You may have already gone through the definition of Power Electronics (i.e. electronics applied at power-level). Similarly, Power Diode is the Diode (semiconductor-based p-n junction diode) which works at High Power and High Voltage level.
NOTE: All the Power Electronics devices act only as Switches in some way, i.e. they will be used just for switch-on and switch-off process.
What is a Power Diode?
Power diode is a two-terminal p-n junction semiconductor device. We use it for the rectification of alternating voltage and current at power-level.
Symbol of Power Diode
It has the same symbol as the conventional low-voltage signal-level diodes. The two terminals are Anode (the positive one) and Cathode (the negative one).
Ratings of Power Diode
- Voltage Rating: upto 4000 V
- Current Rating: upto 3500 A
- Upper Frequency: 1 kHz
- ON State Voltage Drop: nearly 1 V
Types of SWITCHES in Power Electronics
We can control the On and Off states uncontrolled switches by the circuit only. But we cannot control them externally. Diodes are examples of Uncontrolled Switches. For example, a Diode becomes ON when the circuit makes it Forward Biased. On the other hand, it becomes OFF when the circuit makes it Reversed Biased.
For Semi-controlled Switches, we can only control the ON state externally. The thyristor is the example of a Semi-controlled Switch. We can switch ON a thyristor by applying the GATE signal to it. Although, we can not switch off a thyristor by applying the same Gate signal.
We can control both On and Off-states of the switches externally by applying proper gate signals. BJTs, MOSFETs, IGBTs etc. are examples of Controlled Switches.
Construction and Fabrication of Power Diodes
Apart from the similarity of a normal signal-level Diode and a Power diode, there also exists some important differences e.g. the difference in the construction of the two.
Construction of Power Diodes
A normal signal-level diode has only two layers of semiconductor materials n-type layer and p-type layer but a Power diode has one extra layer known as Drift Region thus making total three layers in it namely; p + layer, n – layer and n + layer (+ means highly doped and – means lightly doped e.g. p + means highly doped p-type layer). This extra n – layer is known as Drift Region.
The Drift Region is a lightly doped region with doping concentration of 1014 / cm3 (it means 1014 dopants are introduced per cm3 volume of intrinsic semiconductor). It has a lower doping concentration as compared to the p + and n + regions with 1019 / cm3 doping concentration.
With such low doping concentration, the resistivity of the Drift Region is very high, since for semiconductor materials
Fabrication of Power Diodes
Fabrication means the manufacturing process. The modern-day technique used for the manufacturing of the Diodes is the VLSI technique which stands for Very Large Scale Integration.
Mainly two processes are used in the VLSI technique for manufacturing diodes:
- Epitaxy or Epitaxial growth
In this process, dopants are diffused into the simple undoped Si material to prepare desired p-type or n-type layer. The word Diffusion signifies the same meaning of motion of particles from higher concentration to lower concentration. e.g. the diffusion of donor atoms into the intrinsic Si-semiconductor forms n-type material. And the diffusion of the acceptor atoms into the n-type material forms the p – n junction diode.
The diffusion process is usually carried out at high temperatures around 1000 – 1200°C.
In this process, silane gas (SiH4) is passed over the Si-material to form another deposited layer of Si above the previous one. This new Si-layer has a different crystal structure than the original one. Hence the epitaxially grown Si-layer can be doped with a different concentration of dopants.
To fabricate Power Diodes, an n+ type substrate is taken on which Epitaxy grows another Si-layer. This epitaxial layer is doped with a low concentration of donor atoms by Diffusion to form n- Drift Region. And finally, the acceptor atoms are diffused into n – layer to form the p+ layer and hence the Power diode is ready.
V-I Characteristics (Static Characteristics) of Power Diodes
There is a little difference between the V – I characteristics, sometimes known as Static characteristics, of the signal-level diodes and power diodes.
- We see how the V – I characteristics of the standard diode look like,
It follows the famous Shockley Diode equation:
I = current flowing through the Diode.
Is = Reverse Saturation Current.
VD = Voltage across the Diode.
VT = Thermal Voltage.
n = Ideality Factor of the Diode.
- V – I Characteristics of the Power Diode is given below
It looks similar to the characteristics of the signal-level diode, but the difference lies in the forward region. In the Forward Biased region, the Power Diode has Linear features (shown by the straight line in the above V-I characteristics) instead of the Exponential Curve.
The reason for this linearity is the Drift Region and the large current flowing through the diode. How?
- Since a large current flowing through the power diodes, the exponential curve becomes linear-one because, for the small increase in the voltage beyond 1V (in the above figure), the significant change in current makes the curve linear.
- Also due to the high resistance of the Drift Region, the diode behaves as a resistor in the forward region hence the voltage drop varies linearly with the current.