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A power diode, also known as a high-power diode, is a type of power semiconductor device that, while still having two terminals (anode and cathode), is capable of handling more power than a standard PN junction diode. These are made to handle several kiloamps of current under forward bias with minimal power loss and to block several kilovolts under reverse bias.
Symbolically a power diode is represented as:
Before elaborating more about power diodes, it is important to recall-
What is a Diode?
A normal diode, also referred to as a signal diode, is a PN junction-equipped low-power diode. The unidirectional current flow that a diode offers was already covered in our earlier content on diodes. P and n-type semiconductor materials are combined to create this device, which conducts when forward biassed and blocks current flow when reverse biassed.
Additionally, a power diode has a PN junction that only allows current to flow in one direction. However, it is different from a low-power diode in terms of how it is built and the features it offers. When we refer to features, we are referring to high power diodes’ enhanced level of voltage, current, and power ratings. Additionally, these operate at slower switching speeds than signal diodes.
Basics of PN junction
A p-n junction is the basic component that drives the operation of various power semiconductor devices. We have observed that a p-n junction is created by direct physical contact between p and n-type semiconductor materials, even in signal diodes.
We already know the fundamentals: p-type semiconductors have more holes than electrons, while n-type semiconductors have more electrons than holes. However, different p and n type material concentrations are needed to create power diodes. According to variations in doping concentration, a p-type material can be designated as p+, p, and p-, and an n-type material can be designated as n+, n, and n-.
Accordingly, we can deduce that p+ is a heavily doped p region and n- is a lightly doped n region in general.
Need for Power Diodes?
The signal diodes are primarily employed in low-power rectification circuits. The small-signal diodes, however, do not work well in any of these applications where a large forward-biased current and high reverse-biased blocking voltage are required. This is the case because signal diodes are not designed to handle such a high current, so if a high current is supplied, they will overheat and destroy the device.
Thus, power diodes were developed to address the shortcomings of signal diodes.
The question is now, how does it accomplish this?
In comparison to a signal diode, a power diode has a larger p-n junction region. As a result, it provides a high forward current capability of several hundred amps as well as a significant amount of reverse voltage blocking, up to several thousand volts.
Construction of Power Diodes
The construction of a power diode differs from that of a signal diode, as we have already discussed. A PN junction is all that a signal diode has. However, the construction of power diodes is quite intricate due to their suitability for high voltage and current applications.
The constructional structure of power diodes is shown below:
Considered is a substrate that is heavily doped with n+ and on top of which an n- layer is grown epitaxially. Diffusion of the p+ layer also occurs over the n- region. The cathode connection is made by the n+ substrate, while the anode connection is made by this p+ region.
We have seen when discussing the p-n junction diode that no such structure was present. Thus, the n- region serves as a drift region while the p+ and n+ regions serve as the anode and cathode, respectively. The depletion region is absorbed within the drift region under the reverse biassed condition. When reverse biassed, the breakdown voltage of the diode exhibits a direct proportionality with the thickness of the n-region.
Therefore, the breakdown voltage will be higher the wider the n- region.
The n- region, however, adds up for the diode’s ohmic resistance in forward conduction mode. Due to the significant power dissipation that results, cooling arrangements must be made.
Working of Power Diodes
Power diodes function somewhat similarly to standard diodes. Take a look at the forward biassed state of a power diode in the illustration below, where the battery’s positive terminal is connected to the anode and its negative terminal connects to the cathode.
A majority of carriers (holes) from the p+ region start injecting into the n- drift region as junction J1 becomes forward biassed in this situation. The holes of the p+ region will recombine with the electrons of the n- region when the injection rate is low. However, as the injection rate is increased, holes will enter and rejoin the n+ region’s electrons. It’s known as a double injection. Once the threshold is exceeded, the diode begins conducting heavily because of this flow of carriers and recombination within the drift region.
Now think about the reverse biassed situation, where the battery’s positive terminal is connected to the cathode and its negative terminal forms a connection with the anode.
As the junction becomes reverse biassed, the power diode also stops conducting in this situation, just like a regular diode would. The minority carriers will have trouble penetrating the junction and recombining because the depletion region is extended here up to the drift region.
However, it should be noted that current flow won’t stop right away if the applied potential’s polarity is suddenly changed. A small leakage current (of the order of 100 mA) will also flow through the diode in the opposite direction as a result of the minority charges stored in the junction. The dependence of this reverse current on junction temperature variation is evident.
Impact ionization will occur once the applied potential reaches the breakdown voltage.
What is impact ionization?
The electric field is excited by the reverse applied voltage, which causes the electrons to accelerate. The moving electrons may be able to release more electrons from the covalent bonds of silicon atoms once they have amassed enough kinetic energy. A significant amount of free electrons are produced by this cumulative process,. Which causes a significant reverse current to flow through the device.
The diode could be destroyed as a result of this significant increase in power dissipation caused by the large reverse current. As a result, it is said that using the device in the region of reverse breakdown is forbidden.
I-V Characteristics (vi characteristics of power diode)
Forward current is zero when there is no supply voltage present, but as the supply input rises and reaches the threshold level (of about 0.7 V), some forward current starts to flow through the device. When the threshold value is exceeded, the diode’s current (at 1V) noticeably increases as it begins to conduct. When voltage rises above the threshold in this case, a linear rise in forward current is observed.
Leakage current flows through the device in reverse biassed mode regardless of the applied potential,. But once breakdown is reached, even at roughly constant voltage, a significant amount of reverse current flows.
Reverse Recovery Characteristics of Power diode
As we recently discussed, the diode continues to conduct even after the forward applied voltage is removed because of the charge that has been stored in the semiconductor layer and depletion region. Therefore, the amount of time that this leakage current flows is known as the reverse recovery time, or trr. Up until the leakage current reaches zero, the diode’s blocking capacity is restored.
The trr is the time between the moment forward current vanishes and the moment the reverse recovery current remains only 25% of its peak value IRM.
From the figure, it is clear that
trr = ta + tb
: ta is the interval between zero crossings of forward current and peak reverse current IRM. During ta the charge within the depletion region, is vanished. While tb is the duration from the peak of reverse current IRM to 0.25% IRM. During tb, the charge from the layers of semiconductors is removed.
The ratio of tb and ta is termed as softness factor given by S. It is generally unity, hence such diode with S equal to 1 is called soft recovery diode. While if S>1 hence it is called fast recovery or snappy recovery diode.
Types of Power Diode
The reverse recovery characteristics that power diodes possess are used to categorise them.
- General Purpose Diodes: These have trr values of about 25 microseconds, which is quite high. This diode is used in low-frequency applications like rectification converters that operate at almost 1 KHz. Its voltage rating ranges from 50 volts to five kilovolts,. And its current rating ranges from one amp to several thousand amps.
- Fast Recovery Diodes:- These display trr of only 5 microseconds, which is quite low. Primarily utilised in systems for converting electrical power. Its voltage rating ranges from 50 volts to three kilovolts,. And its current rating ranges from one amp to several thousand amps.
- Schottky Diodes:- These diodes form a metal-semiconductor junction instead of a p-n junction,. With silicon serving as the semiconductor and aluminium serving as the preferred metal. While the reverse voltage rating is roughly 100 V, the current rating ranges from 1 A to 300 A.
Power diodes are primarily used as freewheeling diodes in ac to dc and dc to ac conversion systems, rectification, battery charging, etc. because of their properties. Power diodes are additionally utilised in electroplating, UPS, choppers, SMPS, and induction heating in addition to these.