Insulated-gate bipolar transistor

A bipolar transistor with insulated gate electrode (English insulated - gate bipolar transistor, just IGBT) is a semiconductor device that is used in the power electronics because there are benefits of the bipolar transistor ( good passage behavior, high blocking voltage, robustness) and advantages of a field effect transistor ( almost wattless control) merged. Also advantageous is a certain robustness with respect to short-circuiting, since the IGBT is limited to the load current.

There are a total of four different basic types of IGBTs, which are represented by four different circuit symbols. Depending on the doping of the base material n-and p-channel IGBTs can be produced. These are divided each into a self-conducting and a self-locking type. This property can be selected within the framework of the manufacturing process. In the self-routing switching symbols in IGBTs, also referred to as depletion type, a solid line between the collector terminals (C) and emitter (E) is drawn. This line is shown, stopped at the self-locking type, also called enhancement type. The gate terminal (G ) is used for all types as a control terminal.

Design and operation

IGBTs are a further development of the vertical power MOSFETs. The figure shows a vertical section through an n-channel IGBT.

The IGBT is a four-layered semiconductor device, which is controlled by a gate. It has a generally uniform, highly doped p-type substrate ( n-channel IGBT) with a specially designed p-n junction on the back. On the substrate a lightly doped n -type epitaxial layer is applied and then the p- cathode pans (sometimes highly doped) and highly doped n-type islands introduced by diffusion. Thus, a n pnp structure for an n- channel IGBT. P-channel IGBT according to have a p npn structure.

For the operation of the IGBTs of the p-n junction and the gate are responsible. The result is a Darlington circuit formed of an n -channel FET and a PNP transistor.

To the collector (relative to the emitter ) is applied a positive potential, so that the rear-side passage is located in the forward mode and not in the reverse blocking mode. The forward operation can be divided into two areas: in a lock - and in a passband.

As long as the threshold voltage ( gate-emitter voltage VGE ) is not reached the FET, the IGBT is in the off-state. , The voltage VGE increases, the IGBT reaches the passband. Is formed as in normal MIS field effect transistors below the gate in the p- tub, a cathode conductive n- channel. This allows the electron transport from the emitter into the epitaxial layer. Since the back pn junction is connected in the forward direction, holes are injected into the epitaxial layer from the p substrate, thereby creating a electron-hole plasma which provides the actual conduit. This plasma has to be or disconnects at every changeover, thereby resulting in higher switching losses than in the power MOSFET. Degradation of this plasma, it may also happen that the IGBT turns on again briefly.

As can be seen in the figure, the four layer semiconductor device involves the risk of the parasitic thyristor which is formed of the PNP transistor and a parasitic npn transistor. Similar to CMOS circuits in the IGBT, therefore, can get to the so-called latch-up effect, i.e., the thyristor is ignited, and a current flows, which can not be controlled by the gate.

Properties

  • Via the collector- emitter path of a bipolar transistor as in the IGBT falls from at least the threshold voltage. At rated current that is typically depending on the blocking voltage of 1.7 V to 3 V. This makes them unattractive for low voltages.
  • The conduction losses at high currents are much smaller than comparable field-effect transistors with high blocking voltages.
  • IGBTs have a small on-state resistance.
  • When IGBT is like the FET is a voltage controlled device.
  • Unlike power MOSFETs can not be connected in parallel without any punch-through IGBT (PT- IGBT ) to increase the current carrying capacity. Non-punch -through IGBT (NPT - IGBT ), however, have such power MOSFETs have a positive temperature coefficient and can be connected in parallel. In most IGBT high power modules which is also done.
  • The IGBT is limited only capable of blocking in the reverse direction. Usually a free-wheeling diode is already installed with short switching times between emitter and collector in the case, who runs in the reverse direction. Otherwise, when you need an external free-wheeling diode must be added.

Disadvantages are the opposite power MOSFETs large switching losses, especially when switching off ( tail current ).

The distinctive advantages of IGBTs are the high voltage and current limits: Voltage range up to and including 6.5 kV and currents up to 3600 A, with an output of up to 100 MW. Limited by the switching losses maximum frequency is approximately 200 kHz.

Applications

IGBTs are used, among other high-power applications, since it (currently up to 6.6 kV) equipped with a high forward blocking voltage and high currents can turn ( to about 3 kA). In drive technology (eg in locomotive ) they replace in PWM converters for three-phase machines now largely the previously commonly used circuits with GTO thyristors.

Applications are not limited to:

  • Switching Power Supplies
  • Frequency inverter for drive technology or in induction
  • DC chopper
  • UPS systems
  • Dimmer in conjunction with electronic transformers (eg for low-voltage halogen lamps)
  • Phase Angle
  • High -voltage direct current transmission
  • Inverter
  • Solid State Relays
Short circuit Darlington transistor Latchup Flyback diode Ampere Gate turn-off thyristor Induction furnace Integrated Authority File
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