Semiconductor diode

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I. Introduction

It is designed by Walter H. Schottky. It is a type of semiconductor diode with a low forward voltage drop and a very fast switching action. The voltage drop of Schottky diode is 0.15 to 0.45V. But in case of a normal diode they have a voltage drop of about 0.7 to 1.7V and due to this low voltage drop it becomes a system of higher efficiency. The cat's-whisker detector which is a wireless device used in the early days can be considered as a primitive schottky diodes. Schottky diodes are mainly called as majority carrier devices and therefore it does not have any problem of storage of minority carrier that slow down the other diodes so they have much faster reverse recovery than other p- n junction diodes. Due to these advantages they used in many sections of electronics. It has also been given a number of other names that are also be used from time to time. These names are surface barrier diode, hot carrier or hot electron diode.

II. Construction Of Schottky Diode

A Schottky diode uses mainly a metal-semiconductor junction as a Schottky barrier instead of a semiconductor-semiconductor junction as in used in normal diodes. This Schottky barrier made the voltage drop of Schottky diode very low and hence they have very fast switching time. The Schottky diode can be formed by point contact diode method in which a metal wire is pressed against a clean semiconductor surface. The metal can be deposited on to the metal surface by using a technique known as fabrication process. But we have to see many factors in the fabrication process. The most critical point which we have to see in the manufacturing process is to ensure a clean surface for good contact of the metal with the semiconductor surface. This is mainly by a chemical processes. The metal is normally deposited in a vacuum either by the use of methods known as evaporation or sputtering techniques. But when we have to use silicides instead of a pure metal contact then this is mainly achieved by depositing the metal and then we have to heat the silicides.

III. Problem With The Construction

The construction mainly uses the different chemical processes and hence some kind of problems arises in the construction. Break down effect arises around the edge of the metalised area. This is mainly arises because of the high electric field that are present around the edge of the plate. To overcome this problem a guard ring of P+ semiconductor is used along with an oxide layer around the edge. This layer is fabricated using a diffusion process. In some cases metallic silicides may be used instead of metal. The advantage of Schottky Structure is that the fabrication requires relatively low temperature techniques. It does not need high temperature for impurity diffusion.

IV. Characteristics Of Schottky Diode

The Schottky diode is mainly called as a majority carrier device. This makes its switching speed very high because it does not have any holes and electrons recombination when they enter in the opposite type of region. The RC time can be reduced by making the device much smaller. We make this diode an order of magnitude faster than the normal p-n junction diodes. These are the factors that make the schottky diode much popular for radio frequency application. The current density of Schottky diode is much higher than the other p-n diode.This means that forward voltage drops are much lower making the Schottky diode ideal for power rectification process. But its main drawback is that reverse recovery current is much higher than that of other. The I-V characteristic of Schottky diode mainly shown below. It can be clearly seen that the Schottky diode has the similar forward semiconductor diode characteristic but it has much low voltage drop. At the above certain level the reverse direction breakdown occurs. But the mechanism of the breakdown in much similar to a p-n junction diode.

V. Difference Between Schottky Diode And Ordinary Diode

The reverse recovery time of normal p-n junction diode is about 100ns or it may be also much less than 100ns. But a Schottky diode does not have any kind of reverse recovery time because it has nothing to recover from. In p-n junction diode there is also a reverse recovery current which in result increases the EMI noise in the high power semiconductors. With the use of slight capacitive loading Schottky diodes switching become much faster than other diodes. In Schottky diode there is no case of random recombination of n and p type material. This makes the conduction of Schottky diode much faster than any ordinary Schottky diode. This property of Schottky diode makes the Schottky diode much smaller in area than any other Schottky diode. Due to the smaller area it has much faster transition than other diodes. They can be also called as heart of RF mixers and detectors which operate up to the region of 50 GHz frequency. The current density of Schottky diode is also much higher than the other p-n diode. These properties make the forward voltage drop of Schottky diode much lower and hence they are ideal for power reactification process. In normal diode dominant current arises from recombination in the depletion layer under small forward bias region but in case of Schottky diode the dominant current arise due to electron injection from the semiconductor to the metal layer.

VI. Applications

The Schottky diodes are mostly used in the electronics industry. They have been used as a diode rectifier in the electronics industry. The unique characteristics of schottky diode make it enable for the use in many applications where we cannot use the other diodes.

  • RF mixer and detectors
  • Power rectifier
  • Power OR circuit
  • Solar cell application
  • Voltage clamping

A. Use of Schottky diode in RF mixer and detectors

The Schottky diode is mainly used in the radio frequency application. Schottky diode is used because of its high switching speed and high frequency capabilities. Schottky diode is used in many diode ring mixers which are very high performance in nature. Their low forward voltage drop and low capacitance make schottky diode ideal for RF detectors.

B. Use of Schottky diode in Power Rectifier

Schottky diodes are also used in high power applications such as rectifiers. Due to the properties of high level of current density and low drop in forward voltage it makes it enables for rectifiers. Because of these properties less power is wasted in rectifiers when we use schottky diode in it than any p-n junction diode. Therefore schottky diode increases the efficiency of the power rectifiers and hence less heat is dissipated and hence energy losses in the rectifiers reduce.

C. Schottky diode in power OR circuits

Schottky diode is also used in those electronics devices in which load have been driven by the two different power supplies. For example power supply and the battery supply are the two different loads. For that cases where we use two different power supplies it is necessary to make sure that the power from one supply do not enter into the other supply. This objective is mainly achieved by the use of diodes in the circuit. But to ensure the maximum efficiency of the circuit the voltage drop of the diode must be low. And we know that schottky diode has really low forward voltage drop. Therefore schottky diode is really good for Power OR circuit.

But its main limitation is that it has high reverse leakage current. This may lead to a problem with any type of circuit that we are using. Leakage current in the path of impedance circuit can give rise to false readings of the circuit. This thing must be taken in order when we design a circuit.

D. Schottky diode in solar cell applications

Solar cells are mainly connected with the batteries which are rechargeable in nature. Most the solar cell used lead acid batteries. The batteries are used because power requirement is for the whole day and the main source of energy i.e. sun is not present in 24 hours. In Solar cell there is no reverse charge applied and hence a diode is required in series combination with the solar cell. If there is any kind of voltage drop there will be reduction in efficiency and hence we need a diode having low forward voltage drop. And we know that schottky diode has low forward voltage drop and therefore they are used in solar cells.

E. Schottky diode as a clamp diode

Schottky diodes are also used in a transistor circuit as a clamp diode. It is used to increase the speed of the switching operation. Transistor circuit mainly use two kinds of logic families as a clamp diode and they are low power Schottky which is known as 74LS and Schottky 74S. For the clamping action schottky diodes are put forward between the collector and base junction of transistor. When we have to use the low or 0 output the transistor must be in on state and the base collector junction is forward biased. When we use schottky diode rather than the other diodes then schottky diode takes most of the current and hence the turn off time of the transistor is reduced and hence the speed of the circuit is increased.

VII. Junction Capacitance Of Schottky Diode

In Schottky diode there is a junction which is a type of insulator that separates the two regions which is conductive in nature and hence it is like a parallel plate capacitor. Capacitance of this region is determined by the physical dimensions of the junction as well as the doping profile of the semiconductor layer.

The depletion region thickness depends upon the external voltage which is applied to it. If we make the schottky diode forward bias then it will reduce the thickness of the depletion layer and hence it moves the plates of the capacitor together. And when we make it reverse bias voltages then it will increases the thickness of the depletion layer.

The relation between reverse bias voltage and junction capacitance of schottky diode:-

CJ (VR) = CJ (0) / (1- VR / VI - KT / q) 1/2

CJ = Junction capacitance at reverse bias voltage

VR = reverse bias voltage from external voltage source

CJ (0) = junction capacitance with VR =0

K= Boltzmann's constant

T = absolute temperature

q = charge of an electron

VIII. Limitation Of Schottky Diode

The main limitation of Schottky diode is the low value of reverse voltage which is about 50V or below than the 50V. The another limitation of schottky diode is relatively high value of reverse leakage current in it. The value of reverse leakage current also effects by the temperature. With the increase in temperature the reverse leakage current increase. But now a day the reverse voltage value of the schottky diode is increased which is about 200V. The thermal instability also occurs in the schottky diode because of the increase in reverse current when the temperature is increased.

IX. Designation

Mainly a Schottky diode is generally denoted by the 1N5817 series for 1A rectifiers. Schottky diode is mainly used in the logic family which is 7400 TTL. The 74S, 74LS and 74ALS series schottky diode are used in 7400TTL. They are used as a clamp diode with the collector base junction of the transistor to prevent their saturation and hence they reduce their turn off delays. Small signal Schottky diodes are mainly represented by the series 1N5711, 1SS106, 1SS108, 1N6263, BAT41, BAT 43, BAT45, and BAT49. This schottky diode are mainly used in high frequency applications such as detectors, RFmixers .Schottky diode generally replaced Ge diodes in these kind of applications.

X. New Generation Of Schottky Diode

In 2001 there is an another important invention by a company which is known as Siemens Semiconductor but now a days it is Infenion is a silicon carbide (SiC) Schottky diode. The reverse leakage current is about 40 times lower than a simple schottky diode. The thermal conductivity of Silicon carbide schottky diode is also high than the simple schottky diode. The effect of temperature is also much lower than other diodes and hence its switching speed is much faster than a simple schottky diode. There are some special type of SiC Schottky diode which operates at a junction temperature of 500K and hence they are readily used by the radiation cooling in aerospace application. Firstly Schottky diodes are only available in 300V and 600V. But after the coming of Sic schottky diode they are also available in 1200V..

XI. Features Of SiC Schottky Diode

Siemens Semiconductor now Infineon has just launched its next generation of SiC Schottky diodes. It is mainly called as 3G thinQ! ™ Family. It offers the lowest device capacitance for any given current rating for the use in industry. This further increases the overall system efficiency. It increase switching frequencies to a much higher value whether the load given is much low. It gives the overall benefits to higher frequency designing system. It also gives benefits to increase power density and hence the cooling requirement system cost is much down in it.

A. Features

  • Low cost
  • Very fast switching action
  • No reverse recovery
  • Temperature influence has no effect on to the switching behaviour of schottky diode.
  • Standard operating temperature -55° to 175°C

B. Applications

  • Motor drives
  • Switched-mode power supplies (SMPS)
  • Use in liquid crystal display and plasma displays
  • Use in lighting systems
  • Power factor error correction
  • Solar cell

XII. Conclusion

Schootky diode mainly possess two unique features over an ordinary p-n diode and they are

It is a unipolar device because of the absence of minority charge carries in reverse direction while p-n junction diode is a bipolar device.

Due to the absence of holes in the metal there is no depletion layer or the stored charge in the schottky diode.

Therefore these two main properties make the switching action of the schottky diode much faster than the other diodes. The forward resistance which is offered by the schottky diode is much lower than the other because it has large semiconductor and metal surface area. The schottky diode also has low forward drop and this make it good for high power application. But its main disadvantage is high leakage current and low reverse voltage and hence they are only uses in those circuits where we need low reverse recovery time and high efficiency.

Its new generation SiC schottky diode is used in 3G technology services because it is 40 times much faster than the schottky diode and it has high value of reverse voltage.

Acknowledgement

I am heartily thankful to my teacher, Shiwani Arora whose encouragement, support and guidance from the initial to the final level enabled me to develop an understanding of the term paper which is on Schottky Diode. Lastly, I offer my blessings and regards to all of those who supported me in any respect during the completion of the term paper.

References

[1] Electronics devices and circuit ,J.B. Gupta ,3rd edition

[2] http://en.wikipedia.org/wiki/Schottky_diode

[3] http://www.absoluteastronomy.com/topics/Schottky_diode

[4] http://www.infineon-designlink.com/index.php?article_id=7

[5] http://www.radio-electronics.com/info/data/semicond/schottky_diode/schottky_barrier_diode.php

[6] http://www.electronics-radio.com/articles/electronic_components/diode/schottky-barrier-diode.php

[7] http://b2bhimfr.us.splinder.com/archive/2009-05

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