IGBT(Insulated gate bipolar Thyristor )
What is IGBT?
“INSULATED GATE BIPOLAR TRANSISTOR IS COMMONLY REFERRED AS AN HYBRID VERSION OF POWER MOSFET AND POWER BJT POSSESSING KEY FEATURES AND QUALITIES OF BOTH.”
Power BJT and power MOSFET both have found to be great for power switching applications in power electronics with unique set of qualities.But as we know no component is ideal and all are restricted by some or the other limitations.Let us discuss some setbacks of power BJT and power MOSFET. Before jumping on to limitations,you may want to consider checking out our detailed posts on power MOSFET and power BJT.
Limitations of power BJT
- It needed large base current to operate the BJT in saturation mode.
- Not efficient for fast switching applications.
- Potential risk to BJT due to secondary breakdown.
Limitations of power MOSFET
- Very high on state power dissipation.
- Inefficiency in blocking high reverse voltages.
- Very high on-state voltage drop.
IGBT due to its hybrid nature of structure possess qualities and key features of both power MOSFET and power BJT and compensates limitations of both the devices efficiently.Lets try to understand how IGBT exactly work.
Above shown is the most common way to symbolize IGBT. There are other ways of representing IGBT but we will stick to the conventional way defined by the industry experts. It varies from power BJT in the sense that a gate terminal replaces base terminal while collector and emitter contacts remain the same.That means IGBT acts as a voltage controlled device since its input characteristics is similar to input characteristics of power MOSFET. Output characteristics are similar to that of a power BJT.
IGBT BASIC STRUCTURE
As you can see IGBT is a vertically oriented structure that is identical to enhancement type power MOSFET except for having a p+ layer at the collector terminal instead of n+ layer and extra n+ buffer layer just above the p+ layer.Inclusion of this extra n+ layer depends upon the need of the application. N- drift layer helps to increase breakdown voltage capacity of the device
NOTE:If fast switching is the need of application,then n+ layer is added to the structure to decrease the turn off time of the device.This type of structure is commonly referred as punched through IGBT in power electronics world.
However if symmetrical blocking capacity(i.e. approximately equal blocking capacity of reverse and forward voltages) is the need of the application then non-punch through IGBT are preferred and n+ buffer layer is not included in the structure.
So how does an IGBT work?
Operation and VI characteristics of an IGBT is similar to input characteristics of power MOSFET and output characteristics of PNP bipolar junction transistor.As discussed earlier the input to the base terminal now the gate terminal of MOSFET. Thus as soon as we apply positive gate to source(emitter) voltage an induced n-channel is formed at the gate terminal of MOSFET. Thus conduction in the device starts as soon as induced channel is formed.Till now the conduction is similar to that of a power MOSFET.
Curious to know how the formation of induced channel take place in a power MOSFET right?We have got it covered in our post on power MOSFET.
Remember we had discussed earlier that an IGBT has a p+ region at collector(drain) terminal instead of an n+ region.The presence of n+ region in power MOSFET used to result in on-state voltage and thus power loss.Thus in IGBT a p channel at the collector enhances the conductivity of IGBT and thus overall power loss is reduced to a great extent.To start conduction we have to apply positive voltage across collector(drain) to source(emitter).The electrons start travelling from source(emitter) towards collector(drain) passing through induced channel formed at body region [due to gate to source(emitter) voltage] towards (n-) drift region.Note that we are talking here about flow of electrons not current flow.
Refer to the construction of IGBT above and you will notice that a forward bias will get formed between the p+ and n+ buffer layer.This forward biased PN junction will inject holes into n+ buffer layer.
Now electrons present at n- drift region are attracted towards holes at n+ buffer layer and conduction starts.Thus in this way IGBT effectively speed ups the conductivity process reducing the resistance in conduction to minimum and thus reducing on-state power dissipation due to high voltage loss.
Thus we saw how IGBT compensates for high power dissipation in power MOSFET and how switching speed of the device is increased than that of power BJT. Let’s have a look at some of its applications.
- SMPS(switch mode power supply).
- UPS(uninterrupted power supply).
- Power control circuits like inverters,choppers,etc.
- Inductive heating applications.
- Audio amplifiers in speakers.
- Motor drives in locomotives.
- IGBT are best alternative as power transistors in medium switching applications.