Tunnel Diode - Basics, Operation, VI Characteristics, Equivalent Circuit
Tunnel Diode: Basics, Operation, VI Characteristics
In this post let us discuss about one of the special diode named as tunnel diode. Before that let us understand about the positive resistance and negative resistance region..
What is meant by positive resistance and negative resistance?
If the current flowing through the device/circuit increases when the voltage applied across it increased, then it is called as the device has positive resistance.
If the current flowing through the device/circuit decreases as the voltage applied across it increased, then it is called as the device has negative resistance.
Tunnel Diode - Definition:
A tunnel diode is a special type of PN junction diode that shows the negative resistance between two values of forward voltage (ie, between peak point voltage and valley point voltage).
Tunnel Diode Basics:
The tunnel diode was first introduced by Leo Esaki in 1958. Its characteristics are completely different from the PN junction diode. The symbol of tunnel diode is shown below.Basically the tunnel diode is a normal PN junction diode with heavy doping (adding impurity) of P type and N type semiconductor materials.
- Approximately a tunnel diode is doped 1000 times as heavily as a normal diode.
- Due to this, large number of majority carriers are available in the semiconductor layers.
- As a result, in tunnel diode the depletion layer is very narrow (The initial recombination is occurred using carriers near to junction itself).
- Compare to normal PN junction, the depletion layer of tunnel diode is 100 times narrower.
- The width of depletion layer will be of the order of 10-6cm.
- Most frequently germanium and gallium arsenide are used to make tunnel diodes.
- The ratio IP/IV is the major factor for tunnel diode applications.
- For germanium, it is typically 10-1, while for gallium arsenide, it is closer to 20-1.
- The IP value will be from few microamperes to several hundred amperes. But the VP is limited around to 600mV.
- So if we provide improper voltage even from the internal battery of 1.5V will destroy the diode. (shortly we will see about the IV, IP).
The tunnel diodes (operating in negative resistance region) are used in high speed applications such as in computers, oscillators, switching networks, pulse generators, and amplifiers where switching times are in the order of nanoseconds.
Tunnel Diode Equivalent Circuit:
The equivalent circuit of the tunnel diode is shown below. Ls is mainly due to terminal leads.
Rs is due to ohmic contact at lead-semiconductor junction, semiconductor materials and due to leads.
Cj = Junction diffusion capacitance
Rn = Negative resistance of the region.
Tunnel Diode VI Characteristics:
- As the forward voltage starts to increase, the diode current raises rapidly due to tunnel effect.
- After the point VP, the tunnel effect is reduced and current flow starts to decrease even though increase in voltage( ie negative resistance region).
- After the point VV, the tunnel diode behaves as a normal diode.
- When the forward voltage applied across the tunnel diode increased from zero, electrons from n- region tunnel through the potential barrier to the p- region.
- When the forward voltage (VP) increases, the diode current increases till the peak-point (IP). (ie, positive resistance)
- When voltage increased beyond VP, the diode current decreases till valley point (VV). The tunnel diode exhibits negative resistance from VP to VV.
- When the forward voltage increased beyond valley point, the tunnel diode behaves as a normal diode( exhibiting positive resistance).
Now let us understand how this is happening.
- In tunnel diode, the heavy doping provides large number of majority carrier, which leads to much drift activity in p and n regions.
- Due to this many valence electrons have their energy levels closer to the conduction region.
- So applying a very small forward voltage will cause the diode in conduction.
The process of the electrons in the valence energy band moves to conduction band with little or no applied voltage is known as tunneling. (the valence electrons tunnel through the forbidden energy band)
Tunnel Diode Applications:
- It is used as an ultra-high speed switch with switching speed of the order of nano second or pico seconds.
- As logic memory storage device.
- As microwave oscillator.
- In relaxation oscillator circuit.
- As an amplifier.
Tunnel Diode Advantages:
- Low noise
- Low input power
- Easy of operation
- High speed
- Low cost
- Environmental Immunity
Tunnel Diode Disadvantages:
- Voltage range over which it can be operated is 1V or less.
- Since it is a two terminal device there is no isolation between the input and output circuit.
Necessary Condition for Tunneling:
Two condition to be satisfied for tunneling phenomenon takes place:
- The effective depletion region width near the junction must be small, of the order of 3A° by heavy doping
- There must be equivalent empty energy states on the p-side corresponding to energy levels of electrons on the n-side, for these electrons to tunnel from n-side to p-side.
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