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A Journey from Wave to Particle Nature Of Light

A Journey From Wave To Particle Nature of Light

A Journey From Wave To Particle Nature of Light

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Schottky Diode

Schottky Diode

Introduction:

A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode. A normal diode will have a voltage drop between 0.6 to 1.7 volts, while a Schottky diode voltage drop is usually between 0.15 and 0.45 volts. This lower voltage drop provides better system efficiency and higher switching speed. In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. This Schottky barrier results in both a low forward voltage drops and very fast switching.


A Schottky diode, also known as a hot carrier diode, is a semiconductor diode which has a low forward voltage drop and a very fast switching action. There is a small voltage drop across the diode terminals when current flows through a diode.


In a Schottky diode, a semiconductor–metal junction is formed between a semiconductor and a metal, thus creating a Schottky barrier. The N-type semiconductor acts as the cathode and the metal side acts as the anode of the diode. This Schottky barrier results in both a low forward voltage drop and very fast switching.

2.2. Types of Schottky Diodes

There are many different kinds of Schottky diodes and at Future Electronics we stock many of the most common types categorized by maximum average rectified current, maximum reverse voltage, maximum reverse current, forward voltage, packaging type and maximum peak current. The parametric filters on our website can help refine your search results depending on the required specifications.


The most common sizes for maximum average rectified current are 70 mA, 100 mA, 200 mA and 1 A. We also carry Schottky diodes with maximum average rectified current as high as 300 A. Forward voltage can range from 280 mV to 430 V, with the most common Schottky diode semiconductor chips having a forward voltage of 800 mV or 1 V.

2.3. Circuit symbol

The Schottky circuit symbol used in many circuit schematic diagrams may be that of an ordinary diode symbol. However, it is often necessary to use a specific Schottky diode symbol to signify that a Schottky diode rather than another one must be used because it is essential to the operation of the circuit. Accordingly, a specific Schottky diode symbol has been accepted for use. The circuit symbol is shown below:

Schottky diode symbol

It can be seen from the circuit symbol that it is based on the normal diode one, but with additional elements to the bar across the triangle shape.

2.6. Schottky diode Characteristics:

· The Schottky diode is what is called a majority carrier device. This gives it tremendous advantages in terms of speed because it does not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode. By making the devices small the normal RC type time constants can be reduced, making these diodes an order of magnitude faster than the conventional PN diodes. This factor is the prime reason why they are so popular in radio frequency applications.


· The diode also has a much higher current density than an ordinary PN junction. This means that forward voltage drops are lower making the diode ideal for use in power rectification applications.


· Its main drawback is found in the level of its reverse current which is relatively high. For many uses this may not be a problem, but it is a factor which is worth watching when using it in more exacting applications.


· The Schottky diode has the typical forward semiconductor diode characteristic, but with a much lower turn on voltage. At high current levels it levels off and is limited by the series resistance or the maximum level of current injection. In the reverse direction breakdown occurs above a certain level. The mechanism is similar to the impact ionisation breakdown in a PN junction.

2.7. IV characteristic:

· The IV characteristic is generally that shown below. In the forward direction the current rises exponentially, having a knee or turn on voltage of around 0.2 V. In the reverse direction, there is a greater level of reverse current than that experienced using a more conventional PN junction diode.

The use of a guard ring in the fabrication of the diode has an effect on its performance in both forward and reverse directions. Both forward and reverse characteristics show a better level of performance.


· However the main advantage of incorporating a guard ring into the structure is to improve the reverse breakdown characteristic. There is around a 4 : 1 difference in breakdown voltage between the two - the guard ring providing a distinct improvement in reverse breakdown. Some small signal diodes without a guard ring may have a reverse breakdown of only 5 to 10 V.

2.8. Limitations:

· The most evident limitations of Schottky diodes are their relatively low reverse voltage ratings, and their relatively high reverse leakage current. For silicon-metal Schottky diodes, the reverse voltage is typically 50 V or less. Some higher-voltage designs are available (200 V is considered a high reverse voltage). Reverse leakage current, since it increases with temperature, leads to a thermal instability issue. This often limits the useful reverse voltage to well below the actual rating.


· However that does not mean the Schottky diode is not to be used. The reverse bias breakdown voltage is in fact made to be relatively constant and specific and the diodes are sold and used based on the reverse bias breakdown voltage, for the purpose of being used as a conductor ( a voltage limiter) in reverse bias breakdown mode.


· While higher reverse voltages are achievable, they would present a higher forward breakdown voltage, comparable to other types of standard diodes. Such Schottky diodes would have no advantage unless great switching speed is required.

2.9. Applications of Schottky Diode

Despite the fact that Schottky barrier diodes have many applications in electronics scene, it is actually one of the oldest semiconductor devices in existence. As a metal-semiconductor device, its applications can be tracked back to before 1900 where crystal detectors, cat’s whisker detectors and the like were all effectively Schottky barrier diodes.


The Schottky diodes are widely used in the electronic industry finding many uses as diode rectifier. Its unique properties enable it to be used in a number of applications where other diodes would not be able to provide the same level of performance.


Some of its applications are:


2.9.1. RF mixer and Detector Diode


The Schottky diode has come into its own for radio frequency applications because of its high switching speed and high frequency capability. In view of this Schottky barrier diodes are used in many high performance diode ring mixers. In addition to this their low turn on voltage and high frequency capability and low capacitance make them ideal as RF detectors.


2.9.2. Power Rectifier


The Schottky barrier diodes also used in high power applications, as rectifiers. Their high current density and low forward voltage drop mean that less power is wasted than if ordinary PN junction diodes were used. This increase in efficiency means that less heat has to be dissipated, and smaller heat sinks may be able to be incorporated in the design.


2.9.3. Solar Cell Applications


Solar cells are typically connected to rechargeable batteries, often lead acid batteries because power may be required 24 hours a day and the Sun is not always available. Solar cells do not like the reverse charge applied and therefore a diode is required in series with the solar cells.


Any voltage drop will result in a reduction in efficiency and therefore a low voltage drop of the Schottky diode is particularly useful, and as a result they are the favoured form of diode in this application.


2.9.4. Clamp Diode


Schottky barrier diodes may also be used as a clamp diode in a transistor circuit to speed the operation when used as a switch. They were used in this role in the 74LS (low power Schottky) and 74S (Schottky) families of logic circuits. In these chips the diodes are inserted between the collector and base of the driver transistor to act as a clamp. To produce a low or logic ‘0’ output the transistor is driven hard on, and in this situation the base collector junction in this diode is forward biased. When the Schottky diode is present this takes most of the current and allows the turn off time of the transistor to be greatly reduced, thereby improving the speed of the circuit.

Varactor Diode

1.    Varactor Diode

1.1.          Definition:

A Varactor Diode is:

A p-n junction diode which acts as a variable capacitance under changing reverse bias.

A p-n junction diode that changes its capacitance and the series resistance as the bias applied to the diode is varies.

1.2.          Symbol:

The symbol of Varactor diode is:

The Varactor diode symbol consists of the capacitor symbol at one end of the diode that represents the variable capacitor characteristics of the diode.

1.3.          Explanation:

Varactor diodes are also termed as varicap diodes, in fact, these days they are usually termed as Varactor diodes. Even though the variable capacitance effect can be exhibited by the normal diodes (P-N junction diodes), but, Varactor diodes are preferred for giving the desired capacitance changes as they are special types of diodes. These diodes are specially manufactured and optimized such that they enable a very high range of changes in capacitance. Varactor diodes are again classified into various types based on the Varactor diode junction properties. And, these are termed as abrupt Varactor diodes, gallium-arsenide Varactor diodes, and hyper abrupt Varactor diodes. The Varactor diode also called a varicap or tuning or voltage variable capacitor diode, is a junction diode with a small impurity dose at its junction, which has useful property that its junction or transition capacitance is easily varied electronically.

A Varactor diode is a specially manufactured P-N junction with variable concentration of impurities in its P-N materials. In a conventional diode doping impurities are usually distributed equally throughout the material. Varactor have a very light dose of impurities near the junction. Moving away from the junction the impurity level increases.

1.4.          Circuit Diagram:

It is impossible to show all the circuits in which Varactor / varicap diodes may be used. However, it is worth providing one example to show how these diodes may be used in a typical circuit. Effectively a capacitor is replaced with the Varactor diode, but it is necessary to also ensure that the tune voltage, i.e. the voltage used to set the capacitance of the diode can be inserted into the circuit, and that no voltages such as bias voltages from the circuit itself can affect the Varactor diode.

Within this circuit D1 is the Varactor diode that is used to enable the oscillator to be tuned. C1 prevents the reverse bias for the Varactor or varicap diode being shorted to ground through the inductor, and R1 is a series isolating resistor through which the Varactor diode tuning voltage or bias is applied.

1.5.          Construction & Working:

When any diode is reverse biased, a depletion region is formed, as seen below:

The larger the reverse bias applied across the diode, the width of the depletion layer W becomes wider. Conversely, by decreasing the reverse bias voltage the depletion region width W becomes narrower. This depletion region is devoid of majority carriers and acts like an insulator preventing conduction between the N and P region of the diode, just like a dielectric, which separates the two plates of a capacitor.

When Varactor diode is reverse biased than the neutral region between P and N layers increases and when the reverse biasing decreases then this neutral region is also decreased. From this it is concluded that diode also has the capacity like a capacitor the difference is only that capacity in the capacitor varies due to dielectric between the two plates and in the diode capacity varies with neutral region thus dielectric region of the capacitor can be considered as neutral region of the diode and in this way diode can be considered as capacitor whose capacity changes with the reverse voltage. All the diodes change their capacity with the reverse voltage but some of them are manufactured specially which changes their capacity with reverse voltage of a definite capacity range.

1.6.          Characteristics of Varactor Diode

Varactor diodes, also known as varicap diodes, are a simple electronic component. A type of simple semiconductor diode commonly used in electronics such as parametric amplifiers, filters, oscillators and frequency synthesizers, Varactor diodes have a variable capacitance, which is a

function of the voltage impressed on its terminals. In electronics, Varactor diodes are mostly utilized as voltage-controlled capacitors.

1.6.1.      Operation:

Varactor diodes are constructed in the same way as a capacitor and operate under reverse bias conditions, which gives rise to three current-conducting regions. Currents conduct through positive (P) and negative (N) regions, located at either end of the diode. Near the junction of the P and N regions, a depletion region ensures that no current carriers are available, thus acting as an insulator. Due to this arrangement, a Varactor diode's conductive plates are separated by an insulator like dielectric, much like a capacitor.

1.6.2.   Capacitance:

In electronics, capacitance is the ratio of charge impressed on a given conductor. This characteristic determines a diode's frequency of operation. Any capacitor or conductor's capacitance depends on varying factors such as the area of its conductive plates, the dielectric constant of the insulator between the plates and the distance between the two plates. The width of a Varactor diode's depletion region increases and decreases via changing the level of the diode's reverse bias. In effect, changing this level alters the distance between the capacitor's plates. As the capacitance range of Varactor diodes are controlled by adjusting the gradient and junction width, range changes are applied using reverse voltage. Commonly, Varactor diodes operate at a four-to-one capacitance range.

1.6.3.   Reverse Breakdown:

Varactor diodes are designed to provide voltage-controlled capacitance operation under reverse bias. A diode's reverse breakdown is defined by the minimum reverse voltage required to make the diode conduct in reverse. As reverse bias increases, capacitance decreases; the

maximum voltage that a Varactor diode can withstand is determined by its maximum capacitance level. The reverse bias of most Varactor diodes operates from around a few volts up to about 20 volts, with some rare exceptions operating up to 60 volts. As a Varactor diode's voltage increases, specific energy supplies must be provided for the circuits driving the diode.

The Varactor diodes have the following some other significant characteristics:

• Varactor diodes produces considerably less noise compared to other conventional diodes.
• These diodes are available at low costs.
• Varactor diodes are more reliable.
• The Varactor diodes are small in size and hence, they are very light weight.
• There is no useful purpose of Varactor diode operated when it is operated in forward bias.
• Increase in reverse bias of Varactor diode increases the capacitance as shown in the figure below.

1.6.4.       Important Criteria:

·         Capacitance: Capacitance of the device. Capacitance from a few Pico Farads to hundreds of Pico Farads is provided. ¢

·         Capacitance range: Range of capacitance produced when voltage is varied. ¢ Voltage range: The minimum and maximum voltage that can be applied to the device.

·         Bias current: The bias is always reverse. This means that the Varactor diode does not conduct electricity. If the bias is turned positive, then the device will start conducting.

Other criteria to be considered include:

·         Reverse and breakdown voltage,

·         Leakage current,

·         Junction Temperature.

·         Voltage and other transients must be avoided.

·         Transients can occur if the DC voltage is put off.

1.7.          Application of Varactor Diode:

1.7.1.      Voltage controlled oscillators, VCOs:

 Voltage controlled oscillators are used for a variety of applications. One major area is for the oscillator within a phase locked loop - this are used in almost all radio, cellular and wireless receivers. A Varactor diode is a key component within a VCO.

1.7.2.      RF filters:

Using Varactor diodes it is possible to tune filters. Tracking filters may be needed in receiver front end circuits where they enable the filters to track the incoming received signal frequency. Again this can be controlled using a control voltage. Typically, this might be provided under microprocessor control via a digital to analogue converter

Some other applications are:

2.         It is used in variable resonant tank LC circuit. Here C part is varied using Varactor diode.

3.         AFC (Automatic Frequency Control) where in Varactor diode is used to set LO signal.

4.         Varactor is used as frequency modulator.

5.         It is used as frequency multiplier in microwave receiver LO.

6.         It is used as RF phase shifter.

BROWNIAN MOTION

BROWNIAN MOTION

The erratic random movement of microscopic particles in a fluid, as a result of continuous bombardment from molecules of the surrounding medium.

Brief History:

It was first observed in 1827 by a botanist Brownian. The term “classical Brownian motion” describes the random movement of microscopic particles suspended in a liquid or gas. Brown was investigating the fertilization process in Clarkia pulchella, then a newly discovered species of flowering plant, when he noticed a “rapid oscillatory motion” of the microscopic particles within the pollen grains suspended in water under the microscope. In 1827 the biologist Robert Brown noticed that if you looked at pollen grains in water through a microscope, the pollen jiggles about. He called this jiggling 'Brownian motion', but Brown couldn't work out what was causing it. The first of the three papers that Einstein published in 1905 finally came up with an explanation.

Everything around us is made up of atoms and molecules: the chair you're sitting on, the food you eat, the air you're breathing. The idea of atoms has been around since the time of the ancient Greeks, and a century before Einstein, the great chemist John Dalton had suggested that all chemicals were made of tiny invisible molecules, which in turn were made of even tinier atoms. The problem was that there was no proof of their existence, until Einstein looked into the problem of Brownian motion.

Einstein explanation of Brownian motion:

Einstein realized that the jiggling of the pollen grains seen in Brownian motion was due to molecules of water hitting the tiny pollen grains, like players kicking the ball in a game of football. The pollen grains were visible but the water molecules weren't, so it looked like the grains were bouncing around on their own.

Einstein also showed that it was possible to work out how many molecules were hitting a single pollen grain and how fast the water molecules were moving - all by looking at the pollen grains.

Importantly, Einstein's paper also made predictions about the properties of atoms that could be tested. The French physicist Jean Perrin used Einstein's predictions to work out the size of atoms and remove any remaining doubts about the existence of atoms.

Explanation:

Brownian motion, also called Brownian movement,  any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations. It was named for the Scottish botanist Robert Brown, the first to study such fluctuations (1827).

If a number of particles subject to Brownian motion are present in a given medium and there is no preferred direction for the random oscillations, then over a period of time the particles will tend to be spread evenly throughout the medium. Thus, if A and B are two adjacent regions and, at time t, A contains twice as many particles as B, at that instant the probability of a particle’s leaving A to enter B is twice as great as the probability that a particle will leave B to enter A. The physical process in which a substance tends to spread steadily from regions of high concentration to regions of lower concentration is called diffusion. Diffusion can therefore be considered a macroscopic manifestation of Brownian motion on the microscopic level. Thus, it is possible to study diffusion by simulating the motion of a Brownian particle and computing its average behavior.

The first point we must understand here is that the problem of Brownian motion, that is of randomly moving particles spurred by an infinite thermal energy reservoir, is nothing different to the problem of a random walker who takes either a step right or to the left with two different probabilities. The question as to what the velocity trajectory of a Brownian 

particle  is at position r at time t now maps over to a question of how many steps t, each of length ∆,would a random walker( a drunk person) need to reach a point r to the right( or left) of his starting position. Now if the system is sufficiently randomized, that is the random walker can step to the right or left at any time with an equal probability ½, it is not too difficult to see that his mean displacement will be zero.

Zeamenskey. (2012).Heat and Thermodynamics.Tata McGraw-Hill Education

The phenomenon of Brownian motion can be easily observed in a laboratory if a colloidal solution is examined under an ultra-microscope. As the direction of illumination is perpendicular to the axis of the microscope. The suspended particles in the solution look like bright illuminated spots. These illuminated particles continually move to and fro in a random haphazard way. The particles spin, rise sink and rise again. The movements of the particles is continuous and spontaneous. This nonstop random and haphazard motion of the particles is called Brownian motion. The motion of the particles become more conspicuous in a liquid of lower viscosity. The phenomenon of Brownian movement gives a clear picture of the gaseous state of matter. The motion of the molecules of a gas is similar in nature to the Brownian motion of suspended particles in a colloidal solution.

http://www.britannica.com/EBchecked/topic/81815/Brownian-motion

 Essential Feature:

1.      The motion of each particle is completely irregular and random. No two particles are found to execute the same motion.

2.      The motion is continuous and takes place for ever.

3.      The smaller particles appear to be more agitated than the larger ones.

4.      The motion is independent of the nature of the suspended particles.

5.      The motion become more violent on increasing temperature.

6.      The motion is not modified due to the shaking of colloidal motion.

7.      The Brownian motion cannot be observed with particles of large size.

8.      The motion is more conspicuous in a liquid of lower viscosity.

9.      The laws of kinetic theory of gases are applicable to Brownian motion too.

Lal.B, Subrahmanyam.N. (2008).Heat Thermodynamics And Statistical Physics. S. Chand, 2008

Langevin’s Theory of Brownian Motion:

According to Langevin the force acted on suspended particle is of two types

1.      Frictional Force proportional to the velocity

f()=6

2.      Force due to all external influence of surrounding fluid.

According to this theory viscosity of liquid decreases as temperature increases. Thus the temperature effect is negligible comparable to the effect of viscosity. Diffusion, fluctuations in concentration and Brownian motion represents a single phenomenon. Diffusion is a macroscopic phenomenon while Brownian motion is a microscopic phenomenon.

Einstein’s theory of Brownian motion:

According to Einstein’s theory of transitional Brownian motion the particles lend to diffuse into the medium in course of time. Consequently the diffusion coefficient must be related to the Brownian movement.

Conclusion:

Historically and conceptually, Brownian motion lies intermediate between thermodynamics and statistical mechanics. On the one hand it is a good quantitative model for thermodynamic fluctuations, and on the other hand, as in Einstein’s original treatment, it can be usefully described by a probability distribution and transition probability, which are the stuff of statistical mechanics.

Differential Equation:

The term differential equation was coined by Leibniz in 1676 for a relationship between the two differentials dx and dy for the two variables x and y. A differential equation is an equation which contains a derivative of an unknown function. It tells something about a rate of change, from which we hope to deduce facts about the function. Here is a differential equation. A differential equation is basically a mathematical equation that relates some function with its derivatives. In applications, the functions usually represent physical quantities, the derivatives represent their rates of change, and the equation defines a relationship between the two.  A differential equation contains one or more terms involving derivatives of one variable (the dependent variable, y) with respect to another variable (the independent variable, x).

For example, 

Unlike algebraic equations, the solutions of differential equations are functions and not just numbers. It represents the relationship between a continuously varying quantity and its rate of change. This is very essential in all scientific investigation. Partial differential equations (PDEs) are equations that involve rates of change with respect to continuous variables. The position of a rigid body is specified by six numbers, but the configuration of a fluid is given by the continuous distribution of several parameters, such as the temperature, pressure, and so forth. The dynamics for the rigid body take place in a finite-dimensional configuration space; the dynamics for the fluid occur in an infinite-dimensional configuration space. This distinction usually makes PDEs much harder to solve than ordinary differential equations (ODEs), but here again there will be simple solutions for linear problems. Classic domains where PDEs are used include acoustics, fluid flow, electrodynamics, and heat transfer.