Insulated Gate Bipolar Transistor | IGBT

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.

There was a need for a device that combined the best features of both PMOSFETs and Power BJTs. IGBT was introduced in the early 1980s, quickly becoming popular among engineers for its superior characteristics. IGBT has PMOSFET-like input characteristics and Power BJT-like output characteristics. Its three terminals are Gate, Collector, and Emitter.

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. R_d is the resistance offered by the drift region and R_b 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 (>V_GET) 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 I_c in IGBT constitutes of two components- I_e and I_h. I_e is the current due to injected electrons flowing from collector to emitter through injection layer, drift layer and finally the channel formed. I_h is the hole current flowing from collector to emitter through Q₁ and body resistance R_b. Hence

Although I_h is almost negligible and hence I_c ≈ I_e.
A peculiar phenomenon called latching up occurs in IGBTs when the collector current exceeds a certain threshold (I_CE). The parasitic thyristor latches, and the gate terminal loses control over the collector current, making the IGBT unable to turn off even if the gate voltage drops below V_GET. To turn off the IGBT, typical commutation circuits are needed, similar to forced commutation of thyristors. If not turned off quickly, the device may be 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 V_GE because IGBT is a voltage controlled device unlike BJT which is a current controlled device. When the device is in OFF mode (V_CE is positive and V_GE < V_GET) the reverse voltage is blocked by J₂ and when it is reverse biased, i.e. V_CE 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 V_GE is greater than a threshold value V_GET.

Switching Characteristics of IGBT

The figure below shows the typical switching characteristic of IGBT.

Turn-on time (t_on) consists of two parts: delay time (t_dn) and rise time (t_r). Delay time is when the collector current rises from leakage current (I_CE) to 0.1 IC and the collector-emitter voltage drops from V_CE to 0.9V_CE. Rise time is when the collector current increases from 0.1 I_C to I_C and the collector-emitter voltage falls from 0.9V_CE to 0.1 V_CE.

The turn off time t_off consists of three components, delay time (t_df), initial fall time (t_f1) and final fall time (t_f2). Delay time is defined as time when collector current falls from I_C to 0.9 I_C and V_CE begins to rise. Initial fall time is the time during which collector current falls from 0.9 I_C to 0.2 I_C and collector emitter voltage rises to 0.1 V_CE. The final fall time is defined as time during which collector current falls from 0.2 I_C to 0.1 I_C and 0.1V_CE rises to final value V_CE.

Advantages and Disadvantages of IGBT

Advantages:-
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:-
Disadvantages of IGBT are showing below

• Cost
• Latching-up problem
• High turn off time compared to PMOSFET