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Schottky Diode

Schottky diode is an electronic device that was invented by a German scientist by the name of Walter H. Schottky. Schottky diodes are made from the contact of semiconductor material with metal. This diode also goes by the name of Schottky barrier diode or hot-carrier diode. Schottky diodes have a low forward voltage drop. Their biggest advantage is their fast switching action. The earliest form of Schottky diode is a device called the ‘cat’s whiskers’. This device was invented by an Indian scientist called J. C. Bose in the earlier part of the twentieth century. This device did not gain considerable favour among the intelligentsia owing to its unpredictable and laborious operation. Later this device was used in the second world war as a receiver of radio signals. This launched further interest in semiconductor technology and finally resulted in the invention of the first diodes and transistors in the Bell Labs.

Current flows in a Schottky diode when a sufficiently large forward voltage is applied. Schottky diodes are known for their comparatively lower forward voltage. While for a standard silicon p-n junction diode the forward voltage ranges from 600- 700 mV, for a Schottky diode, this voltage is in the range of 150- 450 mV. This lower forward voltage allows for more efficient operation and faster switching action, which forms the main advantage of a Schottky diode.

Construction

Schottky diode is made by a junction of a metal and a semiconductor. This junction creates a ‘Schottky barrier’. Some typical metals used in the construction are molybdenum, platinum, chromium and tungsten. A mixture of a semiconductor and a metal may also be used, like the silicide of palladium or platinum. The semiconductor used in the construction is an n-type semiconductor. The metal acts as an anode and the semiconductor (which is n-type) acts as a cathode. This means that the current will flow from the metal side to the semiconductor. It does not flow in the opposite direction.

The value of the forward voltage of the diode is determined by the type of metal or semiconductor used. For a p-type semiconductor, the forward voltage is too low. This means that the backward current is significantly large. To prevent this, we generally use the n-type semiconductor. P-type semiconductors are used very rarely. Even if they are used, they are used in combination with a material that can withstand the large backward current without getting damaged. An example of such a material is titanium silicide.

Reverse Recovery Time

Reverse recovery time is the most prominent feature of the Schottky diode and this is what sets it apart from a normal p-n junction diode. Reverse recovery time is when the diode switches from a conducting state to a non-conducting state. For a p-n diode, this recovery time could be of the order of several microseconds. They are limited by the diffusion capacitance of the carriers. These carriers are accumulated in the different regions of the diode when the diode is conducting. This leads to a slowing-down of the switching action. Schottky diodes are a unipolar device and therefore their switching action is much faster.

The reverse recovery time for a Schottky diode is of the order of 100 ps (for a small signal diode). Certain high capacity power diodes can even have recovery time in the region of tens of nanoseconds. Standard p-n junction diodes also have a reverse recovery current which can bring EMI noises to the semiconductor. This is also prevented in the Schottky diode owing to its much faster switching action. We only have to deal with a slight capacitive loading, that only lasts for a fraction of a second. Overall, this is not a major concern for the Schottky diode.

Schottky diodes are a majority carrier device. This means that for an n-type Schottky diode, the majority of carriers are n-type carriers (that are electrons). Similar is the case of the p-type Schottky diode.

Limitations

The major limitation of the Schottky diode is reverse current leakage. The leakage of reverse current can lead to the heating of the device, which leads to insatiable operation. This reverse current also increases with temperature, so if the leakage current is already significant, then the rise in temperature can make the leakage worse. This puts the diode in danger of overheating and getting damaged. This shortcoming can be addressed by making diodes with high reverse voltage.

High reverse voltage forbids the significant leaking of the reverse current, but the switching operation is slowed down. Switching is still much faster than a standard p-n junction diode, so for basic operations, it does not make much of a difference.

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