Tunnel Diode

The tunnel diode is also known as the “Esaki Diode”. It was invented by “Leo Esaki” in 1957, and for this invention, received the he received Nobel in 1973. It exhibits negative resistance, negative resistance means current decreases when voltage increases.

What is a Tunnel Diode?

A tunnel diode is a very heavily doped p-n junction diode. In a Tunnel diode electric current decreases as the applied voltage increases, and at high voltage, it works as an ordinary p-n junction diode. In the Tunnel Diode, electric current is due to the “Tunneling effect”. Due to its fast switching ability, it is used in high-frequency oscillators and amplifiers. Due to the tunneling effect, charge carries moves across the depletion layer at very low voltage. The circuit shown below is the symbolic diagram of the Tunnel diode. In tunnel diode the p-terminal acts as an anode and the n-terminal acts as a cathode.

Tunnel Diode Symbol

Working of Tunnel Diode

Width of the depletion region depends on the level of doping in semiconductor materials, If the doping level is high then the width of the depletion region is thin and if the doping level is low then width of depletion layer is more. In tunnel diode the width of the depletion region is very thin(because tunnel diode is very highly doped semiconductor device), thus when low voltage is applied across the terminal in forward biasing electron moves across the depletion from n-type semiconductor towards p-type semiconductor.

1. Unbiased condition or Unbiased Tunnel Diode

At unbiased condition means when no voltage is applied band of n-type is overlapped with the p-type semiconductor material, due to heavy doping.

Because of overlapping the electron of conduction band of n-type and the hole of valence band of p-type are nearly at same level. thus when temperature increase some electron tunnel from conduction band of n-type to valence band of p-type and some hole tunnel from valence band p-type to conduction band n-type semiconductor material, but the current flow is zero because the number of tunneled electron and holes are same but in opposite direction.

2. In Forward Biased Condition (Some positive voltage is applied across tunnel diode)

When a small positive voltage is applied less than the potential of the depletion region. In this condition some of the electron of conduction band of n-type tunnel in empty valence band of p-type, this creates a small current flow through the diode.

3. At Vp (voltage is increased further)

When voltage is increased a large number of free electron at n-side and holes at p-side generates. Because of the increase in voltage the over lapping is increased. Energy level of conduction band of n-type is exactly equal to the energy level of valence band at p-type result maximum tunnel current flow.

4. If applied voltage is more than peak voltage(V>Vp)

When we further increase the voltage more than the peak voltage, the energy level of conduction band at n-side and the valence band at p-side misalign. Due to this misalignment tunneling of charge carrier decreases, as result decrease in tunnel current.

If the applied voltage is increased largely then the tunnel diode behaves like an ordinary diode, and show zero tunnel current and maximum diode current. If the applied voltage is greater than the voltage of the depletion layer the tunnel current start decreasing till the valley voltage level after that current increase exponentially.

Operating Region of Tunnel Diode

Ohmic region:- Portion of the curve in which current increases linearly is known as ohmic region. In ohmic region tunnel diode behaves like linear diode. Biasing voltage of ohmic region is zero to peak voltage.

Negative resistance region :- Portion of the curve in which tunnel current decreases known as negative resistance region. Biasing voltage is from peak voltage to the valley voltage. Negative resistance region is widely used characteristic of tunnel diode.

Saturation region:- At very high voltage diode attained saturation level in saturation region tunnel diode behaves like a forward biased ordinary diode in which current increases exponentially.

Types of Tunnel Diode

There are different types of tunnel diode based on their structure and material used in their development.

1. Single-Well Tunnel Diode: This type of diode exhibits negative differential resistance and is used in amplifiers and oscillator circuits.
2. Double-Barrier Tunnel Diode: This diode is used in applications such as millimeter wave oscillators because it operate at higher frequencies.
3. Superlattice Tunnel Diode: This is used for more precise control over the tunneling process and can lead to improved performance characteristics.
4. Resonant Tunnel Diode
5. Esaki Inter-band Tunnel Diode: Esaki inter-band tunnel diode is an extension of the traditional tunnel diode is used for higher-frequency applications.

Advantage of Tunnel Diode

• Very small size.
• Easy to fabricate.
• Low cost.
• Low power consumption.
• Long life.
• Low noise.
• High speed.

Disadvantage of Tunnel Diode

• No proper DC isolation.
• Smaller voltage swing.
• Can not be fabricated in large amount.

Applications of Tunnel Diode

• Tunnel diode have fast switching characteristic so it is used in microwave application.
• It is used ii relaxation oscillator circuit.
• Used as logic memory storage devices.
• Used in FM receivers.

Frequently Asked Questions on Tunnel Diode

Q.1: What is negative resistance ?

Answer:

Negative resistance is a phenomenon where current through the device decreases as the voltage increases across the device.

Q.2 What peak current and valley current in tunnel diode?

Answer:

Peak current is the highest current in negative resistance region in voltage-current characteristic and valley current is lowest current in negative resistance region.

Q.3 Define ohmic region of tunnel diode?

Answer:

Ohmic region of tunnel diode’s voltage-current characteristic curve in which tunnel diode shows positive resistance means current through the device increases as voltage increase.

Q.4 List three type of tunnel diode?

Answer:

The name of three different type of tunnel diode is given below-
1. Single-Well Tunnel Diode
2. Double-Barrier Tunnel Diode
3. Esaki Interband Tunnel Diode