What is Diode?

Diode

What is diode?

Definition: 

A diode is defined as two terminal electronic component that only conduct current in one direction (so long as it is operated within a specified voltage level). An ideal diode will have zero resistance in one direction, and infinite resistance in the reverse direction.

 Although in the real world, diode cannot achieve zero OR infinite resistance. Instead, a diode will have negligible resistance in one direction (to allow current flow) and a very high resistance in the reverse direction (to prevent current flow). A diode is effectively like a valve for an electrical circuit.

 Semiconductor diodes are the most common types of diode. These diodes begin conducting electricity only if a certain threshold voltage is present in the forward direction (i.e. the voltage "low resistance" direction). The diode is said to be "forward biased" . When conducting current is in the direction. When the connected within a circuit in the reverse direction (i.e. the "high resistance" direction). The diode is said to be "reverse biased".

A diode only blocks current in the reverse direction ( i.e. when the it is reverse biased) while the reverse voltage is within the specified range. Above this range, the reverse barriers breaks. The voltage at which this breakdown occurs is called as the "reverse breakdown voltage". When the voltage of the circuit is higher than the reverse breakdown voltage, the diode is able to conduct electricity in the reverse direction. (i.e. the "high resistance" direction). This is why in practice we say diodes have a high resistance in the reverse direction- not an infinite resistance.

 A P-N junction is the simplest form of the semiconductor diode. In ideal condition, this P-N junction behaves as a short circuit when it is forward biased, and as an open circuit when it is in the reverse biased. The name diode is derived from "di-ode" which means a device that has two electrodes. Diodes are commonly used in many electronics projects and are included in many of the best Arduino starter kit.

Diode symbol

The symbol of a diode is shown in below figure. The arrowhead points in the direction of conventional current flow in the forward biased condition. That means the anode is connected to p-side and the cathode is connected to the n-side.

 We can create a simple P-N junction diode by doping pentavalent for donor impurity e in one portion and trivalent or acceptor impurity in other portion of the silicon or germanium crystal block. These doping make a P-N junction at the middle part of the block. We can also form a P-N junction by joining a p type and n type semiconductor together with a special fabrication technique. The terminal connected to the p type is the anode and terminal connected to the n type is cathode.

Working principle of diode

A Diodes working principle depends on the interaction of n type and p type semiconductors. An n-type semiconductors has plenty of free electrons and a very few numbers of holes. In other words, we can say that the concentration of free electron is high and that of holes is very low in an n type semiconductor. Free electrons in the n type semiconductor are referred as majority charge carriers, and holes in n type semiconductor are referred to as minority charge carriers.

A p-type semiconductor has a high concentration of holes and a low concentration of free electrons. Holes in a p-type semiconductor are majority charge carrier, and free electrons in p type semiconductor are minority charge carrier. 

Unbiased diode

Now let us see what happens when one n type region and one p-type region come in contact. Here due to concentration differences, majority carriers diffuse from one side to another. As the concentration of holes is high in p-type region and it is low in the n type region, the holes start diffusing from the p-type region to the n type region.

Again the concentration of free electrons is high in the p-type region and it is slow in the p-type region and due to this reason free electron starting diffusing from the n-type region to the p type region.

The free electrons diffusing into the p type region from the n type region would recombine with holes available there and create uncovered negative ions in the p-type region. In the same way, the holes diffusing into the n type region from the p-type region would recombine with free electrons available there and create uncovered positive ions in the n type region.

In this way, there would a layer of negative ions in the p-type side and a layer of positive ions in the n type region appear along the junction line of these two types of semiconductors. The layer of uncovered positive ions and uncovered negative ions from the region in the middle of the diode where no charge career exists since all the charge carrier get recombined here in this region. Due to the lack of charge carriers, this region is called as a "deflection region".

 After the formation of depletion region, there is no more diffusion of charge carrier from one side to the another in the diode. This is due to the electric field appeared across the depletion region will prevent further migration of charge carrier from one side to another.

The potential of the layer of uncovered positive ions in the n-type side would repeal the holes in the p type sides and the potential of the layer of uncovered negative ions in the p-type side would repeal the free electrons in the n type side. That means the potential barrier is created across the junction to prevent further diffusion of charge carriers.

Forward biased diode

Now let us see that what happens if a positive terminal of a source is connected to the p-type side and the negative terminal of the source is connected to the n-type side of the diode and if we increase the voltage of this slowly from zero.

In the beginning, there is no current flow through the diode. This is because although there is an external electrical field applied across the diode, the majority charge carriers still do not get sufficient influence of the external field to cross the depletion region. As we told that the depletion region acts as a potential barrier against the majority charge carriers.

This potential barrier is called as a forward potential barrier. The majority charge carriers start crossing the forward potential barrier only when the value of externally applied voltage across the junction is more than the potential of the forward barrier. For silicon diode, the forward barrier potential is is 0.7 volt and the German for germanium diodes it is is 0.3 volt.

When the externally applied forward voltage across the diode becomes more than the forward barrier potential, the free majority charge carriers start crossing the barrier and contribute the forward diode current. In that situation, the diode would behave as a short circuited path and the forward current gets limited by only externally connected resistors to the diode.

Reverse bias diode

Now let us see what happens if we connect the negative terminal to the voltage source to the p-type side and positive terminal of the voltage source to the n-type side of the diode. At that condition, due to electrostatic attraction of the negative potential of the source, the holes in the p-type region would be shifted more away from the junction leaving more uncovered negative ions at the junction.

In the same way, the free electrons in the n type region would be shifted more away from the junction towards the positive terminal of the voltage source leaving more uncovered positive ions in the junction. As a result of this phenomenon, the depletion region becomes wider. This condition of the diode is called as a reverse biased condition. At that condition, no majority carriers across the junction, and they instead move away from the junction. In this way, a diode blocks the flow of current when it is reverse biased.

There are always same free electrons in the p type semiconductor and some holes in the n-type semiconductor. These opposite charge carriers in a semiconductor are called as minority charge carriers. In the reverse biased condition, the holes find themselves in the n-type side would easily cross the reverse biased depletion region as the field across the depletion region does not present rather it helps minority charge carrier to cross the depletion region.

As a result, there is a tiny current flowing through the diode from positive to the negative side. The amplitude of this current is very small as the number of minority charge carrier in the diode is very small. This current is called the reverse saturation current. 

If the reverse voltage across the diode gets increased beyond the safe value, due to higher electrostatic force and due to higher kinetic energy of minority charge carriers colliding with atoms, a number of covalent bonds get broken to contribute a huge number of free electron hole pair in the diode and the process is cumulative.

The huge number of such generated charge carriers would contribute a huge reverse current in the diode. If this current is not limited by an external resistance connected to the diode circuit, the diode may permanently be destroyed.

Types of diode

1)Zener diode:

Zener diode are widely used as voltage reference and as shunt regulator to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased. A Zener diode conducts when the voltage richest the diode reverse breakdown voltage.

2)P-N junction diode:

A P-N junction diode is a two terminal or two electrodes semiconductor device. Which allow the electric current in only one direction, while block the electric current in opposite or reverse direction. If the diode is forward biased, it allow the circuit electric current flow. On the other hand, if the diode is reverse biased, it block the electric current flow. P-N junction semiconductor diode is also called as a P-N junction diode.

In n type semiconductors, free electrons are the majority charge carrier whereas the p type semiconductors, holes are the majority charge carriers. When the n-type semiconductor is joined with the p type semiconductor, a P-N junction is formed. The p-n junction which is formed when the p-type and n-type semiconductor are joined it is called P-N junction diode.

3)Tunnel diode:

A tunnel diode also known as a esaki diode is a type of semiconductor diode that has effectively negative resistance due to the quantum mechanical effect called tunneling. Tunnel diode have a very heavily doped P-N junction that is about 10 nm wide.

4)Varactor diode:

The diode whose internal capacitance varies with the variation of the reverse voltage such type of diode is known as the varactor diode. It is used for storing the charge. The varactor diode always works in reverse bias and it is a voltage dependent semiconductor device.

5)Schottky diode:

A schottky diode is a metal semiconductor diode with a low forward voltage drop and a very fast switching speed. The schottky diode is another type of semiconductor diode but have the advantage that their forward voltage drop is substantially less than that of the conventional silicon P-N junction diode.

6)Photo-diode:

A photo-diode is a semiconductor device that convert lights into an electrical current. The current is generated when photons are absorbed in the photo-diode. Photo-diodes may contain optical filter, built n lenses, and may have large or small surface areas.

7)PIN diode:

PIN diode is a diode with a wide and intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type region are typically heavily doped because they are used for ohmic contacts.

8)Laser diode:

A laser diode (LD), injection laser diode (ILD), or diode laser is a semiconductor device similar to the light emitting diode. In which the a diode performed directly with electrical current can create lasing conditions at the diodes junction. Laser diode can directly convert electrical energy into light. Driven by voltage, the p-n transition allow for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation in the form of an emitted Photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generate light with the same phase, coherence and wavelength.

9)Avalanche diode: 

An avalanche diode is a special type of semiconductor device designed to operate in reverse breakdown region. Avalanche diodes are used as relief valves (a type of valve used to control the pressure in a system) to protect electrical system from excess voltages.

10)Light emitting diode:

Light emitting diode (LED) is a semiconductor light source that emits light when current flow through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The colour of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor. White light is obtained by using multiple semiconductor or a layer of light emitting phosphor on the semiconductor device.

Resistor color code

Resistor color code

How does the resistor color code work?

Resistors value often indicated with color codes. Practically all leaded resistor with a power rating up to 1 watts are marked with color bands. The coding is defined in the international standard IEC 60062. This standard describes the marking codes for resistors and capacitors. It includes also numerical codes, as for example often used for SMD resistors. The color code is given by several bands. Together they specified the resistance value, the tolerance and sometimes the reliability or failure rate. The number of bands varies from three till 6. As a minimum, two bands indicate the resistance value and one band serves as multiplier. The resistance values are standardized, these values are called preferred value.

Resistor color code chart

The chart below shows how to determine the resistance and tolerance for resistors. The table can also be used to specify the color of the band when the values are known. An automatic resistor calculator can be used to quickly find the resistor values.

Tips for reading resistor code

In the selection below examples are given for different numbers of bands, but first some tips are given to read the color code:

• The reading direction might not always be clear. Sometimes the increased space between band 3 and 4 give away the reading direction. Also, the first band is usually the closest to a lead. A gold or silver band (the tolerance) is always the last band.
• It is a good practice to check the manufacturer's documentation to be sure about the used color coding system. Even better is to measure the resistance with a multi-meter in some cases this might even be the only way to figure out the resistance;

 For example when the color bands are burnt off.

4 band resistor

The four band color code is a most common variation. This resistors have two bands for the resistance value, one multi-meter and one tolerance band.

 In the example on the left these band are green, blue, red and gold. By using the color code chart, One find the green stands for 5 and blue for 6. The value is the 56.100= 5600Ω. The golden band means that the resistor has a tolerance of 5%. The resistance value lies therefore between 5320 and 5880Ω.

 If the tolerance band would be left by blank, the result is a 3 band resistor. This means that the resistance value remains the same, but the tolerance is 20%.

5 band resistor

Resistors with high precision have an extra band to indicate a third significant digit. Therefore, the first three bands indicate the significant digits, the 4th band is the multiply factor and fifth band is represents the tolerance. There are exception to this. For example, sometimes the extra band indicates failure rate (military specification) or temperature coefficient (older or specified specialized resistor)

 Shown an example: brown (1), yellow (4), purple (7), black (x1), green (0.5%) :147Ω0.5%.

6 band resistor

Resistors with 6 band are usually for high-precision resistors that have an additional band to specify the temperature coefficient PPM/K. The most common colour for the 6 band is brown (100 PPM/K). This means that for a temperature change of 10 ˚C, the resistance value can change 0.1%. For special applications where the temperature coefficient is critical other colors.

 Show an example: Orange (3), red (2), brown (1), green (x10), brown (1%), red (50 PPM/K): 3.21 kΩ 1% 50 PPM/K.

Combine Chart for 4 band, 5 band, and 6 band Resistor:

  

Color code explanation exceptions

Reliability band 

Resistors that are produced according to military specifications, sometimes include and extra band to indicate reliability. This is specified in failure rate (%) per 1000 Hours of service. This is rarely used in commercial electronics. Mostly the reliability band can be found on four band resistors. More information about the reliability can be found in the US military handbook MIL-HDBK-199.

Single black band or zero ohm resistor

Resistor with a single black band is called a zero ohm resistor. Principally it is a wire linked with only function of connecting traces on the PCB. Using the resistor package has the advantage of being able to use the same automated machines to place component on the circuit board.

5 band resistor with a 4 band of gold or silver

5 band resistors with 4th band of gold or silver form an exception, and are used on specialized and older resistors. The first two band represent the significant digits, the 3rd the multiple factor, and forth the tolerance and 5th is the temperature coefficient (PPM/K).

Deviating colors

For high voltage resistors often the colors gold and silver are replaced with yellow and grey. This is to prevent having metal particles in the coating.

What is resistor and Its types.

Resistor and Its types

An array of axial-lead resistors:

Electronics symbol:

Type	Passive
Working principle	Electric resistance

 

A resistor is a passive to two-terminal electrical component that implements electrical resistance as a circuit element. 

In electronic circuits ,resistor are used to reduce current flow, adjust signal levels, to divide voltage,bias active elements and terminate transmission line, among other uses. High power resistors that can dissipates many watts  of electrical power as heat, may be used as part of motor controls, in power distribution system or as a test load for generator. Fixed resistors have resistance that only change slightly with temperature, time or operating voltage. Variable resistor can be used to adjust circuit elements ( such as a volume control or a lamp dimmer), or as sensing device for heat, light, humidity, force, or chemical activity.

 Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Practical resistors as discrete component can be composed of various compounds and forms. Resistors are also implemented within an integrated circuit.

Theory of operation with symbol of resistor

The ohm is obtained represented by the Omega (Ω) symbol: The symbol of resistance is a zigzag line as shown in below.

 The letter R is used in equations.

Ohm's law(Ω):

                    The behavior of an ideal resistor is dictated by the relationship specified by ohm's law.
                          V=I.R.
ohm's law states that the voltage (V) across a resistor is proportional to the current (I) where the constant of proportionality is the resistance (R). For example, if 300Ω resistor is attached across the terminal of 12 volt battery, then the current of 12/30=0.04 amperes flows through that resistor.

practical resistors also have some inductance and capacitance which affect the relationship between voltage and current In alternating current circuits.
Ohms(Ω) is the SI unit of electrical resistance, named after George Simon ohm. An ohms is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived unit of mΩ(1 mΩ = 10⁻³ Ω), kiloΩ (1 kΩ = 10³ Ω) and MegaΩ (1 MΩ = 10⁶ Ω) are also in common usages.

Serial and parallel resistors:

The total resistance of resistors connected in series in the sum of their individual resistance values.

                                        
The total resistance of resistor connected in parallel in the reciprocal of the sum of the reciprocals of the individual resistors.

                                           

For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor and a 15 Om resistor produce  1/1/10 + 1/5 + 1/15  ohms of resistance, or 30/11= 2.727 ohms.

Resistor network that is a combination of parallel and series connection can be broken up into smaller part of that are either one or the other. Some complex network of register cannot be resolved in this manner, requiring more sophisticated circuit analysis. generally the Y-∆ transform or Matrix method can be used to solve such problems.

Power dissipation:

At any instant the power P(watt) consumed by a resistor of resistance R ohms is calculated as:  where V(volts) is the voltage across the registers and I (ampere) is the current flowing through it. using ohm's law, the two other forms can be derived. this power is converted into heat which must be dissipated by the registers package before its temperature rises excessively.

Resistors are rated according to their maximum power dissipation. They usually absorb much less than a watt of electrical power and require little attention to their power rating.

Resistors required to dissipate substantial amount of power, particularly used in power supplies, power conversion circuits, and power amplifier, are generally referred to as a power resistors; the designation is loosely applied to resistors with power rating of 1 watt or greater. power registers are physically larger and may not use the preferred values, color code and external packages described below.

If the average power dissipated by a resistor is more than power rating,damage  to the resistor may occur, permanently altering its resistance; this is distinct from the reversible change in resistance due to its temperature coefficient when it warms. Excessive power dissipation may rise the temperature of the resistor to a point where it can burn the circuit board or adjacent component or even cause a fire. There are flameproof resistors that fail (open circuit) before they overheat dangerously.
 since poor air calculation, high altitude, or high operating temperature may occur resistors may be specified with higher rated dissipation than is experienced in service.
All resistors have a maximum voltage rating. this may limit the power dissipation for higher resistance values.

Types of resistors

1. Carbon composition resistor
2. Thermistor
3. Wire wound resistor
4. Metal film resistor
5. Carbon film resistor
6. Variable resistor
7. Varistor
8. Light dependent resistor

Carbon composition resistor

A carbon composition resistor is a very commonly used resistor. These resistors are low cost and easy to construct.
Carbon resistors are mainly made of carbon clay composition covered with a plastic case. The lead of resistor is made of tinned copper.

 The main advantage of this resistor are that they are readily available, low cost, and they are very durable. these resistors are also available in a wide range of values, from as low as 1 ohms to as high as 22 megaohms. For this reasons, carbon composition resistor are often included in many of the best Arduino starter kits.

 The main disadvantage of carbon composition resistor is that they are very temperature-sensitive. The tolerance range in resistance of carbon composition resistor is 5 to 20%. Although this is not a concern for the majority of Electronics project one would experiment with at home.

This type of register has a tendency to produce some electric noise due to the passage of electrical current from one carbon practical to others. where low cost is the main criterion for designing a circuit rather than its for perfection of performance, these resistors are normally used. carbon register are provided with a different colored band on their cylindrical body. these color band are code for the resistance value of resistors along with their tolerance range.

Thermistor

The word thermistor is a thermal resistor. Its resistance value change with the change in the temperature. Most thermistors have a negative temperature coefficient which means its resistance will fall down when the temperature increases. These are normally made of semiconductor materials A resistance upto a few megaohms can be obtained from thermistors. they are used to detect a small temperature change, when there is a temperature change, however small, there will be a large change in the value of the resistance.

Wire wound resistor

In wire wound resistor a wire is manganin constantan is wound around a cylinder of insulating material. the temperature coefficient of resistance of manganin and constantan is almost zero. So resistance variation with temperature of these resistor is negligible. the wounded wire is covered with an insulating cover such as baked enamel. This cover of insulating heat resistible material resist the effect of ambient temperature variation. different sizes and ratings of wire wound Resistors can easily be achieved by using different length and diameters of the wire.

these resistors are easily available for wide range of rating. This range of resistance values vary from 1 ohm to 1 mega ohm. The typical tolerance limit of these Resistors varies from 0.01% to 1%. They can be used for high power applications of 5 to 200W dissipation ratings. The cost of these resistor is much higher than carbon registers. Normally a wire wound resistor is used to where a carbon composition resistor cannot meet the purpose because of its limitations.

The main disadvantage of these resistors is the inductance that arises because of its coil like structure. at high frequency, the behavior of the circuit may be changed due to its reaction. This problem can be solved if one half of the wire is wound in one direction and another half in the opposite direction so that the inductance due to this two halves cancel each other hence the net inductive effect of the resistor becomes Nil. The non inductive wire wound resistor is ideal for high frequency circuit but it is costlier than an ordinary one.

Metal film resistor and carbon film resistor

The resistor is constructed by means of deposition a thin film of a conductive material such as pure carbon or metal on to an insulating core. The desired value of resistance of metal film resistor or carbon film resistor can easily be obtained by either trimming the layer of the thickness or by cutting helical grooves of suitable pitch along the its length.

Metallic contact cap is fitted at both end of the resistor. The caps are in contact with the conductive film or helical grooves. the lead wire is welded to the end caps. Metal film resistor or carbon film resistor can be made up to a value of 10000 mega ohms and size of this type of Resistor is much smaller than a wire wound resistor. because of their constructional features, these register are only non inductive.

The accuracy level of metal film resistor can be of order 1% and they are suitable for high grade applications. carbon film resistor give lower tolerances and smaller values of electrical resistance than those available with metal film. however the carbon film posses a mildly negative temperature coefficient of resistance which is very useful for certain electronic circuits.

Variable Resistor

The variable resistor means its resistance value can be adjusted similar to a potentiometer. There are a rotating shaft and a wiping contact. Basically, there is a resistive semicircular bar or coil and by wiping the contact we change the effective length of the resistive element and hence the resistance gets changed. One example of such Resistors is rheostat.

The variable resistor or rheostat can also be a linear sliding type where the sliding contact move on the resistor element linearly for adjustment of the effective resistance of the resistor.

Nonlinear resistor or Varistor

They are also known as varistors. They are popular for having a non-linear V-I characteristic curve. That is its resistance is not uniform and it does not obey ohm's law. They are made of materials such as silicon carbides, zinc oxide.
There are three types of varistors:

1.         Silicon carbide disc type varistor 
2.         zinc oxide type Varistor
3.         Silicon carbide rod type varistor

Light dependent resistor

Light dependent resistor or LDR will vary in resistance depending on the intensity of light falling on it. This is made of cadmium sulphide which contain a smaller number of electrons when it is not illuminated. when a light rays falls on it, electrons get ejected and hence the conductivity of it increases. Hence it offers low resistance when light falls on it and offers high resistance in the dark.