At a glance of IGBT
In today’s blog, I will explain to you the IGBT in the simplest form. At first, we will learn about the Construction and Classification of such as Punch through IGBT and Non Punch through IGBT. In operation, we will learn about the Creation of inversion layer and Conductivity module, also the characteristics of IGBT.
Introduction to IGBT
IGBT stands for insulated gate bipolar transistor.
IGBT has the combined characteristics of MOSFET and BJT, the input characteristics are similar to MOSFET, and the output characteristics are identical to BJT. Still, it has no secondary breakdown problem like BJT.
The IGBT provides high input impedance and low on-state conduction loss. Also, IGBT is a voltage control device and has a greater turn-off time.
Construction of IGBT
As shown in the above figure, IGBT has a vertically oriented structure used to maximize the area available for current flow. Due to this, the resistance offered by the device is reduced; hence, the on-state power losses are decreased.
IGBT is constructed on a P+ layer substrate. And on the P+ substrate, a high resistivity N- layer is epitaxially grown. (The condensation of gas that produces another compound to form a film on the substrate). The thickness of N- layer decides the voltage blocking capacity of IGBT.
On P+ substrate, the metal layer is deposited to form a collector terminal. The P regions are diffused in epitaxially grown N– layer. Further, the N+ region is diffused in the P region. However, the insulating layer of SiO2 is grown on the surface. This insulating layer is itched to embed metallic emitter and gate terminals. Hence the name of the device shows an insulated gate.
The P+ substrate layer is also called as injector layer because it injects holes into the N – layer. Also, N- the layer is the drift layer, and the P- layer is known as the body of IGBT.
The N- layer between the P+ injection layer and P body region serves to accommodate the depletion layer of the PN junction that is J2.
Buffer layer
In addition to this structure, IGBT consists of a buffer layer that is not essential for the operation but increases efficiency.
The IGBT in which the buffer layer is present is known as punch-through IGBT. And when the buffer layer is absent, then the IGBT is known as non-punch through IGBT.
In the previous blog, we discussed MOSFET; if you can observe the Gate terminal carefully is the same for both devices. Instead of drain and source terminals, the collector and emitter terminals are used as a transistor. The emitter acts as a source terminal, and the collector serves as a drain terminal. However, in IGBT, there is a PNPN thyristors structure between collector and emitter terminals.
Classification of IGBT
As I mentioned earlier, there are two types of IGT according to the presence of the buffer layer.
- Symmetric IGBT (non-punch through)
- Asymmetric IGBT( punch through)
Non – Punch through IGBT(Symmetric)
N+ buffer layer is absent in the non-punch through IGBT.
In the forward blocking mode, when the collector is positive and the emitter is negative, the forward supply applied to junction J1 and J3 are in the forward bias, and junction J2 will be reverse biased.
As junction J2 is reverse biased, it decides the blocking capacity of the device. The junction J2 will be formed between moderately doped P body layer and lightly doped N– drift layer. Also, the thickness of N– layer decides the forward blocking capacity of the device.
When the reverse voltage is applied across the IGBT that is collector is negative, and the emitter is positive. Then the device works in reverse blocking mode. At that time the junction J2 becomes forward biased, and junction J1 and J3 become reverse biased. Hence the junction J1 and J3 decide the reverse blocking capacity of the device.
The source layer, N+, is heavily doped, and the P- layer is thin and moderately doped. Therefore the J1 cannot block the applied reverse voltage. However, the opposite happens with junction J3. It blocks the reverse voltage due to the presence of N– layer.
The drift layer is designed to block large voltage. Still, due to the absence of a buffer layer, this device provides symmetric blocking and hence the name symmetric IGBT. Also, due to this property, symmetric IGBT is used in rectifier circuits.
Punch through IGBT ( Asymmetric)
In this type of IGBT, the buffer layer is present between the drift and injection layers.
When the forward voltage is applied to the terminals, the collector terminal is positive concerning the emitter. The junction J1 and J3 become forward bias, and junction J2 becomes reverse biased. The forward blocking capacity. Junction J2 is formed between body layer (P) and N–, drift layer thickness decides the forward blocking capacity of the device.
If the reverse voltage is applied across the collector and emitter, junction J2 becomes forward biased; however, junction J1 and J3 become reverse biased. The reverse blocking voltage is decided by the J1 and J3 junction.
As I explained in symmetric IGBT, J1 cannot block large voltages. Due to the presence of N+ J3 junction is unable to block large reverse voltage; however, the device will break down at low reverse voltage. Thus it has an asymmetric blocking capacity and fast turn-off speed. Hence this type of IGBT can be used in inverters.
Operating principle of IGBT
The operation of IGBT is divided into two parts
- Creation of inversion layer
- Conductivity module
1. Creation of inversion layer
As the name suggests in the first step of operation, an inversion layer is created inside the IGBT. You all might have to wonder how? So let me explain to you first.
This work creates an inversion layer. when the applied gate to source voltage (VGS) is greater than the threshold gate to source voltage (VGS threshold), beneath the Sio2 layer is an N-type inversion layer is created.
As you can see from the figure, due to the creation of the N- layer in the P-type body layer, there will be an (N+ -N- N–) channel is created. This channel helps electrons to travel and cause the conduction of current.
Before creating the inversion layer, you can imagine that there was no pathway for electrons to flow from emitter to collector, and hence no current conduction happened.
2. Conductivity module
In simple language, all the activity in the device happens inside the drift layer of IGBT in the conductivity module.
After creating the inversion layer, when the forward voltage is applied across the emitter and collector junction, J3 will become forward biased.
As the inversion layer is created, the electron from the emitter will get injected into the drift layer by following the path (N+ -N- N–).
As junction J3 is already forward biased, it will inject the hole in the N+ buffer layer. The electrons present inside the drift region will attract the holes injected by P+ from the buffer layer to the drift layer.
As you can see, the double ejection of electrons and holes takes place in the drift region from both sides; however, this increases the conductivity of the drift layer and reduces the resistance of the device to its minimum.
This conductivity module helps in the reduction of on-state voltage across the IGBT.
Characteristics of IGBT
As I mentioned earlier, IGBT works the same as logic level BJT when moving forward. The only difference between them is that IGBT voltage between gate to source VGS acts as a controlling parameter, and the drain current ID is controlled.
When IGBT is in the off state that is no voltage is applied, the gate current will be zero, and the voltage applied across IGBT is equal to source voltage. However, when the applied voltage VGS is greater than the threshold voltage, the device turns on. The current allows for flowing through the device.
This current is limited by source voltage and load resistance; the off-state voltage across IGBT drops to zero after the current starts flowing. As we applied the positive voltage to the gate and source terminal (VGS), the collector current (drain current) increases with the gate to source voltage increase.
Forward breakdown voltage: Forward breakdown voltage is the voltage at which avalanche breakdown takes place.
When we apply the forward voltage to the device, breakdown occurs; for instance, the forward breakdown voltage and current become very high. Causing large power dissipation, this high power can cause the device to get damaged. Hence the IGBT should always operate below forward breakdown voltage
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