Tunnel Diode-Type of Semiconductor Diode

Tunnel Diode-Type of Semiconductor Diode

Tunnel Diode also known as Esaki Diode is a type of semiconductor diode which provides fast operation in the microwave frequency region. Its working is based on the tunneling effect. The diode was invented in the year 1957 by Leo Esaki. Later in the year 1973 he obtained the Nobel Prize for his work on tunneling effect.  Heavily doped pn junction is one of the characteristic of tunnel diode. Some of the manufacturers of this diode are Sony and General Electric. Their major applications include amplifiers, oscillators, switching circuits, frequency converters, detectors etc.

What is a Tunnel Diode?

Tunnel diode is a pn junction device that exhibits negative resistance. This means that when voltage is increased the current through it decreases in certain regions in the forward direction. Figure below shows the basic symbol of tunnel diode. From the figure it is clearly understood that the tunnel diode is a two terminal device. P type semiconductor in the diode acts as the anode and the n type semiconductor as the cathode. A tunnel diode is doped approximately 1000 times as heavily as a conventional diode. This heavy doping results in a large number of majority carriers. Because of the large number of carriers, most are not used during the initial recombination that produces the depletion layer. As a result, the depletion layer is very narrow. In comparison with the normal diode, the depletion layer of a tunnel diode is 100 times narrower.

Construction of Tunnel Diode

Tunnel diodes are usually made from gallium arsenide, germanium or silicon. These materials have small forbidden gap and good ionic mobility which helps in providing high frequency and speed. For constructing the diode, firstly a small dot having a diameter of 50µm is soldered to the heavily doped pellet of n type of germanium or gallium arsenide or silicon material used. This pellet is then soldered to Kovar pedestal which forms the anode. Through this process, better heat dissipation is achieved. Kovar to the tin dot via a mesh screen acts as the cathode contact. The ceramic body separates both the anode and cathode. Finally, the chip is sealed.

Tunneling Process

When the p and n region of a diode is heavily doped, the depletion region becomes very thin (~10nm). In this case, there is a greater chance for the electrons to tunnel from the conduction band of n region to the valence band of p region. This phenomenon is known as the tunneling effect and is seen mainly in the tunnel diodes.

Operation of Tunnel Diode

In general, a pn junction diode can act as a tunnel diode when it satisfies the below conditions:

• Fermi Level must be within the band of the diode.
• Barrier of the depletion region must be thin obtained by heavily doping the diode,
• At 0K, all the states below the Fermi level must be occupied, while all the states above the Fermi level must be empty.
• There must be filled energy states on the side were the tunneling process starts.

Now let us consider the operation of the tunnel diode under forward and reverse bias conditions.

Under Forward Bias

1. At zero bias, there is no electric current.
2. A small forward bias is applied. Potential barrier is still high and there is no flow of diffusion current through junction. But electrons in the conduction band of n side tunnels to the valence band of p side and hence a current flows.
3. With a high voltage, energy of the electrons in the n region becomes equal to the holes in the valence band of p region and thereby produces high tunneling current.
4. As the forward bias continues, number of electrons in the conduction band of n side decreases compared to the number of holes in the valence band of p side. Due to this the tunnel current flow starts to decrease.
5. When more voltage is applied, tunneling current decreases to zero. But diffusion current flows.
6. When more forward voltage is given, the tunnel diode will act as a normal diode.

Under Reverse Bias

7. Here the electrons in the valence band of p side tunnels to the conduction band of n side. This creates a tunneling current which increases with the reverse voltage. Hence the tunnel diode will act as good conductor in the reverse bias condition. Also the characteristic of the diode at this time is similar to the zener diode with zero breakdown voltage. Tunnel diodes in the reverse biased operation are often called as Back Diodes.

V-I Characteristics of Tunnel Diode

The figure above shows the VI characteristics of the tunnel diode. Here the total current (I) flowing through the diode is given by the equation below.

                              I= I_p (V_p/V) exp (1-(V/ V_p)) + I₀exp (qV/KT)

The first term in the equation represents the tunnel diode current while the second term represents the current of the normal diode. Here I_p-Peak current. V_p- Peak voltage, V- Applied voltage, q- charge, K- Boltzmann constant, T- Temperature.

Also the negative resistance (R) is given by the equation below:

                              R= - [((V/V_p)-1) (I_p/V_p) .exp (1- (V/V_p))]^-1

Advantages

•  Low power dissipation
•  Very high speed
•  Longevity
•  Low noise
•  Environmental immunity
•  Simplicity in fabrication
•  Low cost

Disadvantages

• Low voltage swing
• No isolation between output and input.
• Damages wile overheating.

Applications of Tunnel Diode

• Can be used as oscillators and high frequency trigger devices.
• Good to use for microwave frequency applications.
• It is good for counter circuits, pulse generating circuits, television tuners in the form of an oscillator.
• It can be used to work as an amplifier in many applications due to its better operation and longevity.
• Can be used in space and in highly radioactive environment applications.
• Low voltage high frequency switching applications.
• Can be used as a logic memory storage device.
• Low power dissipations allow them to use in FM receivers also.

Last but not the least, Tunnel diodes are considered to be high speed devices with negative resistance and can be used for microwave applications.