IGBT is a relatively new device in power electronics and before the advent of IGBT, Power MOSFETs and Power BJT were common in use in power electronic applications. Both of these devices possessed some advantages and simultaneously some disadvantages. On one hand, we had bad switching performance, low input impedance, secondary breakdown and current controlled Power BJT and on the other we had excellent conduction characteristics of it. Similarly, we had excellent switching characteristics, high input impedance, voltage controlled PMOSFETs, which also had bad conduction characteristics and problematic parasitic diode at higher ratings. Though the unipolar nature of PMOSFETs leads to low switching times, it also leads to high ON-state resistance as the voltage rating increases.
Thus the need was for such a device which had the goodness of both PMOSFETs and Power BJT and this was when IGBT was introduced in around the early 1980s and became very popular among power electronic engineers because of its superior characteristics. IGBT has PMOSFET like input characteristics and Power BJT like output characteristics and hence its symbol is also an amalgamation of the symbols of the two parent devices. The three terminals of IGBT are Gate, Collector and Emitter. The figure below shows the symbol of IGBT.
IGBT is known by various other names also, such as- Metal Oxide Insulated Gate Transistor (MOSIGT), Gain Modulated Field Effect Transistor (GEMFET), Conductively Modulated Field Effect Transistor (COMFET), Insulated Gate Transistor (IGT).
Structure of IGBT
The structure of IGBT is very much similar to that of PMOSFET, except one layer known as injection layer which is p+ unlike n+ substrate in PMOSFET. This injection layer is the key to the superior characteristics of IGBT. Other layers are called the drift and the body region. The two junctions are labeled J1 and J2. Figure below show the structure of n-channel IGBT.
Upon careful observation of the structure, we’ll find that there exists an n-channel MOSFET and two BJTs- Q1 and Q2 as shown in the figure. Q1 is p+n–p BJT and Q2 is n–pn + BJT. Rd is the resistance offered by the drift region and Rb is the resistance offered by p body region. We can observe that the collector of Q1 is same as base of Q2 and collector of Q2 is same as base of Q1. Hence we can arrive at an equivalent circuit model of IGBT as shown in the figure below.
The two transistor back to back connection forms a parasitic thyristor as shown in the above figure.
N-channel IGBT turns ON when the collector is at a positive potential with respect to emitter and gate also at sufficient positive potential (>VGET) with respect to emitted. This condition leads to the formation of an inversion layer just below the gate, leading to a channel formation and a current begins to flow from collector to emitter.
The collector current Ic in IGBT constitutes of two components- Ie and Ih. Ie is the current due to injected electrons flowing from collector to emitter through injection layer, drift layer and finally the channel formed. Ih is the hole current flowing from collector to emitter through Q1 and body resistance Rb. Hence
Although Ih is almost negligible and hence Ic ≈ Ie.
A peculiar phenomenon is observed in IGBT known as Latching up of IGBT. This occurs when collector current exceeds a certain threshold value (ICE). In this the parasitic thyristor gets latched up and the gate terminal loses control over collector current and IGBT fails to turn off even when gate potential is reduced below VGET. For turning OFF of IGBT now, we need typical commutation circuitry as in the case of forced commutation of thyristors. If the device is not turned off as soon as possible, it may get damaged.
Characteristics of IGBT
Static I-V Characteristics of IGBT
The figure below shows static i-v characteristics of an n-channel IGBT along with a circuit diagram with the parameters marked.
The graph is similar to that of a BJT except that the parameter which is kept constant for a plot is VGE because IGBT is a voltage controlled device unlike BJT which is a current controlled device. When the device is in OFF mode (VCE is positive and VGE < VGET) the reverse voltage is blocked by J2 and when it is reverse biased, i.e. VCE is negative, J1 blocks the voltage.
Transfer Characteristics of IGBT
Figure below shows the transfer characteristic of IGBT, which is exactly same as PMOSFET. The IGBT is in ON-state only after VGE is greater than a threshold value VGET.
Switching Characteristics of IGBT
The figure below shows the typical switching characteristic of IGBT.
Turn on time ton is composed of two components as usual, delay time (tdn) and rise time (tr). Delay time is defined as the time in which collector current rises from leakage current ICE to 0.1 IC (final collector current) and collector emitter voltage falls from VCE to 0.9VCE. Rise time is defined as the time in which collector current rises from 0.1 IC to IC and collector emitter voltage falls from 0.9VCE to 0.1 VCE.
The turn off time toff consists of three components, delay time (tdf), initial fall time (tf1) and final fall time (tf2). Delay time is defined as time when collector current falls from IC to 0.9 IC and VCE begins to rise. Initial fall time is the time during which collector current falls from 0.9 IC to 0.2 IC and collector emitter voltage rises to 0.1 VCE. The final fall time is defined as time during which collector current falls from 0.2 IC to 0.1 IC and 0.1VCE rises to final value VCE.
Advantages and Disadvantages of IGBT
Advantages of IGBT are showing below
- Lower gate drive requirements
- Low switching losses
- Small snubber circuitry requirements
- High input impedance
- Voltage controlled device
- Temperature coefficient of ON state resistance is positive and less than PMOSFET, hence less On-state voltage drop and power loss.
- Enhanced conduction due to bipolar nature
- Better Safe Operating Area
Disadvantages of IGBT are showing below
- Latching-up problem
- High turn off time compared to PMOSFET