IGBT and their Characteristics

Power Electronics

1. IGBT and their Characteristics Name: Muhammad Moin MUET Jamshoro

2. IGBT IGBT is the short form of Insulated Gate Bipolar Transistor. It is a three-terminal semiconductor switching device that can be used for fast switching with high efficiency in many types of electronic devices. These devices are mostly used in amplifiers for switching/processing complex wave patters with pulse width modulation (PWM). The typical symbol of IGBT  along with its image is shown below .

3. We can see that the insulated gate bipolar transistor is a three terminal, trans conductance device that combines an insulated gate N-channel MOSFET input with a PNP bipolar transistor output connected in a type of Darlington configuration. As a result the terminals are labeled as: Collector, Emitter and Gate. Two of its terminals (C-E) are associated with the conductance path which passes current, while its third terminal (G) controls the device.

4. IGBT has three terminals attached to three different metal layers, the metal layer of the gate terminal is insulated from the semiconductors by a layer of silicon dioxide (SIO2).  IGBT is constructed with 4 layers of semiconductor sandwiched together. The layer closer to the collector is the p+ substrate layer above that is the n- layer, another p layer is kept closer to the emitter and inside the p layer, we have the n+ layers. The junction between the p+ layer and n- layer is called the junction J2 and the junction between the n- layer and the p layer is called the junction J1. The structure of IGBT is shown in the figure below .

5. VI Characteristics Of IGBT The input characteristics of IGBT can be understood from the graph below. Initially, when no voltage is applied to the gate pin the IGBT is in turn off condition and no current flows through the collector pin. When the voltage applied to the gate pin exceeds the threshold voltage, the IGBT starts conducting and the collector current I G  starts to flow between the collector and emitter terminals. The collector current increases with respect to the gate voltage as shown in the graph below.

6. The output characteristics of IGBT have three stages, initially, when the Gate Voltage V GE  is zero the device is in the off state, this is called the cutoff region. When V GE  is increased and if it is less than the threshold voltage then there will be a small leakage current flowing through the device, but the device will still be in the cutoff region. When the V GE  is increased beyond the threshold voltage the device goes into the active region and the current starts flowing through the device. The flow of current will increases with an increase in the voltage V GE  as shown in the graph above.  

7. Applications of IGBT: IGBT is mainly used in Power related applications. Standard power BJT’s have very slow response properties whereas MOSFET is suitable for fast switching application, but MOSFET is a costly choice where higher current rating is required. IGBT is suitable for replacing power BJTs and Power MOSFETs. Also, IGBT offers lower ‘ON’ resistance compared with BJTs and due to this property the IGBT is thermal efficient in high power related application. IGBT applications are vast in electronics field. Due to low on resistance, Very high current rating, high switching speed, zero gate drive, IGBTs are used in High power motor control, Inverters, switched mode power supply with high frequency converting areas.

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