Posted on : Sun , 03 2014 by : virusi
Tunnel Diode construction :
Tunnel Diode or Esaki Diode is the p-n junction device that exhibits negative resistance. That means when the voltage is increased the current the current through it decreases.
Tunneling is an effect that is caused by quantum mechanical effects when electrons pass through a potential barrier. It can be visualized in very basic terms by them “tunneling” through the energy barrier. The tunneling only occurs under certain conditions. It occurs within tunnel diodes because of the very high doping levels employed (1000 times greater than a conventional junction diode). Because of heavy doping depletion layer width is reduced to an extremely small value of 1/10000 m.
Reverse breakdown voltage is also reduced to very small value~0 resulting in appearance of the diode to be broken for any reverse voltage and a negative resistance section is produced in the volt-ampere characteristics of the diode.
For the forward bias situation there are a number of different areas. For voltages up to Vpe, electrons from the conduction band find increasing availability of empty states in the valence band and the level of current increases up to a point where the current equals Ipe.
Once this point is reach, it is found that number of empty states available for electrons with the level of energy they are given by the increased voltage level starts to fall. This means that the current level falls in line with this. The overall current level falls away relatively swiftly, dropping to near zero.
As the current from the tunneling effect falls, so the diffusion current, which is the same action as occurs in a normal PN junction diode starts to increase and steadily becomes the dominant mechanism.
Let’s take a look at same tunnel diode advantages :
– Very high speed : The high speed of operation means that the tunnel diode can be used for microwave RF applications. The switching speed is up to 5 GHz.
– Longevity : Studies have been undertaken of the tunnel diode and its performance has been shown to remain stable over long periods of time, where other semiconductor devices may have degraded.
One of the main reasons for the early success of the tunnel diode was its high speed of operation and the high frequencies it could handle. This resulted from the fact that while many other devices are slowed down by the presence of minority carriers, the tunnel diode only uses majority carriers, i.e. holes in an n-type material and electrons in a p-type material. The minority carriers slow down the operation of a device and as a result their speed is slower. Also the tunneling effect is inherently very fast.
One of the major minuses of the tunnel diode is that they have a low tunneling current and this means that they are low power devices. While this may be acceptable for low noise amplifiers, it is a significant drawback when they are sued in oscillators as further amplification is needed and this can only be undertaken by devices that have a higher power capability, i.e. not tunnel diodes. The second disadvantage is that they are problems with the reproducibility of the devices resulting in low yields and therefore higher production costs.
Tunnel Diode V-I characteristic :
The characteristic curve for a tunnel diode shows an area of negative resistance. When forward biased the current in the diode rises at first, but later it can be seen to fall with increasing voltage, before finally rising again.
The reason for this is that there are a number of different components to forming the overall curve :
– Normal diode current : This is the ‘normal’ current that would flow through a PN junction diode.
– Tunneling current : This is the current that arises as a result of the tunneling effect.
– Excess current : This is a third element of current that contributes to the overall current within the diode. It results from what may be termed excess current that results from tunneling though bulk states in the energy gap, and means that the valley current does not fall to zero.
PRACTICAL EXAMPLE :
– In your browser allow Java SE 7.
– Lower you java security settings (Go to Control Panel >> Java >> Security and set the security level to medium) .
– Edit Site List (Go to Control Panel >> Java >> Security and click on Edit Site List… and add eagerlearning.org in the list).
Let’s check a simple example with a tunnel diode connected in series with a resistor to a AC voltage source and check the V-I curve of the tunnel diode.
If you want to change resistor or AC voltage value than you should double click on the element and insert the desired value. First Diagram is showing Tunnel Diode IV characteristic, you can play with the resistance value and see how the V-I characteristic is changing. The second diagram shows the I and V that is flowing through the Tunnel Diode.