KingAnupamDutta: 10/09/16

Diode

What Is a Diode?

Answer:- A diode is a specialized electronic component with two electrodes called the anode and the cathode. Most diodes are made with semiconductor materials such as silicon, germanium, or selenium. Some diodes are comprised of metal electrodes in a chamber evacuated or filled with a pure elemental gas at low pressure. Diodes can be used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulatores, and oscillators.

Description :- a diode is a two-terminal electronic component that conducts primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance to the flow of current in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. A vacuum tube diode has two electrodes, a plate (anode) and a heated cathode. Semiconductor diodes were the first semiconductor electronic devices. The discovery of crystals' rectifying abilities was made by German physicist Ferdinand Braun in 1874. The first semiconductor diodes, called cat's whisker diodes, developed around 1906, were made of mineral crystals such as galena. Today, most diodes are made of silicon, but other semiconductors such as selenium or germanium are sometimes used.

Which way does power flow through a diode?

Answer:- The diode will not conduct with a reverse voltage (a reverse bias) applied. Once the voltage is applied in the forward direction (positive to anode and negative to cathode) again, current will flow; in this case as the voltage is increased more current flows.

Once you graduate from the simple, passive components that are resistors, capacitors, and inductors, it’s time to step on up to the wonderful world of semiconductors. One of the most widely used semiconductor components is the diode.

Ideal Diodes

The key function of an ideal diode is to control the direction of current-flow. Current passing through a diode can only go in one direction, called the forward direction. Current trying to flow the reverse direction is blocked. They’re like the one-way valve of electronics.

If the voltage across a diode is negative, no current can flow*, and the ideal diode looks like an open circuit. In such a situation, the diode is said to be off or reverse biased.

As long as the voltage across the diode isn’t negative, it’ll “turn on” and conduct current. Ideally* a diode would act like a short circuit (0V across it) if it was conducting current. When a diode is conducting current it’s forward biased(electronics jargon for “on”).

The current-voltage relationship of an ideal diode. Any negative voltage produces zero current – an open circuit. As long as the voltage is non-negative the diode looks like a short circuit.

Ideal Diode Characteristics
Operation Mode	On (Forward biased)	Off (Reverse biased)
Current Through	I>0	I=0
Voltage Across	V=0	V<0
Diode looks like	Short circuit	*Open circuit*

Circuit Symbol of A Diode

Every diode has two terminals – connections on each end of the component – and those terminals are polarized, meaning the two terminals are distinctly different. It’s important not to mix the connections on a diode up. The positive end of a diode is called the anode, and the negative end is called the cathode. Current can flow from the anode end to the cathode, but not the other direction. If you forget which way current flows through a diode, try to remember the mnemonic ACID: “anode current in diode” (also anode cathode is diode).

The circuit symbol of a standard diode is a triangle butting up against a line. As we’ll cover in the later in this tutorial, there are a variety of diode types, but usually their circuit symbol will look something like this:

Current-Voltage Relationship For Diodes

The most important diode characteristic is its current-voltage (i-v) relationship. This defines what the current running through a component is, given what voltage is measured across it. Resistors, for example, have a simple, linear i-vrelationship…Ohm’s Law. The i-v curve of a diode, though, is entirely non-linear. It looks something like this:

The current-voltage relationship of a diode. In order to exaggerate a few important points on the plot, the scales in both the positive and negative halves are not equal.

Forward bias: When the voltage across the diode is positive the diode is “on” and current can run through. The voltage should be greater than the forward voltage (VF) in order for the current to be anything significant.
Reverse bias: This is the “off” mode of the diode, where the voltage is less than VF but greater than -VBR. In this mode current flow is (mostly) blocked, and the diode is off. A very small amount of current (on the order of nA) – called reverse saturation current – is able to flow in reverse through the diode.
Breakdown: When the voltage applied across the diode is very large and negative, lots of current will be able to flow in the reverse direction, from cathode to anode.

Types of Diodes

Light Emitting Diode (LED): It is one of the most popular type of diodes and when this diode permits the transfer of electric current between the electrodes, light is produced. In most of the diodes, the light (infrared) cannot be seen as they are at frequencies that do not permit visibility. When the diode is switched on or forward biased, the electrons recombine with the holes and release energy in the form of light (electroluminescence). The color of light depends on the energy gap of the semiconductor.
Avalanche Diode: This type of diode operates in the reverse bias, and used avalanche effect for its operation. The avalanche breakdown takes place across the entire PN junction, when the voltage drop is constant and is independent of current. Generally, the avalanche diode is used for photo-detection, wherein high levels of sensitivity can be obtained by the avalanche process.
Laser Diode: This type of diode is different from the LED type, as it produces coherent light. These diodes find their application in DVD and CD drives, laser pointers, etc. Laser diodes are more expensive than LEDs. However, they are cheaper than other forms of laser generators. Moreover, these laser diodes have limited life.
Schottky Diodes: These diodes feature lower forward voltage drop as compared to the ordinary silicon PN junction diodes. The voltage drop may be somewhere between 0.15 and 0.4 volts at low currents, as compared to the 0.6 volts for a silicon diode. In order to achieve this performance, these diodes are constructed differently from normal diodes, with metal to semiconductor contact. Schottky diodes are used in RF applications, rectifier applications and clamping diodes.
Zener diode: This type of diode provides a stable reference voltage, thus is a very useful type and is used in vast quantities. The diode runs in reverse bias, and breaks down on the arrival of a certain voltage. A stable voltage is produced, if the current through the resistor is limited. In power supplies, these diodes are widely used to provide a reference voltage.
Photodiode: Photodiodes are used to detect light and feature wide, transparent junctions. Generally, these diodes operate in reverse bias, wherein even small amounts of current flow, resulting from the light, can be detected with ease. Photodiodes can also be used to generate electricity, used as solar cells and even in photometry.
Varicap Diode or Varactor Diode: This type of diode feature a reverse bias placed upon it, which varies the width of the depletion layer as per the voltage placed across the diode. This diode acts as a capacitor and capacitor plates are formed by the extent of conduction regions and the depletion region as the insulating dielectric. By altering the bias on the diode, the width of the depletion region changes, thereby varying the capacitance.
Rectifier Diode: These diodes are used to rectify alternating power inputs in power supplies. They can rectify current levels that range from an amp upwards. If low voltage drops are required, then Schottky diodes can be used, however, generally these diodes are PN junction diodes.

. Small signal or Small current diode - These diodes assumes that the operating point is not affected because the signal is small
· Large signal diodes - The operating point in these diodes get affected as the signal is large.

. Transient voltage suppression diodes - This diode is used to protect the electronics that are sensitive against voltage spikes.

· Gold doped diodes - These diodes use gold as the dopant and can operate at signal frequencies even if the forward voltage drop increases.
· Super barrier diodes - These are also called as the rectifier diodes. This diodes have the property of low reverse leakage current as that of normal p-n junction diode and low forward voltage drop as that of Schottky diode with surge handling ability.
· Point contact diodes - The construction of this diode is simpler and are used in analog applications and as a detector in radio receivers. This diode is built of n – type semiconductor and few conducting metals placed to be in contact with the semiconductor. Some metals move from towards the semiconductor to form small region of p- tpye semiconductor near the contact.
· Peltier diodes - This diode is used as heat engine and sensor for thermoelectric cooling.
· Gunn diode - This diode is made of materials like GaAs or InP that exhibit a negative differential resistance region.
· Crystal diode - These are a type of point contact diodes which are also called as Cat’s whisker diode. This didoe comprises of a thin sharpened metal wire which is pressed against the semiconducting crystal. The metal wire is the anode and the semconducting crystal is the cathode. These diodes are obsolete.
· Avalanche diode - This diode conducts in reverse bias condition where the reverse bias volage applied across the p-n junction creates a wave of ionization leading to the flow of large current. These didoes are designed to breakdown at specific reverse voltage in order to avoid any damage.
· Silicon controlled rectifier - As the name implies this diode can be controlled or triggered to the ON condition due to the application of small voltage. They belong to the family of Tyristors and is used in various fields of DC motor control, generator field regulation, lighting system control and variable frequency drive . This is three terminal device with anode, cathode and third controled lead or gate.
· Vaccum diodes - This diode is two electrode vacuum tube which can tolerate high inverse voltages.