The insulated gate bipolar transistor (IGBT) is a semiconductor device developed with combined characteristics of MOSFET and BJT. It has emitter-collector characteristics as BJT and control features of MOSFET. IGBTs have high OFF-state and low ON-state voltage characteristics of BJT and high input impedance characteristics of MOSFET. In this article let us see the characteristics of IGBT.

Static V-I Characteristics of IGBT :

The steady-state V-I characteristics of n-channel IGBT are shown below. The V-I characteristics of IGBT are plotted between output or collector current I_C and collector-emitter V_CE for different values of gate-to-emitter voltage V_GE.

The IGBT is turned ON when the gate voltage applied is greater than the threshold value and can be turned OFF by reducing gate voltage below the threshold value. The output current I_C will increase with an increase in voltage V_GE. The V_CE breakdown is the forward breakdown voltage above which the current and voltage through the device will high resulting in huge power dissipation.

Transfer Characteristics of IGBT :

The transfer characteristics of IGBT are drawn between output collector current I_C and gate-emitter voltage V_GE as shown below. These characteristics are similar to a power MOSFET that IGBT will start conducting only when applied V_GE is greater than threshold value V_GE(th).

Switching Characteristics of IGBT :

The below shows the switching characteristics of IGBT. A positive voltage is applied across the gate-emitter terminals to turn IGBT. When gate voltage becomes more than the threshold value, the collector current I_C starts flowing and the collector-emitter voltage V_CE starts falling.

The delay by gate-emitter voltage V_GE to reach the threshold value and I_C starts increasing is known as a turn-ON delay (t_d(on)). The rise time (t_r) is defined as the time in which collector current I_C reaches its full value and collector-emitter voltage V_CE falls to the minimum value. The turn ON time of IGBT is,

In order to turn OFF the MOSFET, the gate voltage is reduced. When the gate-emitter voltage reaches the value equal to V_GE1 (voltage at which IGBT comes out saturation), V_CE starts increasing. The time taken to reduce the voltage to V_GE1 is known as turn-OFF delay t_d(off).

When V_CE reaches supply voltage, collector current I_C starts reducing at a rapid rate and falls until gate voltage V_GE reaches to threshold value V_GE(th). This rapid decrease in collector current is basically due to internal MOSFET. The V_GE goes to zero and becomes negative.

Even after gate voltage reaches zero, collector current I_C flows for a while. This flow of collector current is due to stored carriers and it is internal BJT current. Thus the turn-OFF time of IGBT is higher than MOSFET and is given as,

Where

• t_rv = Voltage rise time
• t_fi1 = MOSFET current fall time
• t_fi2 = BJT current fall time.

A voltage in the range of +10V to +15V is usually applied to the gate to turn ON an IGBT and can be turned OFF by bringing the gate voltage to zero value. Some of the advantages of IGBT are low ON-state voltage drop, simple gate drive, low switching losses, etc. Though IGBTs are more expensive than BJT, their advantages make them favored in many applications.