Tunnel Diode Circuit with Operations and Applications

A Tunnel Diode is also known as Eskari diode and it is a highly doped semiconductor that is capable of very fast operation. Leo Esaki invented the Tunnel diode in August 1957. The Germanium material is basically used to make tunnel diodes. They can also be made from gallium arsenide and silicon materials. Actually, they are used in frequency detectors and converters. The Tunnel diode exhibits negative resistance in their operating range. Therefore, it can be used as an amplifier, oscillators and in any switching circuits.

What is a Tunnel Diode?

Tunnel Diode is the P-N junction device that exhibits negative resistance. When the voltage is increased than the current flowing through it decreases. It works on the principle of the Tunneling effect. Metal-Insulator-Metal (MIM) diode is another type of Tunnel diode, but its present application appears to be limited to research environments due to inherit sensitivities, its applications considered to be very limited to research environments. There is one more diode called Metal-Insulator-Insulator-Metal (MIIM) diode which includes an additional insulator layer. The tunnel diode is a two-terminal device with n-type semiconductor as the cathode and p-type semiconductor as an anode. The tunnel diode circuit symbol is as shown below.

Tunnel Diode Working Phenomenon

Based on the classical mechanics’ theory, a particle must acquire energy which is equal to the potential energy barrier height, if it has to move from one side of the barrier to the other. Otherwise, energy has to be supplied from some external source, so the N-sided electrons of the junction can jump over the junction barrier to reach the P-side of the junction. If the barrier is thin such as in tunnel diode, according to the Schrodinger equation implies that there is a large amount of probability and then an electron will penetrate through the barrier. This process will happen without any energy loss on the part of the electron. The behavior of the quantum mechanical indicates tunneling. The high-impurity P-N junction devices are called as tunnel-diodes. The tunneling phenomenon provides a majority carrier effect.

P∝exp⁡(-A* E_b *W)

Where,

‘E’ is the energy of the barrier,
‘P’ is the probability that the particle crosses the barrier,
‘W’ is the width of the barrier

Construction of Tunnel Diode

The diode has a ceramic body and a hermetically sealing lid on top. A small tin dot is alloyed or soldered to a heavily doped pellet of n-type Ge. The pellet is soldered to anode contact which is used for heat dissipation. The tin-dot is connected to the cathode contact via a mesh screen is used to reduce the inductance.

Operation and its Characteristics

The operation of the tunnel diode mainly includes two biasing methods such as forward and reverse

Forward Bias Condition

Under the forward bias condition, as voltage increases, then-current decreases and thus become increasingly misaligned, known as negative resistance. An increase in voltage will lead to operating as a normal diode where the conduction of electrons travels across the P-N junction diode. The negative resistance region is the most important operating region for a Tunnel diode. The Tunnel diode and normal P-N junction diode characteristics are different from each other.

Reverse Bias Condition

Under the reverse condition, the tunnel diode acts as a back diode or backward diode. With zero offset voltage, it can act as a fast rectifier. In reverse bias condition, the empty states on the n-side aligned with the filled states on the p-side. In the reverse direction, the electrons will tunnel through a potential barrier. Because of its high doping concentrations, tunnel diode acts as an excellent conductor.

The forward resistance is very small because of its tunneling effect. An increase in voltage will lead to an increase in the current until it reaches peak current. But if the voltage increased beyond the peak voltage then current will decrease automatically. This negative resistance region prevails till the valley point. The current through the diode is minimum at valley point. The tunnel diode acts as a normal diode if it is beyond the valley point.

Current Components in a Tunnel Diode

The total current of a tunnel diode is given below

                       I_t = I _tun + I _diode + I _excess

The current flowing in the tunnel diode is the same as the current flowing in the normal PN junction diode which is given below

I_diode = I_do * ( exp (?*  V_t ) ) -1

I_do – Reverse saturation current

V_t – Voltage equivalent of temperature

V – Voltage across the diode

η – Correction factor 1 for Ge and 2 for Si

Due to the parasitic tunneling via impurities, the excess current will be developed and it is an additional current by which the valley point can be determined. The tunneling current is as given below

        I_tun = (V/R₀) * exp (- ( V/V₀)^m )

Where, V₀ = 0.1 to 0.5 volts and m= 1 to 3

R₀=Tunnel diode resistance

Peak Current, Peak Voltage of Tunnel Diode

The peak voltage and peak current of a tunnel diode are maximum. Typically for a Tunnel diode, the cut in voltage is more than the peak voltage. And the excess current and diode current can be considered to be negligible.

For a minimum or maximum diode current

V = V_peak, dI_tun/dV=0

(1/R₀) * (exp (- (V/V₀)^m) – (m * (V/V₀)^m * exp (- (V/V₀)^m) = 0

Then, 1 – m * (V/V₀)^m = 0

Vpeak = ((1/m) ^(1/m) ) * V₀ * exp (-1/m)

Maximum Negative Resistance of a Tunnel Diode

The negative resistance of a small signal is given below

R_n = 1 / (dI/dV)  = R₀ / (1 – (m * (V/V₀)^m) * exp (-(V/V₀)^m) / R₀ = 0

If dI/dV= 0, R_n is maximum, then

(m * (V/V₀)^m) * exp (-(V/V₀)^m) / R₀ = 0

If V= V₀ * (1 + 1/m)^(1/m)  then  will be maximum, so the equation will be

(R_n)_max = – (R₀ * ( (exp(1+m)) / m )) / m

Tunnel Diode Applications

• Due to the tunneling mechanism, it is used as an ultra high speed switch.
• The switching time is of the order of nanoseconds or even picoseconds.
• Due to the triple valued feature of its curve from current, it is used as a logic memory storage device.
• Due to extremely small capacitance, inductance and negative resistance, it is used as a microwave oscillator at a frequency of about 10 GHz.
• Due to its negative resistance, it is used as a relaxation oscillator circuit.

Advantages of Tunnel Diode

• Low cost
• Low noise
• Ease of operation
• High speed
• Low power
• Insensitive to nuclear radiations

Disadvantages of Tunnel Diode

• Being a two-terminal device, it provides no isolation between output and input circuits.
• The voltage range, which can be operated properly in 1 volt or below.

This is all about the Tunnel Diode circuit with operations, circuit diagram and its applications. We believe that the information given in this article is helpful for you for a better understanding of this project. Furthermore, any queries regarding this article or any help in implementing the electrical and electronics projects, you can feel free to approach us by connecting in the comment section below. Here is a question for you, what is the main principle of the Tunneling Effect?

Photo Credits:

• Tunnel Diode wikimedia
• Tunnel Diode Characteristics blogspot
• Types of Tunnel Diode engineeringelectronics