What is diode ?
A diode can be discussed as a two-terminal electronic device that only conducts current in one direction (so long as it is working within a required voltage level). An ideal diode always has zero resistance when it is forward biased or one direction, and infinite resistance in the reverse bias.
basically , all the electronic devices require DC energy supply but it is nearly impossible to produce DC power so, we need an alternative to get some DC energy that’s why the usage of diodes comes into the mind to transfer AC power to DC power. A diode is a small electronic component mostly used in almost all the circuits to enable the flow of current in only one direction (unidirectional device). We can easily say that the diode was the reason to build the electronic components using semiconductor materials. Few applications of diodes are rectification, amplification, electronic switch, conversion of electrical energy into light energy, and light energy into electrical energy.
History of Diode:
In the year 1940 at Bell Labs, Russell Ohl was working with a silicon crystal to find out its properties. One day accidentally when the crystal of silicon which has a crack in it was exposed to the light, he found the movement of current through the silicon crystal and that was called diode, which was the starting of the semiconductor era.
Although in the actual world, diodes never achieve zero or infinite resistance. Instead, a diode will have very small resistance in uni or one direction (to allow current flow), and very large resistance in the opposite or reverse direction (to block current flow). A diode is effectively like a valve for an electrical circuit.
Semiconductor diodes are the most basic type of diode. These diodes start conducting current only if some threshold voltage is present in the forward direction (i.e. the “low resistance” direction). The diode is said to be “forward biased” when conducting current in a direction. When connected within a circuit in the reverse direction (i.e. the “high resistance” direction), the diode is said to be “reverse biased”.
When the voltage of the circuit is greater than the reverse breakdown voltage, the diode will ready to conduct electricity in the reverse direction (i.e. the “high resistance” direction). This is why we say diodes have much high resistance in the reverse direction – not an infinite resistance.
A PN junction is the basic or simplest form of the semiconductor diode. ideally, this PN junction behaves as a short circuit (zero resistance ) when it is forward biassed, and as an open circuit (infinity resistance) when it is in the reverse biased. The name diode is derived from “di–ode” which describes a device that has two electrodes. Diodes are commonly used in thousands of electronics projects and are included in many of the Arduino starter kits.
Construction of Diode:
Solid materials are generally classified into three types namely conductors, insulators and semi-conductors. Conductors have a maximum number of free electrons, Insulators have a minimum number of free electrons (negligible such that flow of current is not at all possible) whereas semi-conductors can be either conductors or insulators depending upon the potential applied to it. Semi-conductors that are in common use are Silicon and Germanium. Silicon is preferred because it is abundantly available on the earth and it gives a better thermal range.
Semi-conductors are further classified into two types as Intrinsic and Extrinsic semiconductors.
These are also called as pure semi-conductors where charge carriers (electrons and holes) are in equal quantity at the room temperature. So current conduction takes place by both holes and electrons equally.
In order to increase the number of holes or electrons in a material, we go for extrinsic semi-conductors where impurities (other than silicon and germanium or simply trivalent or pentavalent materials) are added to the silicon. This process of adding impurities to the pure semi-conductors is called as Doping.
Formation of P and N-type semiconductors:
If pentavalent elements (number of valence electrons are five) are added to the Si or Ge then there are free electrons are available. As the electrons (negatively charged carriers) are more in number these are called as N-type semiconductor. In N-type, semi-conductor electrons are majority charge carriers and holes are minority charge carriers.
Few pentavalent elements are Phosphorous, Arsenic, Antimony, and Bismuth. Since these have excess valance electron and are ready to pair with the external positively charged particle these elements are called as Donors.
Similarly, if trivalent elements like Boron, Aluminium, Indium, and Gallium are added to Si or Ge, a hole is created because a number of valence electrons in it are three. Since a hole is ready to accept an electron and get paired it is called as Acceptors. As the number of holes are excess in newly formed material these are called as P-type semiconductors. In P-type semi-conductor holes are majority charge carriers and electrons are minority charge carriers.
Working Principle of Diode
A diode’s working phenomena depend on the interaction of n-type and p-type semiconductors. An n-type semiconductor has huge number of free electrons and very little numbers of holes. In simple words, we can say that the concentration of free electrons is a hug, and holes are very few in an n-type semiconductor.
Free electrons in the n-type semiconductor are said to as majority charge carriers, and holes in the n-type semiconductor are referred to as minority charge carriers.
A p-type semiconductor is a reverse of N-type semiconductor it has a huge concentration of holes and a small concentration of free electrons. Holes in the p-type are majority charge carriers, and free electrons in the p-type are minority charge carriers.
Now let us observe what results when one n-type region and one p-type region come in contact. Here due to concentration differences, majority carriers diffuse from one side diode to another side of the diode. As the concentration of holes is huge in the p-type area and it is small in the n-type region, the holes start diffusing from the p-type semiconductor region to the n-type semiconductor region.
Again the concentration of free electrons is high in the n-type semiconductor region and it is very low in the p-type region and due to this reason, free electrons begin diffusing from the n-type region to the p-type region.
The free electrons diffusing into the p-type semiconductor region from the n-type region would recombine with holes available there and create uncovered negative ions in the p-type area. 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(electrons) in the p-type side and a layer of positive ions (holes) in the n-type region appear along the junction line of these two types of semiconductors. The layers of uncovered positive ions and uncovered negative ions form a region in the exact middle of the semiconductor diode where no charge carrier exists since all the charge carriers get recombined here in this region. Due to the short or lack or missing of charge carriers, this region is called the depletion region.
After the formation of the depletion region, there is no more diffusion of charge carriers from both sides of the diode. This is because the electric field that appeared across the depletion region will stop further moving of charge carriers from one side to another.
The potential of the layer of uncovered positive ions in the n-type side would repeal or force the holes in the p-type side and the potential of the layer of uncovered negative ions in the p-type side would repeal and force the free electrons in the n-type side. That means a potential barrier is created across the junction to stops further diffusion of charge carriers.
P-N Junction Theory:
We have seen how a diode is made with P-type and N-type semi-conductors but we need to know what happens inside it to form such a unique property of allowing current in the only single direction and what happens at the exact point of contact initially at its junction.
Initially, when both the materials are joined together (without any external voltage applied) the excess electrons in the N-type and excess holes in the P-type will get attracted to each other and gets recombined where the formation of immobile ions (Donor ion and Acceptor ion) takes place as shown in below picture. These immobile ions resists the flow of electrons or holes through it which now acts as a barrier in between the two materials (formation of barrier means the immobile ions diffuses into P and N regions). The barrier which is now formed is called as Depletion region. The width of the depletion region in this case depends upon the doping concentration in the materials.
If the doping concentration is equal in both the materials then the immobile ions diffuse into both the P and N materials equally.
What if the doping concentration differs with each other?
Well, if the doping differs the width of depletion region also differs. Its diffusion is more into the lightly doped region and less into the heavily doped region.
Now let’s see the behavior and characteristics of the semiconductor diode when the proper voltage is applied.
The potential of the layer of uncovered positive ions in the n-type side would start to repeal the holes in the p-type side and the potential of the layer of uncovered negative ions in the p-type side would repeal and force the free electrons in the n-type side. That means a potential barrier is created across the junction to prevent or stops further diffusion of charge carriers.
Forward Biased Diode
Now let’s observe what happens if a positive terminal of a battery is connected to the p-type region of the diode and the negative terminal of the source is connected to the n-type region of the diode and if we gradually increase the voltage from zero.
At the start, there is zero current flow through the diode. This is due to there is an externally applied electrical field across the diode, the majority charge carriers still do not have enough influence of the external field to move across the depletion region. As we know that the depletion region behaves as a potential barrier against the majority charge carriers.
This potential barrier is known as a forward potential barrier. The majority charge carriers begin crossing the forward potential barrier if and only if when the value of externally applied voltage across the junction is greater than the potential of the forward barrier. For silicon Si diodes, the forward barrier potential is 0.7 volt at normal temperature and for germanium diodes, it is 0.3 volt.
When the externally supplied forward voltage across the PN junction diode becomes greater than the forward barrier potential, the free majority charge carriers begin crossing the barrier and contribute the forward diode current. In that situation, the diode would act as a short-circuited or zero resistance path, and the forward current can be limited by only externally connected resistors to the diode.
Reverse Biased Diode
Now let us see what results occur if we connect the negative terminal of the battery to the p-type of the diode side and the positive terminal of the battery to the n-type side of the diode. In that situation, due to electrostatic attraction of the negative potential of the battery, the holes in the p-type side would be moving away from the junction producing more uncovered negative ions at the junction.
In the same pattern, the free electrons in the n-type side would be moved more away from the junction towards the positive terminal of the battery producing more uncovered positive ions in the junction.
As a result of this phenomenon, the depletion region will be wider. This condition of a PN junction diode is called the reverse biased condition. At that condition, no majority carriers move a crosses the junction, and they instead move away from the junction. In this pattern, a diode blocks and resist the flow of charges when it is reverse biased.
As we already told at the start of this blog that there are always some free electrons in the p-type material and some holes in the n-type material. These opposite charge carriers in a semiconductor are known as minority charge carriers.
In the reverse-biased situation, the holes found in the n-type side will easily cross the reverse-biased depletion region of the diode as the field across the depletion region does not appear rather it helps minority charge carriers to cross the depletion region.
As a result, there is a much small current flowing by the diode from positive to the negative region. The amplitude of this current is much small. This current is called reverse saturation current.
If the reverse voltage across a diode becomes increased beyond a safe point, because of the higher electrostatic force and due to greater kinetic energy of minority charge carriers colliding with atoms, a number of covalent bonds are broken to contribute a large number of free electron-hole pairs in the PN junction diode and the process is cumulative.
A large number of such produced charge carriers would contribute a huge reverse current in the PN junction diode. If this dangerous current is not limited with the external resistance connected to the diode circuit, the diode may be destroyed.
Forward Characteristics of P-N Junction:
When the positive terminal of the voltage source is connected to P-type of the diode and the negative terminal is connected to the N-type of the diode is called forward bias of P-N junction is shown figure below.
If value of this external voltage source becomes more than the value of the potential barrier, nearly 0.7 volts for silicon and 0.3V for Ge, the potential barrier is crossed and the current begins flowing because of to movement of electrons across the junction and same for the holes.
Reverse Characteristics of P-N Junction:
When a positive terminal of the voltage source is given to the N-type side of diode and negative terminal of the voltage source to the P-type of the diode, it is said to be in reverse bias condition.
P-N Junction Reverse Characteristics Circuit
When a positive terminal of voltage is connected to the N-type of the diode, the electrons move towards the positive electrode and the negative terminal of the voltage source connected with the P-type of diode makes the holes move towards the negative electrode. As a result, the electrons cross the junction to combine with the holes in the opposite side of the junction and vice versa. As a result, a depletion layer is born, having a hug impedance path with a large potential barrier.
Different Types of Diodes:
There are a large number of diodes whose construction is the same but the type of material used are different. For example, if we assume a Light Emitting diode it is made using Aluminium, Gallium and Arsenide materials which when excited releases energy in the form of photons. Similarly, changing in diode’s properties like internal capacitance, threshold voltage etc are considered and a particular diode is designed based on those.
Here we have explained various types of diodes with their working, symbol, and applications:
- Zener diode
- LASER diode
- Varactor diode
- Schottky diode
- Tunnel diode
- PIN diode etc.
Let’s see the working principle and construction of these devices briefly.
Zener diode is a special type of Diode that allows the flow of charges in the forward direction the same as the rectifier diode but at the same time, it can permit the opposite or reverse flow of current also when the voltage is greater than the breakdown value of the Zener diode. This is mostly one to two volts greater than the rated voltage of the Zener and is also called as the Zener voltage or Avalanche point. The Zener was named, Clarence Zener who discovered the most of electrical properties of the Zener diode. Zener diodes find hug applications in voltage regulation and to protect electronic devices from voltage fluctuations. Zener diodes are mostly used as voltage references and as shunt regulators to regulate the voltage across all types of circuits.
The Zener diode uses its PN junction in reverse bias condition to work as a Zener Effect. During the Zener effect or Zener breakdown, the Zener holds the voltage near to a constant value called the Zener voltage. The conventional diode has the same property of reverse bias, but if the reverse bias voltage is exceeded to the rated value, the diode will be subjected to the flow of high current and it may be destroyed. The Zener diode, on the opposite side, is specially manufactured to have a small breakdown voltage known Zener voltage. The Zener diode also exhibits the special property of a controlled breakdown and allows the current to keep the voltage across the Zener diode near to the breakdown voltage.
For example, a 10 volt Zener will drop 10 volts across a wide range of reverse currents.
When the Zener diode is the reverse biased mode, its PN junction will experience an Avalanche breakdown and the Zener diode start conducts in the reverse direction. Under the influence of the applied electric field, the valance electrons will be accelerated to knock and release other electrons. This ends in the Avalanche effect. When this occurs, a small change in the voltage will result in a large current flow. The Zener break down depends on the applied electric field as well as the thickness of the layer on which the voltage is applied.
The Zener diode requires a current limiting resistor in series to it to restrict the current flow through the Zener. mostly the Zener current is fixed as 5 mA.
For example, if a 10 V Zener is used with a 12 volt supply, a 400 Ohms (Near value is 470 Ohms) is ideal to keep the Zener current as 5 mA. If the power supply is 12 volts, there are 10 volts across the Zener diode and 2 volts across the series resistor. With 2 volts across the 400 ohms resistor, then the current through the series resistor and Zener diode will be 5 mA. So as a rule 220 Ohms to 1K resistors are used in series with the Zener diode depending upon the source voltage. If the current through the Zener diode is insufficient, the output voltage will be unregulated and less than the nominal breakdown voltage.
The following formula is useful to determine the current through the Zener:
Zener = (VIn – V Out) / R Ohms
Two conditions of Resistor R must be satisfied
It always must be a low value to permit sufficient current through the Zener diode.
The power rating of the resistor always must be very high enough to protect the Zener diode.
Light Emitting Diode (LED):
construction of LED is similar to a simple diode but various combinations of semiconductors are used to produce different colors. It works in forward biased mode. When the electron-hole recombination takes place a resultant photon or light energy is released, if the forward voltage is furthermore increased more photons will be released and light intensity also more increases but the voltage should not exceed its threshold value else the LED diode gets damaged.
To generate different colors, the combinations are used AlGaAs (Aluminium Gallium Arsenide) – red and infrared, GaP (Gallium Phosphide) – yellow and green, InGaN (Indium Gallium Nitride) – blue and ultra-violet LEDs etc. Check a Simple LED circuit here.
LED: A cut at the base of the LED represents cathode.
For an IR LED we can see its light through a camera.
LASER stands for Light Amplification by Stimulated Emission of Radiation. A P-N junction is formed by two layers of doped Gallium Arsenide where a High reflective coating is applied to one end of the junction and a partially reflective coating at the other end. When the diode is forward biased similar to LED it releases photons, these hit other atoms such that photons will be released excessively when a photon hits the reflective coating and strikes back the junction again more photons releases, this process repeats and a high-intensity beam of light is released in only one direction. The laser diode needs a Driver circuit to work exactly as we want.
In a photodiode, the current through it depends upon the photon or light energy falls on the P-N junction. It is always operated in reverse bias. As discussed earlier, little leakage current flows through a diode when diode is reverse biased which is here called dark current. As the current is due to lack of light (darkness) it is called so. This diode is constructed in such a way that when light strikes the junction it is enough to break the electron-hole pairs and generate electrons which increases the reverse leakage current. Here you can check Photodiode working with IR LED.
It is also known as Varicap (variable capacitor) diode. It always operates in reverse biased mode. The general definition of a capacitor is that it is an electronic component which stores the energy it has separation of conducting plate with an insulator when an ordinary diode is reverse biased the width of the depletion region continuously increases, as the depletion region represents an insulator or a dielectric it can now behave as a capacitor. The changing of reverse voltage causes separation of P and N regions to vary this leads the diode to work as a variable capacitor.
Since capacitance can be increase with a decreasing in distance between the two plates, so it means large reverse voltage produces low capacitance and vice-versa.
An N-type semiconductor is combined to the metal (gold, silver) such that large energy level electrons present in the diode these are named as hot carriers so this diode is also known as a hot-carrier diode. It does not have minority carriers and zero depletion region exists rather a metal semi-conductor junction is present when this diode is forward biased mode it behaves as a conductor but the charge has very high energy levels which are very usefull in fast switching particularly in digital circuits these are also used in microwave applications.
The P-type and N-type regions in this diode are heavily doped such the presence of the depletion layer is very narrow. It exhibits a negative resistance region which is very useful and almost it used in an oscillator and microwave amplifiers. When this diode is forward biased initially, since the depletion region is much small the electrons tunnel through it, the current increases rapidly with a little change in voltage. When the voltage is further increased, due to the large electrons at the junction, the width of the depletion region begins to increase because the blockage of forwarding current (where the negative resistance region forms) when the forward voltage is furthermore increased it acts as a ordanary diode.
In this type of diode, the P-type and N-type regions are separated by an intrinsic semiconductor materail. When the diode is reverse biased it behaves as a constantly valued capacitor. In forward bias condition, it behaves as a variable resistance that is controlled by current. normally it is used in microwave applications which are to be controlled by DC source.
Fast Recovery Diode
The Fast Recovery Diode is a new type of semiconductor diode. This diode has very good switching properties and a very short reverse recovery time and is normally used as a rectifier diode in high frequency switching power supplies.
The fast recovery semiconductor diode is characterized by its very small recovery time, which makes it suitable for high frequency (such as line frequency in TV) rectification. The fast recovery diode has the most important characteristic that determines its performance – the reverse recovery time. The reverse recovery time is explained as the diode transitions from the forward conduction position to the off state, begins from the output pulse falling to the 0 line. The time needed until the reverse power supply backs to 10% of the high reverse current.
Ultra-fast recovery diodes (SRDs) are developed on the basis of fast recovery diodes, the important difference is that the reverse recovery time is shorter. The reverse recovery time of an ordinary fast recovery diode is several hundred nanoseconds, and the reverse recovery time of an ultra-fast recovery diode (SRD) is mostly several tens of nanoseconds. The smaller the value, the larger the operating frequency of the fast recovery diode.
When the operating frequency is in the range of 10 to 100 k Hz, the normal rectifier diodes change the forward and reverse voltages slower than the reverse recovery time, and the normal rectifier diodes do not normally perform the unidirectional conduction of current and perform the rectification operation. At this time, it is must use a fast recovery rectifier diode to be competent. therefore, a rectifier semiconductor diode that is powered by a switching power supply like; a color TV is normally a fast recovery diode and does not be replaced by an ordinary rectifier diode. Otherwise, the appliance may not work we need.
Transient Voltage Suppression Diode
The transient voltage suppression diode is abbreviated as T V P tube (transient-voltage-suppressor). It is a semiconductor device made on the basis of the process of the Zener diode and is normally used in a fast overvoltage protection circuit for voltage source. It can be widely used in PC, electronic instruments, tele equipment, household appliances, and on-board or marine and automotive electronic equipment for field operations, and can be easily used as a protection element for over-voltage shock caused by mane made operation or an electric shock to equipment.
Transient voltage suppression diodes can be distributed into four categories according to their peak pulse power: 50 () w, 1000 W, 1500 W, 5000 w. Each class is divided into many types according to its ordinary voltage.
When the voltage at both ends is larger than the required value, the transient voltage suppression diode will turn on instantaneously, and the resistance at both ends will change from high resistance to low resistance at high speed, thereby absorbing a very huge current and the voltage across the tube. Clamp at a predetermined value.
The avalanche diode is normally a microwave power device mode on the basis of the Zener process technology, which can produce high-frequency oscillation under the act of an applied voltage.
basically The avalanche diode uses an avalanche breakdown to inject carriers into the crystal. Since the carrier takes some time to pass through the semiconductor wafer, its current lags behind the voltage and a delay time produce. If the transit time is properly controlled, then the current is A negative resistance effect that occurs in the voltage relationship, producing high-frequency oscillation. It is normally used in microwave communications, radar, remote control, telemetry, instrumentation, and other equipment.
The bidirectional trigger diode is also known as a two-terminal AC device (DIAC). It is a silicon bidirectional voltage-triggered switching device. When the voltage applied across the DIAC exceeds its breakdown voltage, both ends are turned on, and flowing of current will continue until the current is interrupted or dropped to a little holding current of the device. Turn it off again. DIAC are commonly used in overvoltage protection circuits, phase shifting circuits, thyristor trigger circuits, and timing circuits.
One-way Half-wave Rectifier Circuit
The diode is an automatic switch. When u2 is positive the half cycle, the power supply is automatically connected to the load. When u2 is a negative half cycle, the power supply and load are automatically cut off due to the reverse baise property of a diode. Since this circuit produces output only in the positive half cycle of u2, it is known as a half-wave rectification circuit. If the polarity of the rectifier diode is changed or make opposite, a negative DC ripple voltage can be obtained.
Full Wave Rectifier Circuit
Set the voltage on the secondary part of the transformer to:
1) When u2 is a positive half cycle, the potential at point A is the highest, the potential at point V is the lowest, diodes V1 and V3 are turned on, V2 and V4 are turned off, and the current path is A→V1→RL→V3→B.
2) When u2 is a negative half cycle, the potential at point B is the highest, the potential at point A is the lowest, diodes V2 and V4 are on, V1 and V3 are off, and the current path is B→V2→RL→V4→A.
It can be seen that in a period of change of u2, the current from top to bottom always flows through the load RL, and the waveform of both current and voltage is a full-wave pulsating DC voltage and current.
Identification and Detection of Diode
1. Diode Identification
The identification of the diode is simple: the negative pole of the small-power diode is normally marked with a color ring on the surface; few diodes also use the “P” and “N” symbols to find the polarity of the diode, “P” for the positive pole and “N” for the negative pole. Metal-encapsulated diodes mostly have a diode symbol printed on the surface with the same polarity; LEDs normally use the hight of the pins to detect the positive and negative poles, the long legs are positive, and the short legs are negative.
The surface of the bridge rectifier is mostly marked with the internal circuit structure or the name of the AC input and the DC output. The AC input is normally indicated by “AC” or “~”; the DC output is normally indicated by the “+” and “~” symbols.
because of the variety of shapes of the chip diodes, the polarity is also marked by a variety of ways: in the leaded chip diode, the end of the tube with a white color ring is the negative electrode; in the chip diode with lead and colorless ring The longer end of the lead terminal is the positive electrode; in the leadless chip diode, the end of the ribbon or the notched end is the negative electrode.
Main Parameters of Diode
A specification used to detect the performance and range of a diode, known as the diode’s parameters. Different types of diodes have different characteristic parameters. For beginners, you should understand the following main parameters:
1. Maximum Rectifier Current (IF)
It refers to the maximum forward average current value that the diode is allowed to pass during long-term continuous operation. The value is connected to the PN junction region and external heat dissipation conditions. When the current flows through the tube, the die heats up, and the temperature increases. When the temperature allowable limit exceeds (about 141 for the silicon tube and about 90 for the manifold), the die is overheated and destroyed. Therefore, under the particular heat dissipation conditions, the diode should not exceed the maximum rectified current value of the diode. For example, the normally used IN4001-4007 type germanium diodes have a rated forward operating current of 1A.
2. Highest Reverse Working Voltage (Udrm)
When the reverse voltage applied across the terminal of the diode is high enough, the tube may break down and loses its unidirectional conductivity. In order to ensure safe use, the highest reverse operating voltage value is defined. For example, the IN4001 diode has a reverse withstand voltage of 50V and the IN4007 has a reverse withstand voltage of 1000V.
3. Reverse Current (Idrm)
Reverse current refers to the reverse current flowing through the diode under room temperature (25 ° C) and the largest reverse voltage. The less the reverse current, the better the unidirectional conductivity of the tube. It is worth noting that the reverse current has a near relationship with the temperature, and the reverse current is doubled for every 10 °C enhance in temperature. For example, the 2AP1 type germanium diode has a reverse current of 250uA at 25°C, a rise in temperature to 35°C, a reverse current of 500uA, and so on. At 75°C, its reverse current has reached 8mA. Not only it can lose its unidirectional conductivity, but it also overheats and damages the tube. Another example is the 2CP10 silicon diode, which has a reverse current of the only 5uA at 25°C and a reverse current of 160uA when the temperature rises to 75°C. Therefore, silicon diodes perform better stability at high temperatures as compare to the germanium diodes.
4. Dynamic Resistance (Rd)
The ratio of the change in voltage closer to the static operating point Q of the diode characteristic curve to the amount of change in the corresponding current.
Watch Video to understand this topic
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