Diodes are one of the most important elements of power electronics circuits. This special element has a unique working principle.
The diode is a two-element semiconductor device containing an anode and a cathode and providing unidirectional conduction. A diode allows the current to flow in one direction but not the other. The many types are used in such devices as rectifiers, detectors, peak clippers, mixers, modulators, amplifiers, oscillators, and test instruments.
In the circuit symbols, the cathode is shown as a bar and the anode as a triangle. On some circuit diagrams, the anode of a diode may also be indicated by the letter ‘a’ and the cathode by the letter ‘k’.
To understand the working principle of a diode we should know what a semiconductor is.
Materials that permit the flow of electrons are called conductors. Materials that block the flow of electrons are called insulators. Materials whose conductivity falls between those of conductors and insulators are called semiconductors. Semiconductors are “part-time” conductors whose conductivity can be controlled. Semiconductors have been quite useful in the field of electronics.
Diodes are made from semiconductor materials, mainly silicon, with various compounds (combinations of more than one element) and metals added depending on the function of the diode. Early types of semiconductor diodes were made from Selenium and Germanium, but these diode types have been almost totally replaced by more modern silicon designs.
Silicon is the most common material used to build semiconductor devices. Si is the main ingredient of sand and it is estimated that a cubic mile of seawater contains 15,000 tons of Si. Si is spun and grown into a crystalline structure and cut into wafers to make electronic devices.
Atoms in a pure silicon wafer contain four electrons in outer orbit (called valence electrons). Germanium is another semiconductor material with four valence electrons.
In the crystalline lattice structure of Si, the valence electrons of every Si atom are locked up in covalent bonds with the valence electrons of four neighboring Si atoms. To make useful semiconductor devices, materials such as phosphorus (P) and boron (B) are added to Si to change Si’s conductivity.
Pentavalent impurities such as phosphorus, arsenic, antimony, and bismuth have 5 valence electrons.
When phosphorus impurity is added to Si, every phosphorus atom’s four valence electrons are locked up in a covalent bond with valence electrons of four neighboring Si atoms. However, the 5th valence electron of the phosphorus atom does not find a binding electron and thus remains free to float. When a voltage is applied across the silicon-phosphorus mixture, free electrons migrate toward the positive voltage end.
When phosphorus is added to Si to yield the above effect, we say that Si is doped with phosphorus. The resulting mixture is called N-type silicon (N: negative charge carrier silicon).
The pentavalent impurities are referred to as donor impurities.
Trivalent impurities e.g., boron, aluminum, indium, and gallium have 3 valence electrons.
When boron is added to Si, every boron atom’s three valence electrons are locked up in a covalent bond with valence electrons of three neighboring Si atoms. However, a vacant spot “hole” is created within the covalent bond between one boron atom and a neighboring Si atom. The holes are considered to be positive charge carriers.
When a voltage is applied across the silicon-boron mixture, a hole moves toward the negative voltage end while a neighboring electron fills in its place.
When boron is added to Si to yield the above effect, we say that Si is doped with boron. The resulting mixture is called P-type silicon (P: positive charge carrier silicon).
The trivalent impurities are referred to as acceptor impurities.
The hole of the boron atom points towards the negative terminal. The electron of the neighboring silicon atom points toward the positive terminal. The electron from the neighboring silicon atom falls into the boron atom filling the hole in the boron atom and creating a “new” hole in the silicon atom. It appears as though a hole moves toward the negative terminal!
How does a Diode work?
The diode is created from something called a PN junction. 2 semiconductor materials are put together. This component offers extremely low resistance to current flow in one direction and extremely high resistance to current flow in the other. This characteristic allows the diode to be used in applications that require a circuit to behave differently according to the direction of current flowing in it.
An ideal diode would pass an infinite current in one direction and no current at all in the other direction. Besides, the diode would start to conduct current when the smallest of voltages was present. In practice, a small voltage must be applied before conduction takes place. Furthermore, a small leakage current will flow in the reverse direction. This leakage current is usually a very small fraction of the current that flows in the forward direction.
If the P-type semiconductor material is made positive relative to the N-type material by an amount greater than its forward threshold voltage (about 0.6 V if the material is silicon and 0.2 V if the material is germanium), the diode will freely pass current. If, on the other hand, the P-type material is made negative relative to the N-type material, virtually no current will flow unless the applied voltage exceeds the maximum (breakdown) voltage that the device can withstand. Note that a normal diode will be destroyed if its reverse breakdown voltage is exceeded.
The connection to the P-type material is referred to as the anode while that to the N-type material is called the cathode. With no externally applied potential, electrons from the N-type material will cross into the P-type region and fill some of the vacant holes. This action will result in the production of a region on either side of the junction in which there are no free charge carriers. This zone is known as the depletion region.
In forward-biased condition, the diode freely passes the current. In this reverse-biased condition, the diode passes a negligible amount of current. In the freely conducting forward-biased state, the diode acts rather like a closed switch. In the reverse-biased state, the diode acts as an open switch.
If a positive voltage is applied to the P-type material, the free positive charge carriers will be repelled and they will move away from the positive potential towards the junction. Likewise, the negative potential applied to the N-type material will cause the free negative charge carriers to move away from the negative potential towards the junction.
When the positive and negative charge carriers arrive at the junction, they will attract one another and combine (recall that unlike charges attract). As each negative and positive charge carrier combines at the junction, a new negative and positive charge carrier will be introduced to the semiconductor material from the voltage source. As these new charge carriers enter the semiconductor material, they will move toward the junction and combine. Thus, current flow is established and it will continue for as long as the voltage is applied.
Typical diode characteristic
The forward threshold voltage must be exceeded before the diode will conduct. The forward threshold voltage must be high enough to completely remove the depletion layer and force charge carriers to move across the junction. With silicon diodes, this forward threshold voltage is approximately 0.6 V to 0.7 V. With germanium diodes, the forward threshold voltage is approximately 0.2 V to 0.3 V.
The figure below shows typical characteristics for small germanium and silicon diodes. It is worth noting that diodes are limited by the amount of forwarding current and reverse voltage they can withstand. This limit is based on the physical size and construction of the diode.
In the case of a reverse-biased diode, the P-type material is negatively biased relative to the N-type material. In this case, the negative potential applied to the P-type material attracts the positive charge carriers, drawing them away from the junction. Likewise, the positive potential applied to the N-type material attracts the negative charge carriers away from the junction. This leaves the junction area depleted; virtually no charge carriers exist. Therefore, the junction area becomes an insulator, and current flow is inhibited. The reverse bias potential may be increased to the reverse breakdown voltage for which the particular diode is rated. As in the case of the maximum forward current rating, the reverse breakdown voltage is specified by the manufacturer. The reverse breakdown voltage is usually very much higher than the forward threshold voltage. A typical general-purpose diode may be specified as having a forward threshold voltage of 0.6 V and a reverse breakdown voltage of 200 V. If the latter is exceeded, the diode may suffer irreversible damage.
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