Operating principle of an IGBT
Operating Principle of an IGBT:
- The input side the IGBT behaves essentially as a MOSFET. Therefore, when the gate emitter voltage is less then the threshold voltage no inversion layer is formed in the p type body region and the device is in the off state.
- The forward voltage applied between the collector and the emitter drops almost entirely across the junction J2. Very small leakage current flows through the device under this condition. In terms of the equivalent current, when the gate emitter voltage is lower than the threshold voltage the driving MOSFET of the Darlington configuration remains off and hence the output p-n-p transistor also remains off.
- When the gate emitter voltage exceeds the threshold, an inversion layer forms in the p type body region under the gate. This inversion layer (channel) shorts the emitter and the drain drift layer and an electron current flow from the emitter through this channel to the drain drift region.
- This in turn causes substantial hole injection from the p type collector to the drain drift region. A portion of these holes recombine with the electrons arriving at the drain drift region through the channel.
- The rest of the holes cross the drift region to reach the p type body where they are collected by the source metallization.
- The n type drain drift region acts as the base of the output p-n-p transistor. The doping level and the thickness of this layer determines the current gain “∝”of the p-n-p transistor. This is intentionally kept low so that most of the device current flows through the MOSFET and not the output p-n-p transistor collector.
- This helps to reduce the voltage drop across the “body” spreading resistance shown in Fig 7.2 (b) and eliminate the possibility of static latch up of the IGBT.
- The total on state voltage drop across a conducting IGBT has three components. The voltage drop across J1 follows the usual exponential law of a pn junction.
- The next component of the voltage drop is due to the drain drift region resistance. This component in an IGBT is considerably lower compared to a MOSFET due to strong conductivity modulation by the injected minority carriers from the collector.
- This is the main reason for reduced voltage drop across an IGBT compared to an equivalent MOSFET. The last component of the voltage drop across an IGBT is due to the channel resistance and its magnitude is equal to that of a comparable MOSFET.