What is a tunnel diode?

A tunnel diode—also called Esaki diode because Leo Esaki invented it in 1957—is a heavily doped PN junction diode that exhibits negative resistance and high conductivity due to the tunneling effect. In signal diodes (small signal diodes and rectifier diodes), charge carriers gradually overcome the depletion region. In a tunnel diode, the charge carriers spontaneously overcome the barrier.

Another distinct characteristic of tunnel diodes is their negative resistance. A tunnel diode conducts in both directions, and the current through the diode decreases with the increase in the voltage.

The concentration of impurity atoms in a typical signal diode is one part in 10⁸. While in a tunnel diode, the concentration of impurity atoms is one part in 10³. Due to such heavy doping of the depletion region, a tunnel diode is highly conductive in both forward and reverse bias regions. Its conductivity changes spontaneously that making it suitable for high-frequency switching. That is why tunnel diodes are used as high-speed switches, as well as high-frequency amplifiers and microwave oscillator circuits.

Symbol of tunnel diode
The tunnel diode is shown below in a circuit using either of the two electrical symbols.

Construction of tunnel diode
Unlike typical signal diodes, tunnel diodes are not constructed from silicon. The materials used in constructing a tunnel diode are gallium antimonide, gallium arsenide, and germanium.  In a tunnel diode, as the forward voltage is applied, the current increases spontaneously to a peak value, then with the increase in voltage, it reduces to a minimum level called the valley point. The peak current ratio to the valley current is too small in silicon substrate so it is not appropriate to construct a tunnel diode; however, the ratio of peak current to the valley current is maximum in the case of germanium.

Apart from material selection, the second important factor in the construction of a tunnel diode is doping level. The heavily doped PN junction is almost 1000 times greater than in a normal diode. The depletion region in a tunnel diode becomes extremely narrow due to this heavy doping.

VI characteristics of tunnel diode
Like a semiconductor diode, a tunnel diode is also a two-terminal device. The voltage can be applied to a tunnel diode in either forward or reverse bias. When a forward voltage is applied to a tunnel diode, the current through the diode starts increasing spontaneously due to the tunneling effect. This is called tunneling current. It instantly reaches a peak value called the peak point. The voltage at the peak point is called peak voltage, and the current is called peak current. This is the maximum current that the tunnel diode can conduct.

As the voltage increases, the current through the tunnel diode starts dropping. It slowly reduces to a minimum value called valley current. This is called the reverse resistance region of the tunnel diode. The peak current and valley current ratio is most significant in a tunnel diode’s switching operation. The peak current and valley current of some common tunnel diodes are listed in the table below.

After reaching the valley point, the current increases as the voltage increases, similar to a large-signal diode. The voltage at the point the current recovers to one-fourth of peak point current is called forward saturation voltage (V_FS). The current increases till a maximum forward voltage is reached.

When a reverse bias is applied, the tunnel diode conducts a high value of reverse saturation current due to heavy doping—it breaks down like a normal diode at a peak reverse voltage. The peak inverse voltage of all the tunnel diode models listed above is 40V.

Equivalent circuit of tunnel diode
Tunnel diodes are used as high-speed switches. The most significant characteristic of a tunnel diode is its negative resistance region. The drop of conductivity in the negative resistance region of a tunnel diode determines its switching operation. That is why, for circuit analysis of a tunnel diode, you use a small-signal diode model with negative resistance. The following is an equivalent circuit of a tunnel diode.

In the above equivalent circuit, -Ro is diode resistance in the negative resistance region, Ls is series inductance due to leads, and C is junction capacitance.

Working of a tunnel diode
In a typical signal diode, the depletion region is wide enough to avoid free charge carriers passing through it. In unbiased conditions, the electrons passing from the n-type region to the p-type region and holes passing from the p-type region to the n-type region are minority charge carriers present in the depletion region. In a tunnel diode, the depletion region is too narrow such that even most charge carriers can punch through the depletion region to the other sides. There is virtually no depletion region between p-type and n-type regions. In other words, we can say that the conduction band and valence band overlap in a tunnel diode.

In unbiased conditions, electrons and holes gain kinetic energy due to temperature and move across the junction freely. However, the electron and hole current are equal and cancel each other. No net current flows through the diode in an unbiased condition. When a small forward voltage is applied to the tunnel diode, it is not able to overcome the potential barrier of the narrow depletion region. Still, some majority charge carriers can punch through the depletion region due to the tunneling effect. This causes a small amount of forward current to pass through the diode.

As the voltage increases, the tunnel current starts increasing rapidly due to the free movement of charge carriers. The valence band of the n-type region overlaps the conduction band of the p-type region leading to a rise in current. As the voltage increases, more electrons and holes are generated, causing a high current to flow through the diode. The flow of current also alters the energy levels of the valence band and conduction band of both regions. The current reaches a peak point when the valence band of the n-type region equals the conduction band of the p-type region.

Any further increase in voltage creates a strong electrostatic field due to ions present in the depletion region. This causes misalignment of the valence and conduction bands resulting in the fall of current across the diode. The current falls to a valley point where it is minimum. When the forward voltage is still increased, the valence and conduction band’s overlapping is completely diminished, and the tunnel diode starts behaving like a normal diode.

A significant amount of reverse saturation current flows through the diode due to the tunneling effect in reverse bias conditions. The tunnel diode remains in a state of conduction in reverse bias due to the large reverse current and because of the heavy doping, the peak inverse voltage is tiny compared to the maximum forward voltage. For example, the tunnel diodes listed in the table above have a maximum forward voltage up to 500 V; the PIV rating for all of them is just 40 V.

Advantages of tunnel diode
These are low-cost and durable devices suitable for high-frequency and high-speed applications. They are easy to fabricate and offer high-speed switching operation with low signal noise.

Disadvantages of tunnel diode
Tunnel diodes are low-power devices and have a swing in output voltage. Like any diode, a tunnel diode’s output and input circuit cannot be isolated as a two-terminal device. It is also not possible to fabricate tunnel diodes on a large scale.

Applications of tunnel diode
A tunnel diode is a low-power device that is not frequently used in electronic circuits. Due to high-speed switching operation, they are used in small current high-frequency applications. The negative resistance region of the tunnel diode is utilized to design oscillator circuits and reflection amplifiers. As the tunnel diode remains in the conduction state in both forward and reverse bias, it is also used as a frequency converter. In digital circuits, these diodes are used as logic memory storage.

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Filed Under: Featured, Tutorials, What Is
Tagged With: Tunnel Diode, tunnelling effect
 

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