunnel diodes work by using semiconducting substances with incredibly large doping levels, resulting in the depletion layer between the p

The depletion layer between the p-n junctions becomes about 1000 times narrower than in the fastest silicon diodes.

Once the tunnel diode is forward biased, a process called electron flow "tunnelling" begins across the p-n junction.

"Tunneling" in doped semiconductors is actually a method that is not readily understood using traditional atomic assumptions, and probably cannot be covered in this small article.

Tunnel Diode Forward Voltage Versus Current

In testing the relationship between the forward voltage UF and the current IF of the tunnel diode, we can find that the device has a negative resistance characteristic between the peak voltage Up and the valley voltage Uv as shown in the figure below.

Thus, when the diode is powered up in the shaded region of its IF-UF curve, the forward current decreases as the voltage rises. The resistance of the diode is unquestionably negative and is usually denoted as -Rd.

The design presented in this paper takes advantage of the above qualities of the tunnel diode by implementing a set of serially connected tunnel diode devices that charge a battery from solar heat (not a solar panel).

As shown in the figure below, seven or more gallium antimony-antimony (GISp) tunnel diodes are connected in series and clamped to a large heat sink, which helps to prevent their power from being dissipated (the tunnel diodes get colder as the UF rises or increases).

Heat sinks are used to efficiently accumulate solar heat or any other form of heat that may be applied, the energy of which needs to be converted into charging current in order to charge the proposed NiCd batteries.
Conversion of heat to electricity (thermoelectric) using a tunnel diode
The principle of operation of this particular configuration is actually quite simple. Imagine that a normal, natural resistor R is able to discharge a battery by a current I = V/R. This means that the negative resistor will be able to initiate the charging process of the same battery simply because the sign of I is reversed, i.e., -I = V/-R.
Similarly, if the normal resistor allows P = PR watts to dissipate heat, the negative resistor will be able to supply the same amount of watts to the load: P = -It-R.
When the load itself is a voltage source with relatively low internal resistance, the negative resistor must of course produce a greater voltage level for the charging current Ic to flow, given by the formula:
ic= δ [ σ (uf) - ubat] / σ (rd) + rbat
Referring to the note Σ(Rd), it is immediately understood that all diodes in the string sequence must operate in the -Rd region, mainly because any individual diode with +Rd characteristics may terminate the target.
Testing the Tunneling Diodes
To ensure that all diodes exhibit negative resistance, a simple test circuit can be designed as shown below.

Note that the meter should be specified to indicate the polarity of the current, as it may well happen that the IP:IV ratio (tunneling slope) of a particular diode is very excessive, leading to accidental charging of the cell when a small forward bias is achieved.
The analysis must be carried out at atmospheric temperatures below 7°C (try using a cleaned fridge) and the UF-IF curve for each diode is noted by accurately increasing the forward bias through the potentiometer and noting the resulting IF amplitude as shown by the meter readings.
Next, bring the FM radio close enough to ensure that the diode being tested does not oscillate at 94.67284 MHz (frequency, for a GISp with a doping level of 10-7).
If this is found to occur, the particular diode may not be suitable for this application. Determine the OF range that guarantees -Rd for almost all diodes. Depending on the manufacturing threshold of the diodes in the available lot, this range may be as small as 180 to 230
mV.
Application Circuits
The power generated from the heat of the tunnel diode can be used to charge a small NiCd battery.
First determine the number of diodes required to charge the battery by the minimum current: for the UF selection described above, at least seven diodes must be connected in series to provide approximately 45 when heated to the following temperature level
mA charging current:
Γ ［ -Σ (Rd)If] [ δ (Rth-j) - RΘ]. √ (Td+Ta)°C
Or when the thermal resistance of the heat sink does not exceed 35.3 K/W and is mounted in peak sunlight (Ta 5°C), about 26°C. To obtain a charge from this heat sink, it is recommended to use the same heat sink.
26°C. In order to obtain maximum efficiency from this Ni-Cd charger, the heat sink must be dark-coloured for optimal heat exchange with the diode.
In addition, it must not be magnetic, considering that any type of external field, either induced or magnetic, causes an unstable stimulation of the charge carriers in the tunnel.
As a result, this can bring about unsuspecting pipeline effects; electrons can be knocked off the P -n junction on the substrate, which can accumulate around the diode terminals, triggering potentially dangerous voltages, depending on the metal enclosure.
Unfortunately, several tunneling diodes of the BA7891NG type are very sensitive to the tiniest of magnetic fields, and tests have proved that these diodes need to be kept at a level relative to the earth's surface to prevent this.