Insulated Gate Bipolar Transistor
What is an 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.
As mentioned earlier an IGBT is a fusion between a BJT and MOSFET. The symbol of the IGBT also represents the same, as you can see the input side represents a MOSFET with a Gate terminal and the output side represents a BJT with Collector and Emitter. The Collector and the Emitter are the conduction terminals and the gate is the control terminal with which the switching operation is controlled.
Internal Structure of IGBT
IGBT can be constructed with the equivalent circuit that consists of two transistors and MOSFET, as the IGBT posses the output of the below combination of the PNP transistor, NPN transistor, and MOSFET. IGBT combines the low saturation voltage of a transistor with the high input impedance and switching speed of a MOSFET. The outcome obtained from this combination delivers the output switching and conduction characteristics of a bipolar transistor, but the voltage is controlled like a MOSFET.
Since IGBT is the combination of MOSFET and BJT they are also called by different names. The different names of IGBT are Insulated Gate Transistor( IGT), Metal Oxide Insulated Gate Transistor (MOSIGT), Gain Modulated Field Effect Transistor (GEMFET), Conductively Modulated Field Effect Transistor (COMFET).
Working of IGBT
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.
To understand the working of the IGBT, consider a voltage source VG connected positively to the Gate terminal with respect to the Emitter. Consider other voltage source VCC connected across The Emitter and the Collector, where Collector is kept positive with respect to the Emitter. Due to the voltage source VCC the junction J1 will be forward-biased whereas the junction J2 will be reverse biased. Since J2 is in reverse bias there will not be any current flow inside the IGBT(from collector to emitter).
Initially, consider that there is no voltage applied to the Gate terminal, at this stage the IGBT will be in a non-conductive state. Now if we increase the applied gate voltage, due to the capacitance effect on the SiO2 layer the negative ions will get accumulated on the upper side of the layer and the positive ions will get accumulated on the lower side of the SiO2 layer. This will cause the insertion of negative charge carriers in the p region, higher the applied voltage VG greater the insertion of negatively charged carriers. This will lead to a formation of the channel between the J2 junction which allows the flow of current from collector to emitter. The flow of current is represented as the current path in the picture, when the applied Gate voltage VG increases the amount of current flow from the collector to the emitter also increases.
Types of IGBT
The IGBT is classified as two types based on the n+ buffer layer, the IGBTs that are having the n+ buffer layer is called the Punch through IGBT (PT-IGBT), the IGBTs that does not have an n+ buffer layer are called the Non-Punch Through- IGBT (NPT- IGBT).
Based on their characteristics the NPT- IGBT, and PT-IGBT are named as symmetrical and nonsymmetrical IGBTs. The symmetrical IGBTs are the ones that have equal forward and reverse breakdown voltage. The asymmetric IGBTs are the ones that have a reverse breakdown voltage less than the forward breakdown voltage. The symmetrical IGBTs are mostly used in AC circuits, whereas the asymmetrical IGBTs are mostly used in DC circuits because they don’t need to support voltage in the reverse direction.
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