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At a Glance of power diode

Today’s blog Discusses power diode in which we will deeply study working, construction, characteristics of power diode, and its classification of power diode, Punch Through Diode, and Non- Punch Through Diode.

The following block diagram will show the road map of the blog

working, construction, characteristics of power diode, and its classification of power diode, Punch Through Diode, and Non- Punch Through Diode.

Introduction

A power diode is a “power level” counterpart of the Low power signal diodes.

When the power diode is forward bias, it serves high current, i.e., KA (Kilo-Ampere). And when it is reverse biased, it blocks several KA of current. Power diode, as the name suggests it has high power handling capacity and low switching speed.

A small amount of change in the PN junction diode will significantly affect the operating characteristics. Due to which is used to perform Rectification and freewheeling the energy feedback.

Basically, the PN junction diode and power diode are like Nolan’s Batman and Batman from the Snyder cut. Basically, they serve the same purpose, but it’s a matter of power and strength.

Conditions for Construction of Power Diode

As mentioned in the introduction, the power diode has a high Reverse blocking voltage. To achieve this, two conditions must be fulfilled

Wide depletion layer to restrict the maximum electric field strength
Space charge density in the depletion region must remain low to get a wide depletion region for maximum electric field strength.

Space charge density can be explained as the electrons from the N- side occupying the P- side holes and vice versa on both sides of the junction to create a small space of net charge.

These two conditions can be nullified by adding a lightly doped drift layer of required thickness between two heavily doped P and N layers.

Construction of Power Diode

From the above fig, you can observe that the Power Diode construction shows that it is similar to the PN junction diode. Just the extra drift region is added between heavily doped N⁺ and P⁺ regions. (just like in terms of Batman, it’s the difference of suits that makes Snyder’s Batman more powerful.)

The structure of the power diode is of vertical nature. Thus, it allows more power and increases surface area for forwarding current, hence reducing forward resistance and state power loss.

The PN junction is formed by defusing heavily doped P⁺region, which acts as an Anode into N^– layer which acts as a Cathode.

P⁺Layer

Anode Metallization is connected to the highly doped P-type material. This P-type heavily doped layer has a thickness of 10μm, and 10¹⁹ cm³ number of impurities are added in the P⁺ region. This layer acts as an Anode.

N⁺ substrate Layer

Cathode Metallization is connected to the highly doped N-type material;

this N-type heavily doped layer has a thickness of 250-300 μm, and the 10¹⁹ cm³ number of impurities are added in the N⁺ region.

This layer acts as the cathode.

The impurity atom density of a cathode and anode is 10¹⁹ cm^3, and it is identical in magnitude.

N^–Epitaxial Layer

Lightly doped N^–Epitaxial Layer of specified width (the width depends upon Breakdown Voltage.) is grown on N⁺heavily doped substrate acts as a Cathode.

For the low power diode, i.e., for PN junction, the diode Drift Layer is absent.

There are two essential functions of Drift Layer

To increase voltage handling capacity.
To control reverse breakdown voltage, as the width of the drift layer increases, it can handle high reverse breakdown voltage.

As the drift layer increases, PIV voltage is also increased. PIV stands for Peak Inverse Voltage. It is the maximum Voltage that a diode rectifier can block.

As the drift layer is lightly doped, the resistivity is more with the drift layer, causing more on-state voltage drop. On-state voltage drop can be defined as when the power diode is conducting in forward bias. Practically there is a voltage drop across the device.

Larger the cross-section area of the drift layer, the higher the current handling capacity is. The larger the width of the depletion layer, the higher the voltage handling capacity.

Working of Power Diode

The operating principle of the power diode is similar to the PN junction diode. A diode conducts when Anode voltage is higher than Cathode voltage. When the Cathode voltage is higher than the Anode voltage, the diode works in blocking mode.

Forward Bias condition

When the Anode terminal is connected to the positive terminal and the cathode terminal is connected to the negative terminal, the power diode becomes forward biased.

There will be an injection of the excess p-type carrier into the n- side. All excess p-type carriers recombine with n-type carriers in the drift region at the low level of injections.

However, at a high injection level (i.e., when a large forward current is applied), the excess p-type carrier reaches the n+ junction and attracts electrons from the n+ cathode. This leads to electron injection into the drift region across the n+ junction.

This is called “double injection” in this Excess p and n-type carriers defuse and recombine inside the drift region. Suppose the width of the drift region is less than the diffusion length of carries. In that case, the distribution of excess carriers in the drift region will be fairly flat.

The Conductivity of the drift region will be significantly enhanced; consequently, this is also known as conductivity modulation.

Reverse Bias Condition

When the diode is Reverse Biased, only a small leakage current (less than 100mA for a rated forward current above 1000A) flows in the reverse direction (i.e., from cathode to anode).

This reverse current is independent of the applied reverse Voltage when the applied reverse Voltage reaches the breakdown voltage. Reverse current increases rapidly due to impact ionization and consequent avalanche multiplication process.

When the reverse current is limited by an external circuit, Voltage across the device does not increase any further. Excessive power loss and consequent increase in the junction temperature due to continued operation in the reverse breakdown region quickly destroys the diode. The continued operation in reverse bias condition should be avoided.

Classification of Power Diode

The above fig: Power Diode can be classified into two categories based on Reverse Recovery Time and Penetration of Depletion Region.

Based on Reverse Recovery Time

1. General Purpose Diode

The general-purpose diode is the PN junction power diode used in low-frequency applications. It can be turned off quickly, so it is also used in the high-temperature application.

The general purpose PN junction power diode has a high recovery time of 20μs -30μs. The Conversion and Rectification Frequency is low.

2. Fast Recovery Diode

As the name suggests, this diode has a low recovery time compared to the general-purpose PN junction diode. It is generally used for High-frequency applications due to fast recovery. This diode has a quick recovery time of 2μs – 3μs.

3. Schottky Diode

In our previous blog, we learned about the Schottky diode in detail.

The Schottky diode is a semiconductor diode having two terminals formed by diffusing an N-type semiconductor over metal.

The current flow is due to only the majority carriers (electrons); hence, it is a unipolar device.

It is named after the German physicist Walter H. Schottky. This diode is also known as Schottky barrier diode or hot–carrier diode.

The Schottky diode works while connected to forward bias, the same as the PN junction diode. There are three main differences between the PN junction diode and the Schottky diode.

Low forward drop – for the Schottky diode, it is 0.3 to 0.5, but for the PN junction diode, it is 0.6 to 0.7.
Dissipates less power and generates less heat
Reverse recovery time is high (recovery time states how fast the diode responds to switching); the switching speed is also high.

If we connect the diode to supply and load, it will act as a rectifier while providing current to the load.

Based on Penetration of Depletion Region.

1. Punch Through Diode

The punch-through diode contains three regions of alternate conductivity type.

Punch Through characteristics of this diode is similar to the Breakdown characteristics of the Zener diode when the applied Voltage across the diode is equal to the Punch through Voltage.

2. Non- Punch Through Diode

The width of the depletion region of the breakdown does not penetrate (punch through) into the neighboring N⁺Layer.

Difference between Punch Through Diode and Non- Punch Through Diode

Punch Through Diode	Non- Punch Through Diode
The Punch through diode depletion layer spans the entire drift region, and it is in contact with N⁺ Cathode.	The non – punch-through diode depletion region boundary doesn’t reach the end of the drift layer.
In the Punch through construction, Field strength is more uniform.	The electric field strength is maximum at the P⁺junction and decreases to zero at the depletion region.
Drift region doping is low	Drift region doping is high
Punch through a diode does not carry the penalty of high conduction losses due to conductivity modulation.	Non – punch-through diode carries the penalty of high conduction losses.
Due to the reduced width of the depletion region, the on-state voltage drop is low.	On-state voltage drop is high as compared to punch through the diode.

Characteristics of Power Diode

Forward bias

When the power diode is forward biased, that is, anode potential is higher than cathode potential. The characteristics behavior for this condition is the same as the PN junction diode. The slight change is that it is on state voltage drop is more, i.e., of 1v.

The relation between slop and resistance is inversely proportional to each other. When the diode is forward biased, the on-state voltage drop increases, which causes an increase in the resistance. This ohmic drop makes forward I-V characteristics of the power diode more linear.

Important terms related to forward bias characteristics of power diode

I. Maximum RMS forward current –

It is the maximum allowable RMS value of forwarding current.

Conduction power loss can be determined by the RMS value of forwarding current.

II. Maximum average forward current –

The maximum average value of half-cycle sine wave current allowed to flow through the diode in the forward direction.

The maximum average forward current of the power diode is needed because it is often used to supply DC voltage in rectifier circuits.

III. Average Forward power loss –

Average forward power loss can be explained as a function of average forward current for different conduction angles.

It is crucial for designing cooling arrangements.

Reverse Bias

When the power diode is reverse biased, that cathode potential is higher than the anode potential. Only a tiny amount of leakage current will flow at the start. Still, after the voltage diode breakdown happens, the high current starts flowing through the diode.

Important terms related to forward bias characteristics of power diode

I. DC blocking voltage

Maximum direct Voltage can be applied in the reverse direction across the device for an indefinite period.

This DC blocking Voltage helps select a freewheeling diode for DC-DC Chopper and DC – AC voltage source inverter.

II. RMS Reverse voltage

The RMS value of power frequency (50Hz/60Hz) and the wave voltage can be directly applied across the device.

RMS reverse voltage helps to select diodes for AC – DC rectifier

Switching characteristics of power diode

Power Diodes take less time to change conduction from reverse bias to forward bias condition (switch ON) and vice versa (switch OFF). The behavior of the current flowing through the diode and Voltage applied across it during these switching periods are essential due to the following reasons.

• Higher voltage/current may be caused by a diode switching at different circuits using the diode.

• Voltage and current are present during the switching operation of a diode. For every switching time, a slight loss occurs at the diode. At high switching frequency, this may cause the overall power loss in the diode.

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