The Bipolar Junction Transistor (BJT) and the MOSFET are the most popular and widely used power electronic switch devices. We’ve already gone over how BJTs and MOSFETs work and how they’re used in circuits in great detail. However, both of these components had some limitations when used in very high current applications. As a result, we moved on to another popular power electronic switching device known as the insulated-gate bipolar transistor or IGBT.
Think of an IGBT as a fusion of a BJT and a MOSFET; these components have the input characteristics of a BJT and the output characteristics of a MOSFET. In this article, we’ll go over the fundamentals of IGBTs, how they work, and how to incorporate them into your circuit designs.
What is an IGBT?
Insulated Gate Bipolar Transistor (IGBT) is a three-terminal semiconductor switching device that can be used in a variety of electronic devices to perform fast switching with high efficiency. These devices are primarily used in amplifiers to switch/process complex wave patterns with pulse width modulation (PWM).
The typical IGBT symbol and image are shown below.
As previously stated, an IGBT is a hybrid of a BJT and a MOSFET. The IGBT symbol depicts the same thing; 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 conduction terminals are the Collector and Emitter, while the gate is the control terminal that controls the switching operation.
The Internal Structure of an IGBT
Since the IGBT has the output of the following combination of the PNP transistor, NPN transistor, and MOSFET, it can be built with the equivalent circuit of two transistors and MOSFET. IGBTs combine the low saturation voltage of a transistor with the high input impedance and switching speed of a MOSFET.
The result of this combination has the output switching and conduction characteristics of a bipolar transistor, but the voltage is controlled like a MOSFET.
Because an IGBT is a combination of a MOSFET and a BJT, they are also known by different names. Insulated Gate Transistor (IGT), Metal Oxide Insulated Gate Transistor (MOSIGT), Gain Modulated Field Effect Transistor (GEMFET), and Conductively Modulated Field Effect Transistor are the various names for IGBT (COMFET).
IGBTs have three terminals connected to three different metal layers, with the metal layer of the gate terminal insulated from the semiconductors by a layer of silicon dioxide (SIO2). IGBTs are built with four layers of semiconductor sandwiched together.
The p+ substrate layer is closest to the collector, followed by the n- layer, another p layer is kept closer to the emitter, and the n+ layers are kept inside the p-layer. The junction between the p+ layer and the n- layer is known as J2, and the junction between the n- layer and the p layer is known as J1. The figure below depicts the structure of an IGBT.
Consider a voltage source VG connected positively to the Gate terminal with respect to the Emitter to understand how the IGBT works. Consider another voltage source VCC connected across the Emitter and Collector, with the Collector kept positive in relation to the Emitter.
The junction J1 will be forward biased as a result of the voltage source VCC, whereas the junction J2 will be reverse biased. There will be no current flow inside the IGBT because J2 is in reverse bias (from collector to emitter).
Since no voltage is applied to the Gate terminal, the IGBT will be in a non-conductive state at this point. If we increase the applied gate voltage, the negative ions will accumulate on the upper side of the SiO2 layer due to the capacitance effect, and the positive ions will accumulate on the lower side of the SiO2 layer due to the capacitance effect on the SiO2 layer. This will result in the insertion of negative charge carriers in the p region; the higher the applied voltage VG, the more negatively charged carriers will be inserted.
This will result in the formation of a channel between the J2 junction, allowing current to flow from collector to emitter. The current path in the diagram represents the flow of current; as the Gate voltage VG increases, so does the amount of current flowing from the collector to the emitter.
Types of IGBT
The IGBT is classified into two types based on the n+ buffer layer; those with an n+ buffer layer are known as Punch through IGBTs (PT-IGBTs), while those without an n+ buffer layer are known as Non-Punch Through-IGBTs (NPT- IGBT).
The NPT-IGBT and PT-IGBT are referred to as symmetrical and nonsymmetrical IGBTs, respectively, based on their characteristics. IGBTs with symmetrical breakdown voltages have equal forward and reverse breakdown voltages. Asymmetric IGBTs have a reverse breakdown voltage that is less than the forward breakdown voltage. Because they do not need to support voltage in the reverse direction, symmetrical IGBTs are mostly used in AC circuits, whereas asymmetrical IGBTs are mostly used in DC circuits.
Operation of IGBT as a Circuit
Since IGBT is a combination of BJT and MOSFET, let’s look into their operations as a circuit diagram here. The below diagram shows the internal circuit of IGBT which includes two BJT and one MOSFET and a JFET. The Gate, Collector, and Emitter pins of the IGBT are marked below.
The collector of the PNP transistor is connected to the NPN transistor via a JFET, which connects the collector of the PNP transistor and the base of the PNP transistor. These transistors are connected in such a way that they form a parasitic thyristor with a negative feedback loop.
The Resistor RB is used to short the base and emitter terminals of the NPN transistor, preventing the thyristor from latching up and causing the IGBT to latch up. The JFET used here represents the current structure between any two IGBT cells and allows the MOSFET and supports the majority of the voltage.
Switching Characteristics of IGBT
The IGBT is a voltage-controlled device that can be driven by an IGBT driver. Because these are unidirectional devices, they can only switch current in one direction: from collector to emitter.
The gate volt VG is applied to the gate pin to switch a motor (M) from a supply voltage V+ in a typical IGBT switching circuit shown below. Rs is used to limit the current flowing through the motor.
The graph below explains the input characteristics of an IGBT. When no voltage is applied to the gate pin, the IGBT is turned off and no current flows through the collector pin.
When the voltage applied to the gate pin exceeds the threshold voltage, the IGBT begins to conduct and the collector current IG begins to flow between the collector and emitter terminals. The collector current increases with respect to the gate voltage.
The output characteristics of an IGBT are divided into three stages. Initially, when the Gate Voltage VGE is zero, the device is in the off state, which is referred to as the cutoff region. When VGE is increased, if it is less than the threshold voltage, a small leakage current flows through the device, but it remains in the cutoff region. When the VGE voltage is raised above the threshold voltage, the device enters the active region and current begins to flow through it.
IGBTs are used in a variety of applications, including AC and DC motor drives, Unregulated Power Supplies (UPS), Switch Mode Power Supplies (SMPS), traction motor control and induction heating, IGBT inverters, which combine an isolated-gate FET for the control input and a bipolar power transistor as a switch in a single device, and so on.