What is a Diode? Working Principle & Types | Different Types of Resistors

What is a Diode?

A diode is a device that only allows unidirectional flow of current if operated within a rated specified voltage level.

A diode only blocks current in the reverse direction while the reverse voltage is within a limited range otherwise reverse barrier breaks and the voltage at which this breakdown occurs is called the reverse breakdown voltage. The diode acts as a valve in the electronic and electrical circuits. A P-N junction is the simplest form of the diode which behaves as an ideal short circuit when it is forward-biased and behaves as an ideal open circuit when it is in the reverse biased. Besides simple PN junction diodes, there are different types of diodes although the fundamental principles are more or less the same. So a particular arrangement of diodes can convert AC to pulsating DC, and hence, it is sometimes also called a rectifier.

The name diode is derived from “di-ode” which means a device having two electrodes.

Fig. Diode

Working Principle of Diode

Unbiased Diode

The N side will have a significant number of electrons, and very few holes (due to thermal excitation) whereas the p side will have a high concentration of holes and very few electrons. Due to this, a process called diffusion takes place. In this process free electrons from the n side will diffuse (spread) into the p side and recombine with holes present there, leaving positive immobile (not moveable) ions on the n side and creating negative immobile ions on the p side of the diode. Hence, there will be uncovered positive donor ions on the n-type side near the junction edge.

Similarly, there will be uncovered negative acceptor ions on the p-type side near the junction edge. Due to this, numbers of positive ions and negative ions will accumulate on the n-side and p-side respectively. This region so formed is called a depletion region due to the “depletion” of free carriers in the region. Due to the presence of these positive and negative ions, a static electric field called barrier potential is created across the PN junction of the diode. It is called “barrier potential” because it acts as a barrier and opposes the further migration of holes and electrons across the junction.

Fig. Unbiased Diode

Forward Biased Diode

In a PN junction diode when the forward voltage is applied i.e. 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, the diode is said to be in forwarding biased condition. We know that there is a barrier potential across the junction. This barrier potential is directed in the opposite of the forward applied voltage. So a diode can only allow current to flow in the forward direction when the forward applied voltage is more than the barrier potential of the junction. This voltage is called forward-biased voltage. For the silicon diode, it is 0.7 volts. For the germanium diode, it is 0.3 volts.

When the forward applied voltage is more than this forward-biased voltage, there will be a forward current in the diode, and the diode will become short-circuited. Hence, there will be no more voltage drop across the diode beyond this forward-biased voltage, and the forward current is only limited by the external resistance connected in series with the diode.

Thus, if the forward applied voltage increases from zero, the diode will start conducting only after this voltage reaches just above the barrier potential or forward-biased voltage of the junction. The time, taken by this input voltage to reach that value or in other words, the time, taken by this input voltage to overcome the forward-biased voltage is called recovery time.

Fig. Forward Biased Diode

Reverse Biased Diode

Now if the diode is reverse biased i.e. positive terminal of the source is connected to the n-type end, and the negative terminal of the source is connected to the p-type end of the diode, there will be no current through the diode except reverse saturation current. This is because at the reverse biased condition the depilation layer of the junction becomes wider with increasing reverse biased voltage. Although there is a tiny current flowing from the n-type end to the p-type end in the diode due to minority carriers.

This tiny current is called reverse saturation current. Minority carriers are mainly thermally generated electrons and holes in p-type semiconductors and n-type semiconductors respectively. Now if the reverse applied voltage across the diode is continually increased, then after a certain applied voltage the depletion layer will destroy which will cause a huge reverse current to flow through the diode. If this current is not externally limited and it reaches beyond the safe value, the diode may be permanently destroyed. This is because, as the magnitude of the reverse voltage increases, the kinetic energy of the minority charge carriers also increases.

These fast-moving electrons collide with the other atoms in the device to knock off some more electrons from them. The electrons so released further release much more electrons from the atoms by breaking the covalent bonds. This process is termed carrier multiplication and leads to a considerable increase in the flow of current through the p-n junction. The associated phenomenon is called Avalanche Breakdown.

Fig. Reverse Biased Diode

Types of Diode

The types of diode are as follow-

  1. Zener diode
  2. P-N junction diode
  3. Tunnel diode
  4. Varactor diode
  5. Schottky diode
  6. Photodiode
  7. PIN diode
  8. Laser diode
  9. Avalanche diode
  10. Light emitting diode

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Different Types of Resistors

An electric resistor is a two-terminal passive component specifically used to oppose and limit current. A resistor works on the principle of Ohm’s Law which states that voltage across the terminals of a resistor is directly proportional to the current flowing through it.

Ohm’s Law: V = IR

where V is the voltage applied across resistor,

             I is the current flowing through it,

             and R is the constant called resistance.

The unit of resistance is ohms.

Types of Resistors:

Resistors can be broadly classified based on the following criteria: the type of material used, the power rating and resistance value.

1.        Fixed resistors.

In some scenarios, an electrical circuit may need a lesser amount of current to flow through it than the input value. Fixed resistors are used in these situations to limit the flow of current.

Carbon Composition Resistors:

These resistors are cylindrical rods which are a mixture of carbon granules and powdered ceramic. The resistor value depends on the composition of the ceramic material. A higher quantity of ceramic content will result in more resistance. Since the rod is coated with an insulated material, there are chances of damage due to excessive heat caused by soldering.

High current and voltage can also damage the resistor. These factors bring irreversible changes in the resistance power of these resistors. This type of resistor is rarely used nowadays due to their high cost and are only preferred in power supply and welding circuits.

Fig. Carbon film resistors:

This resistor is formed by depositing a carbon film layer on an insulating substrate. Helical cuts are then made through the carbon film to trace a long and helical resistive path. The resistance can be varied by using different resistivity carbon material and modifying the shape of the resistor. The helical resistive path make these resistors highly inductive and of little use for RF applications.

They exhibit a temperature coefficient between -100 and -900 ppm/ °C. The carbon film is protected either by a conformal epoxy coating or a ceramic tube. The operation of these resistors requires high pulse stability.

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