Diode
In general, all the electronic devices need DC power supply but it is impossible to generate DC power so, we need an alternative to get some DC power thus the usage of diodes comes into the picture to convert AC power to DC power. A diode is a tiny electronic component used in almost all the electronic circuits to enable the flow of current in only one direction (unidirectional device). We can say that the usage of semiconductor materials to build the electronic components was started with diodes. Before the invention of diode there were with vacuum tubes, where the applications of both these devices are similar but the size occupied by the vacuum tube will be much greater than the diodes. The construction of vacuum tubes is a bit complex and they are difficult to maintain when compared with the semiconductor diodes. Few applications of diodes are rectification, amplification, electronic switch, conversion of electrical energy into light energy and light energy into electrical energy.
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 silicon crystal which has a crack in it was exposed to the sunlight, he found the flow of current through the crystal and that was later called as diode, which was the beginning of the semiconductor era.
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 which are in general 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 semi-conductors.
Intrinsic Semi-conductors:
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.
Extrinsic Semiconductors:
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.
N-Type Semiconductor:
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.
P-Type Semiconductor
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.
Now, if we join the two types of semi-conductors P-type and N-type together then a new device is formed called as P-N junction diode. Since a junction forms between a P type and N type material it is called as P-N junction.
The word diode can be explained as ‘Di’ means two and ‘ode’ is obtained from electrode. As the newly formed component can have two terminals or electrodes (one connected to P-type and the other to the N-type) it is called as diode or P-N junction diode or semi-conductor diode.
The terminal connected to P-type material is called Anode and the terminal connected to N-type material is called Cathode.
The symbolic representation of the diode is as follows.
The arrow indicates the flow of current through it when the diode is in forward biased mode, the dash or the block at the tip of the arrow indicates the blockage of current from the opposite direction.
We have seen how a diode is made with P and N semi-conductors but we need to know what happens inside it to form a unique property of allowing current in only one direction and what happens at the exact point of contact initially at its junction.
Junction Formation:
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 diffuses 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 of the diode when proper voltage is applied.
To make the diode conduct first we need to break the barrier formed in the path. To break a barrier within a normal diode a minimum of +0.7 Volts (for Silicon) and +0.3 Volts (for Germanium) external voltage should be applied to the terminals. These voltages are called as Cut-in voltage or Offset voltage or Break-point voltage or firing voltage or Threshold voltage. Until these voltages are very less, current flows through the diode (ideally zero).
If a positive terminal of the battery or a voltage source is applied to the anode or P region of the diode and negative terminal to the cathode or N region of the diode, it is said to be forward biased.
Due to forward bias, majority charge carriers in both regions get repelled (because positive voltage is applied to P region and negative to N region) and enter into the depletion region. Hence immobile ions get back lost carriers become neutral and move to undepleted region hence width of the barrier decreases gradually, when the applied voltage is greater than or equal to cut-in voltage, entire barrier is destructed and the electrons and holes are now free to cross the junction which then forms a closed circuit and enables the flow of current.
Diode in forward bias acts as closed switch and has a forward resistance of few ohms (around 20Ω).
If negative terminal of a voltage source is applied to the anode or P region of the diode and positive terminal to the cathode or N region of the diode, it is said to be Reverse biased.
When such voltage is applied, the majority charge carriers in both regions get attracted towards source such that large numbers of immobile ions are created and enter into the P and N regions. Therefore, width of the depletion region also increases gradually which is now difficult for the electrons and holes to cross the junction so open circuit forms and current flows. But if we go on increase the voltage, at a point barrier or depletion region cannot withhold the external force and the junction breaks down which may sometimes cause the normal diode damage permanently. To overcome this we can heavily dope the regions and put the diode safe, this application can be seen in zener diodes.
The reverse voltage at which the diode conducts is called as Break down voltage.
As the diode in reverse bias acts as an open switch, its resistance is in the order of Megaohms.
When the reverse voltage is applied to the diode small amount of current flows in the circuit due to the minority charge carriers which is generally called as Reverse saturation current. These currents are also called as Leakage currents because even when the diode is open circuited, current exists in circuit so it is termed as leakage.
