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Basic Electronics

Diode, Construction, Working, Operation, Space Charge, Characteristics, & Parameters

Diode Introduction:

Diode, Construction, Working, Operation, Space Charge, Characteristics, & Parameters– By “di” we mean two while “odes” have been extracted from electrodes. Thus, by a diode, we mean two electrodes.  Therefore, a tube or valve consisting of two electrodes is called a diode. In 1883, American scientist Mr. Edison was busy in experiments for rectification/improvement in the electric bulb. He shove a metal plate in the bulb and pushed one of its ends out, so that bulb does not turn black due to the heat generated by the filament and at the same time, the filament also does not become weak. When he turns the bulb on, no dark area was created inside the bulb, however, he received an electric shock as a result of his hand suddenly touching the metal plate. By taking accurate measurements with the help of a voltmeter, he observed that there exists an electric pressure on the plate. Afterward, when he connected this plate with a positive supply through an ampere meter, the flow of current became evident. However, when he connected this plate with a negative supply, the flow of current stopped. From this, he came to know that current can be passed through a metal plate just from one direction. On the basis of this theory, the diode valve was invented, which was converted to the form of diode by Mr. Fleming in 1904.


Diode Construction

From a construction point of view, the diode is the simplest electron tube of all other vacuum tubes or valves, which basically consists of 2 electrodes. One of the two electrodes is called cathode or emitter, the task of which is the emission of electrons. The other one is called an electrode, anode, or collector. Anode is also mostly called as plate. Its function is to collect or assemble electrons emitted from the emitter. Cathode exists in the middle of the diode valve while anode or plate surrounds/encircles the cathode in a cylindrical shape. That’s anode or plate exists around a heater or cathode. As is evident from the structure of the diode tube reflected in figure 1.

Diode
figure 1

Both electrodes are enclosed in a glass or metal envelope which has completely been emptied from the air. Its emitter may either be directly handed or indirectly handed. The Anode of the tube normally resembles a hollow metal cylinder. It is usually made with nickel, molybdenum, graphite, tantalum, or iron. Anode’s surface is normally scratchy and a bit blackish, so that heat produced by the collision of electrons on it could be emitted through the method of heat……….and its temperature also does not increase. The cathode is kept on a constant temperature. Anode pulls electrons found on the surface of cathode only when a positive voltage is provided to anode or plate through cathode. Electrons move from cathode towards the anode (in space/ vacuum) while the flow of current remains in opposite direction.

In order to close the mouth of tube, its base is made from Bakelite or a hard insulator. Brass-coated pins are being fixed at the base, which is connected through the interior cathode. The number of connection pins depends on the number of electrodes.

Keep in mind that the anode should be bigger in size so that it can emit heat easily and at the same time does not heat up.



Diode Operation or Working

The function of a diode depends on a fundamental electricity law that like (similar) charges repels each other and unlike charges attract each other towards it. Electrons emitted from the cathode always have a negative charge and anode attracts these electrons towards it or repels it. This depends on whether a positive charge is being provided to the anode through a cathode or a negative one.

Diode conducts just in one direction, which means that it allows electrons to pass only in one direction (from cathode to anode), provided anode is positive with respect to cathode. It doesn’t conduct from the opposite direction at all; therefore, it acts as a switch, which offers very little resistance in one direction. It is a non-linear device (that’s it does not operate under Ohm’s law) and it is mostly used for rectification and detection purposes.

When potential difference (voltage) is provided between diode’s cathode and anode through a battery or some other source, electricity is created inside the tube or valve. The lines of forces of this field always spread from the negatively charged element to the positively charged element. As electrons are negative charged particles (or negatively electric charged), therefore they move in the direction of lines of forces in the electric field. (It is reminded that in some of the old books, lines of forces have been shown moving from positive to the negative direction, just like the flow of conventional current is being deemed as flow of charges in opposite direction of electrons instead flow of electrons)

The basic function of diode can be explained with the help of a circuit of a simple diode as given in figure 2.

Diode
figure 2

In this figure, a battery is placed between anode and cathode in such a manner that anode is negative with respect to the cathode (that’s anode is connected with negative while cathode with the positive terminal of the battery). Thus, lines of forces of the field are established from anode to cathode inside the valve. Now when voltage is supplied to heater element (H), cathode starts emitting electrons in large quantity due to heat, however negatively charged anode repels these cathodes emitted electrons back towards cathode with great force. (I.e. two alike charges repel each other). In this way, the entire empty space between anode and cathode is filled with these electrons and in fact, electrons do not reach anode at all. Consequently, the tube acts as an open circuit, and the meter fixed between the anode and cathode’s outer circuit reflects zero value as is evident from the figure.


Now if the battery connection is reversed and the anode is made positive with respect to the cathode, as shown in figure 3, now the lines of forces in the electric field will spread from the cathode to the anode’s direction. As a consequence of supplying voltage again to the heater, a profuse emission of electrons from the cathode will kick off. However, in such a situation, positive charged anode will attract cathode emitted electrons with extreme force towards it (we know that attraction always occurs between opposing charges), due to which electrons will strike with anode at a great speed. As electric current is produced due to moving charges (or the flow of charges is called electric current), therefore, the flow of electric current starts as a result of flow of electrons from the cathode to anode, which is named as anode current.  This electron current after reaching anode, through an eternal circuit made with connecting wires, reaches the positive terminal of the battery by passing through a meter continuously. Positive terminal of the battery absorbs electrons coming or reaching towards the positive terminal and an equal number of electrons reach cathode after getting emitted from the negative terminal of the battery. Thus, through emission, the number of electrons emitted by the cathode is equally received back by the cathode through some supply source.  Through a meter installed in the outer circuit, measurement of electrons moving from the anode and returning to the cathode is done. So long as an emitting temperature (temperature on which electrons emission starts) is supplied to cathodes, and the anode is kept positive compared to cathode, a continuous flow of electrons from the cathode to anode inside the tube and anode to cathode in the outer circuit goes on consistently

The summary of diode tube operation explained above is as follows:

  1. An electron current in diode (or anode current) flows only when its anode is kept positive with respect to the cathode. When an anode is negative with respect to the cathode, the flow of current in the diode stops.
  2. The flow of electrons in diode always occurs from cathode to anode and there is no flow from anode to cathode. This is also called unidirectional or unilateral conduction.
  3. A diode can function as a switch or valve due to its unidirectional character or a diode can be treated as a switch, so that when the anode is positive in respect to the cathode, it can automatically pass anode current, while when the anode is negative with respect to the cathode, automatically disrupts the flow of anode current or stops anode current. This characteristic of diodes enables it to act as a rectifier or convert alternating current to direct current. In other words, the process of rectification can be carried out by using a one-dimensional conducting attribute of diodes.


Space Charge

When a cathode is heated in a diode to emission temperature (i.e. the temperature on which the flow of electrons starts), it starts emitting electrons and these emitted electrons fill up the space around cathode with negative charge. This negative charge found around cathode, repels further electrons emitting from cathode. In this way, these electrons emitted from cathodes, reach the anode. A stage comes, when the repelling field created by electrons existing around space becomes so huge, that it starts repelling electrons towards cathode. Such a repelling field is called space charge.

The total number of electrons emitted from the cathode of a diode always remains uniform at a specific temperature. There is no influence of anode voltage (the voltage existing between anode and cathode) on the number of electrons emitted from cathode. The term space charge is used for a cloud of electrons that is created between two electrodes i.e. cathode and anode in an empty space (inter-electrode space). As such a cloud comes into being as a result of negatively charged electrons; therefore, cloud creates a negative charge in empty space found between the electrodes, which repel further cathode emitted electrons towards the cathode again. Thus, such a negative charge repels a significant portion of emitted electrons to the cathode once again and prevents others from reaching the anode.

If a very low voltage is supplied on the anode, it attracts very few electrons found near the anode, due to which very small anode current flows. In such a situation, there is a significant influence of space charge on a major portion of electrons. In such a situation, space charge thwarts bulk of electrons from reaching the anode. When anode voltage is increased, it attracts a very large number of electrons through space charge towards it and a very small quantity of electrons diverts towards the cathode again. In case anode voltage is increased to a reasonable limit, a point will come when anode will attract all electrons emitted from the cathode towards it and the effect of space charge will cease completely. The process of diode has been elaborated in figure 4. There will be no increase in anode current which passes through the anode tube, if anode voltage is increased further and the emission of electrons from cathodes will limit the extreme flow of current.


Diode Characteristics

Characteristics of diode refers to relationship between current passing through the circuit of an anode (plate), (which is also known as anode current and it is denoted by Ibor Ip) and the voltage supplied on anode (which is also called anode voltage and denoted as Eb or Ep). In other words, the relation between a diodes’ anode current and its anode voltage reflects the characteristics of diode (Parallel voltage provided by anode and cathode are called anode voltage) (i.e. what is the effect on anode plate current of keeping the cathode temperature constant and fluctuating the voltage of anode plate. Further, what changes take place in the anode current, by keeping the anode voltage constant and changing the cathode temperatures? Such a study reflects characteristics of diodes.

When cathode is heated to a certain temperature, it emits considerable quantity of electrons according to temperature. Afterwards, anode/plate voltage is increased gradually; due to which anode’s force of attraction on electrons also increases. Very little attraction force is generated on anode when it is supplied a low positive voltage, due to which all electrons emitted from cathode, cannot reach anode and most of the electrons remain on the surface of cathodes (it means that low anode voltage results in low anode current) Its main reason is the overwhelming influence of space charge. Due to space charge, negative charged electrons repel further electrons emitted from cathode to cathode again. Resultantly, space charge works as barrier for electrons emitted from the cathodes. Electrons, which are able to reach anode by crossing space charge barrier due to the attraction of anode, result in the flow of anode current, the value of which can be measured through reading from an ampere meter fitted in the circuit. If the supply of voltage to anode is increased, the positive field generated by anode/ plate becomes large further, due to the influence of which, the effect of space charge further diminishes. In this manner, anode current also enhances. Now if anode voltage is gradually increased, a point comes when anode attracts all electrons emitted from cathode, towards it (that’s all electrons being found on cathode, reach anode), due to which anode current swiftly increases to its maximum value (remember, this became possible due to the fact that anode voltage has overpowered space charge). After this point, if anode voltage is further increased, there will be no increase in anode current (anode current becomes stable and it has no impact of an increase in anode voltage). This happens due to saturation and this condition is called saturation of tube. If further increase is required in anode current, it can be possible only if operating temperature of cathode is increased, instead of increasing voltage in anode. The mutual relationship between the voltage of anode (plate) and anode current has been shown in figure 4(a). It has to be kept in mind that when tube has been functioning in a state of saturation, it is completely free from space charge. Increasing the voltage value of anode might harm cathode as well. Normally, thermionic valves are operated in space charge limited region.

Diode
Figure 3

As opposed to the explanation provided above, if the voltage of anode (plate) is retained constant, and the temperature of cathode is increased gradually, the anode (plate) current will also increase with an increase in temperature. This has been demonstrated in fig (b).  The emission of electrons from the cathode at a low temperature (T3) is low and all emitted electrons move to the plate due to the attraction of the anode/ plate, the result being that no space charge effect is created. However, the electrons emitted through the cathode, also increase with a gradual increase in temperature, and as anodes’ attraction force is permanent, therefore all electrons will not be able to reach the anode due to this attraction, as a result, the space charge effect will build up and current will reach to a constant value. Remember that the level of characteristics curves depends on cathode’s temperature apart from the cathode material (or emission characteristics). If tungsten electrode has been used as a cathode, low current will be achieved. And if oxide-coated cathode has been used, relatively more currents would be achieved. Circuit reflecting characteristics of diode has been shown in the below diagram.

 

Diode
Figure 4

 

 


Diode Parameters

From the detail given above, it is evident that plate current in a diode valve depends on its plate voltage and cathode’s temperature or a plate’s current is controlled through plate voltage and cathodes’ temperature. However, it is necessary that following electric parameters or constants must be taken care of while using diode valves. (Parameter means a constant, changes in the value of which depends on its usage)

Plate Dissipation

Emission of electrons from cathode and its attraction towards plate generates temperature on the plate. This heat is called power loss of plate and it commonly refers to the depletion (wastage) of power on the plate.

Maximum Plate Current

A diode tube or valve can become ineffective as follows.

  1. When anode/ plate gets excessively heated due to the bombardment of so many electrons
  2. Cathode might get damaged due to emission of large quantity of electrons
  3. If diode is used as a rectifier and too much load is exerted on rectifier’s output, extreme current may damage rectifier.

Safe plate current is maximum forward plate current on which neither excessive bombardment of electrons on plate nor excessive emission of electrons from the cathode occurs (i.e. maximum current, which diode can safely pass through without inflicting damage on anode or cathode, is called maximum forward current)

Peak Inverse Voltage

Maximum voltages, which can be provided on a diode from the inverse direction without any sort of damage, is called maximum or peak inverse voltage. It is denoted by PIV.

Plate Resistance

Total internal resistance of a tube from cathode to anode is called plate resistance. It is denoted by Rp. However, mutual ratio of plate voltage and plate currentis also called plate resistance.

Plate Resistance (Rp) = Plate Voltage (Ep) /Plate Current (Ip)


Uses

When diode is used as a rectifier, it converts input AC to output DC, that’s it allows current to move in just one direction (cathode to anode), similarly AC pulsating converts or rectifies to DC. Apart from rectification, diode also uses tube as a detector. The process of separating two or more than two form of AC or DC supplies / voltage to a specific type is called detection. When diode is used as a detector, it separates alternating high frequency to low frequency (audio frequency) signals. Moreover, also used for supply of high voltage to high vacuum diode tubes’ testing, x-ray apparatus and for fulfilling other high voltage requirements.

Diode tubes are also used as frequency changer (generating a frequency via addition or subtraction of two different frequencies), shaper (cutting some part of voltage for changing its view form), for stabilizing the performance of electronic appliances through discriminator circuits (which make changes in amplitude according to frequency fluctuations) and ratio detector circuits.

Summary

  1. A diode is a tube consisting of two electrodes –one of these is cathode (which works as a electron emitter) and the other one is anode or plate(which works as an electron collector)
  2. An electric field is formed between cathode and anode as a result of diode supplying anode with positive charge, which attracts electrons emitted from cathode towards the anode.
  3. Anode current, flows due to the flow of electrons from cathode to anode and these electrons return to cathode again through an outer circuit.
  4. When anode positive relative to cathode, anode current flows in diode at that time. When anode is negative relative to cathode, no current passes through diode.
  5. The electron current in a diode always flows from cathode towards anode. This process is called unidirectional conduction or unilateral flow.
  6. A cloud of electrons created in the empty space between cathode and anode, is called space charge. Space charge is negative; therefore, it has a repellent-like impact on electrons emitted from the cathode. That’s space charge pushes electrons emitted from cathode back to cathode).
  7. The value of anode current depends on the power of electric field generated through space charge and positive anode voltage.
  8. On low anode voltages, negative space charge limits the flow of electrons and thus anode current is fully controlled via anode voltage. In such a situation, anode current remains independent from cathode’s temperatures.
  9. Space charge ceases on high anode voltages and anode current reaches its saturation value, which is equivalent to full emission current. In this case, anode current is independent of anode voltage which then reaches a constant value on a specific cathode temperature.
  10. In space charge limited region, anode current is proportionately 2/3 of anode voltage.


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Engr Fahad

My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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