Analog Electronics, Diode, Electronics

Tunnel Diode- Working, V-I Characteristics & Practical Applications

After reading this article, you will able:

  • To Understand the Tunnel Diode in General.
  • To Understand the Working of Tunnel Diode.
  • To Understand the Tunneling Effect in Tunnel Diode.
  • To Understand the VI Characteristics of Tunnel Diode.
  • To Understand the Working of Tunnel Diode Oscillator.
  • To Understand the Advantages. Disadvantages and Applications of Tunnel Diode.

What is Tunnel Diode?


A Tunnel Diode is a special type of PN junction that exhibits negative resistance. Which means whenever the voltage increases the current will be decreased.

  • Tunnel Diode is highly doped PN junction Diode with impurities, That is the reason it exhibits negative resistance in forward bias such a diode is known as Tunnel Diode.
  • It is sometimes known as Esaki Diode named after Leo Esaki.
  • The tunnel diode is very helpful device because it provides very fast switching.

Symbol of Tunnel Diode:

The tunnel diode is a two terminal device one terminal is Cathode and another Anode. Its cathode is shaped like T letter.


Working of Tunnel Diode

To understand the Working of Tunnel-Diode, we have to understand first tunneling effect because the working of tunnel diode D1 is totally depending on Tunneling Effect.

What is Tunneling Effect?

The heavy doping provides a large number of majority carriers. Due to these large numbers of carriers, there is much drift velocity in both P and N region. Due to the drift activity valance electrons have raised their energy levels very close to the conduction region, which means, it takes only a very small applied forward voltage to cause conduction.

“The movement of valence electrons from the valence energy band to the conduction band with little or no applied forward voltage is called tunneling.

Valence electrons seem to tunnel through the forbidden energy band

I hope this is clear enough?

Tunnel Diode is heavily doped more than 1000 times as compared to an Ordinary diode, due to that it has very narrower depletion.

now lets us understand working clearly:

  • As the forward voltage is first applied, the diode current rises rapidly due to the tunneling effect.
  • As the voltage across the diode is further increased, the tunneling effect is reduced and current flow starts to decrease. The tunnel diode is said to have entered the negative resistance region.
  • From now onwards, the tunnel-diode behaves same as an ordinary diode.

V-I Characteristics of Tunnel Diode.

In Forward Bias:

Step 1: When no voltage is applied there is no current flow.V-I Characteristics of Tunnel Diode

Step 2: A small forward bias is applied. The potential barrier is still very high there is no noticeable injection. However, electrons in the conduction band of the n region will tunnel to the valence band in p region. This will create a forward bias tunnel current.

V-I Characteristics of Tunnel Diode

Step 3: With a larger voltage the energy of the majority of electrons in the n-type is equal to the (holes) in the valence band of p-type; at this point, it produces maximum tunneling current.

V-I Characteristics of Tunnel Diode
Here tunneling current is maximum

Step 4: As the forward bias is continuously increasing, the number of electrons in the n-type is directly opposite to the empty states (Holes) in the valence band. Therefore tunneling current will start decreasing.

V-I Characteristics of Tunnel Diode
current is decreased

Step 5: As the more forward bias is applied, the tunneling current drops to zero (inside the junction).

Step 6: With further voltage increase, the diode behaves like an ordinary diode and the  V-I characteristic is similar to that of an ordinary PN junction diode.

  • The tunnel diode is mostly not used in this region because now onward tunnel diode is working on positive resistance region which means increasing forward bias voltages current will increase.

    V-I Characteristics of Tunnel Diode
    V-I Characteristics of Tunnel Diode

Working of Tunnel Diode Oscillator:

As we studied a tunnel D1 is always operated in the negative resistance region. When tunnel D1 diode is operated in the negative resistance region, it operates similar to the oscillator (it will convert the dc voltage to ac voltages).

Fig: given below shows a parallel resonant LC circuit, here RP is parallel series winding resistance of a coil.

When applying voltages to the tank circuit it is set to produce oscillations as shown in Fig: (ii), the oscillations are damped oscillations (with respect to time oscillations are reducing). It is because energy is lost in the parallel series winding resistance of tank circuit.

working of tunnel diode oscillator

Placing tunnel D1 diode series with the LC or tank circuit and making tunnel diode biased at the center of the negative resistance region of its characteristic as shown in Fig: below, at the output undamped oscillations are produced (voltages will increase with respect to time).

tunnel diode oscillator

It is because the positive resistance region of tank circuit counterattacks the negative resistance of tunnel  D1 diode.

The circuit shown in above Fig. is called negative resistance oscillator or tunnel diode oscillator. The tunnel diode oscillator has one big drawback. While the circuit works awesome at the extreme high frequencies (greater than megahertz range MHz), but it cannot be used efficiently for lower frequencies.

Advantages of Tunnel Diode:

  • It is one of the useful diodes; these diodes have robust nature.
  • Tunnel-Diode provides very high resistance to high nuclear radiations and magnetic fields.
  • Another unique feature is its low-temperature use, which makes it an ideal device.
  • It has a high speed of operation due to that is mostly used. It provides high speed of operation due to fact that the tunneling takes place near to the speed of light.
  • These type of diodes are very low costly
  • It has a very long life of the operation.
  • It produces very low noise
  • It has very low power dissipation

It is simple to fabricate

Few Disadvantages

Following are the few disadvantages of Tunnel diode:

  • Voltage swing in these devices is very low as compared to other diodes.
  • It provides very week isolations in the circuit.
  • The output power level of tunnel D1 diode is very restricted; it’s in the range of few watts because the applied DC voltages must be less than the depletion voltages of a diode.


Practical Applications of Tunnel Diode: 

Some of the practical applications of Tunnel diode are:

  • The tunnel D1 diode is mostly used as relaxation oscillator circuit.
  • It is used as an whenever a high switching is needed mostly in control system because it has a switching time of the nanoseconds or picoseconds.
  • The tunnel D1 is used as microwave oscillator, which has the frequency above than 10GHz.
  • They have also used in the amplifier.
  • It is used as Memory storing device.
  • It is also used in military types of equipment like sonars and radars.
  • It is used in nuclear magnetic resource machine.
  • It is also used in satellite communication types of equipment.
  • A tunnel has very small capacitance, inductance and also it has negative resistance for that reason it is used in microwave oscillators and relaxation oscillators.


This is all about Tunnel Diode- Working, V-I Characteristics & Practical Applications, We believe that the information provided in this article is easily understandable and you guys have learned from this article. Furthermore, if you have any query regarding this article or any other article or you need any help in your studies or projects related to electronics and electrical engineering, you can feel free to contact us by connecting in the comment section below.

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