Diode – Definition, PN Junction Diode, Construction, Working with characteristics
PN Junction Diode, Construction, Working, and V-I characteristics
Introduction and history of Diode
Generally, all the electronic devices work on DC power supply but it’s impossible to generate DC power so, the usage of diodes comes into the picture to convert AC power supply to DC power supply. A diode is defined as a semiconductor electronic device used in almost all the electronic circuits to enable the flow of current in only one direction (it’s a unidirectional device). Before the innovation of diode vacuum tubes was in use, the applications of both these devices are similar but the size occupied by the vacuum tube was 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. In the year 1940 at Bell Laboratories, Russell Ohl was working with a silicon crystal to find out its properties. One day accidentally when the silicon crystal has a crack in it was exposed to the sunlight, he found the flow of current through the crystal and later it was called a diode, it was the beginning of the semiconductor era.
Important applications of diodes are as rectifier, amplifier, electronic switch, conversion of electrical energy into light energy and light energy into electrical energy.
Construction of Diode:
Introduction of semiconductors materials
Basically materials are classified into conductors, insulators, and semiconductors. Conductors possess a large number of free electrons at room temperature; Insulators have no free electrons whereas semi-conductors can either be conductors or insulators depending upon the potential applied. Elementary Semi-conductors materials which are generally used are Silicon and Germanium. Silicon is preferred over germanium because it is abundantly available on the earth and it gives a better thermal range.
Semi-conductors are further classified as Intrinsic and Extrinsic semiconductors.
Intrinsic type of semiconductor
It is pure form of the semiconductor. In intrinsic type of semiconductor charge carriers (electrons and holes) are in equal quantity at the room temperature. So current conduction takes place by both holes and electrons equally.
The semiconductor materials in which external impurity atoms are added to making it more conductive is known as the extrinsic semiconductor. The conductivity of the intrinsic semiconductor is zero at room temperature. Whereas to increase the number of holes or electrons in an extrinsic semi-conductors material trivalent or pentavalent impurity atoms are added to the pure semiconductors. This process of adding impurities to the pure semi-conductors is known as Doping.
Extrinsic semiconductor materials: Further classified into p-type and n-type materials
Formation of n-type semiconductor, +ve ion with free electron
If pentavalent impurity atoms (number of valence electrons are five) are added to the pure Si or Ge then every pentavalent atom will form four covalent bonds and one electron is left free. As the electrons (negatively charged carriers) are in majority, these are called as N-type semiconductor. In N-type, semi-conductor electrons are majority charge carriers and holes are minority charge carriers.
Examples of pentavalent impurity atoms are Phosphorous, Arsenic, Antimony, and Bismuth. Since these have five valance electrons are ready to pair with the external positively charged particle leaving one free electron named as a donor.
Similarly, if trivalent elements like Boron, Aluminium, Indium, and Gallium are added to Si or Ge, causing the formation of holes. Since a hole is ready to accept an electron and get paired it is called Acceptors. As the number of holes is excess in newly formed material these are called P-type semiconductors. In P-type semi-conductor holes are majority charge carriers and electrons are minority charge carriers.
Formation of a p-type semiconductor, -ve ion with a hole
Formation of P-N junction
p-n junction diode
From the symbolic representation, arrow specifies the direction of current flow from anode to the cathode through it when the diode is in forward biased mode, whereas blocking the current flow in the opposite direction
Biasing conditions for the p-n Junction Diode: There are three biasing conditions for p-n junction diode is based on the applied voltage:
Zero bias: When there is no external voltage applied to the p-n junction diode.
Forward bias: When the positive terminal of the battery is connected to the p-type while the negative terminal is connected to the n-type.
Reverse bias: When the negative terminal of the battery is connected to the p-type and the positive is connected to the n-type.
Junction Formation, No bias condition
Initially, when both the materials are combined together (without any external voltage applied) the excess electrons in the N-type material and excess holes in the P-type material will get attracted to each other and gets recombined where the formation of immobile ions (Donar ion in n-type and Acceptor ion in p-type) takes place as shown in below fig. This forms a potential barrier across the junction and resists the flow of electrons or holes through the junction. Formation of the barrier means the immobile ions diffuse into P and N regions. This potential barrier which is formed is called as Depletion region. The width of the depletion region, in this case, depends upon the doping concentration of the p, n materials.
If the doping concentration is equal in both the region then the immobile ions diffuse equally into both the materials. If the doping concentration differs in p and n regions then the width of the depletion region also differs. It diffuses more into the lightly doped region and less into the heavily doped region.
Diode in Forward Bias:
To make the diode conducting first need to break the potential barrier formed in the path. To break a barrier within a normal diode, a minimum of +0.7 Volts (for Si) and +0.3 Volts (for Ge) voltage should be applied to the terminals. These voltages are called as Cut-in voltage or Offset voltage or Break-point voltage or Threshold voltage. Below these voltages levels very less, current flows through the diode (ideally zero).
If the positive terminal of the battery is connected to the anode or P-type of the diode and negative terminal is connected to the cathode or N-type of the diode, then it is said forward biased. When the p-n junction is forward biased, the built-in electric field (2) and the applied electric field (1) are in opposite directions. When both the electric fields add up, the resultant electric field has a magnitude that is lesser than the built-in electric field. This results in a less resistive and thin depletion layer. The depletion layer’s resistance becomes negligible when the applied voltage is larger. In the case of silicon, at the voltage of 0.7 V, the resistance of the depletion region becomes totally negligible and the current flows through the junction Diode in forwarding bias condition acts as a closed switch and has a forward resistance of around 20Ω.
The diode in Reverse bias condition: If -ve terminal of a battery is connected to the anode or P-type of the diode and +ve terminal to the cathode or N-type of the diode, then it is said to be Reverse biased.
In this condition, majority charge carriers in both the regions get attracted towards the supply, Therefore, the width of the depletion region increases gradually which is now difficult for the electrons and holes to cross the junction causes a very small current to flows known as leakage current. But if we go on an increase the reverse voltage, at a point barrier or depletion region cannot withstand the external force and the junction breaks down which may sometimes cause the normal diode damaged permanently. To overcome this situation we can heavily dope the regions and put the diode safe, this condition can be seen in Zener diodes.
The reverse voltage at which the diode conducts is called Break down voltage.
As the diode in reverse biasing acts as an open switch, its resistance is in the order of Megaohms (MΏ). When the reverse voltage is applied to the diode a very small amount of current flows in the circuit due to the minority carriers which is called Reverse saturation current.
V-I Characteristics of diode
The formula used in the p-n junction depends upon the built-in potential difference created by the electric field is given as:
E0=VT ln (NA-ND)/ni2
E0 is the zero bias junction potential
VT is the thermal voltage, 26mV at room temperature
ND and NA are the impurity concentrations
ni is the intrinsic concentration.