Introduction to IGBT-Insulated Gate Bipolar Transistors
Insulated gate bipolar transistor (IGBT) is a new high conductance MOS gate-controlled power switch. The fabrication process is similar to that of an N-channel power MOSFET but employs an N-epitaxial layer grown on a P+ substrate. In operation the epitaxial region is conductivity modulated (by excess holes and electrons) thereby eliminating a major component of the on-resistance. For example, on-resistance values have been reduced by a factor of about 10 compared with those of conventional N-channel power MOSFET of comparable size and voltage capability.
Vertical MOSFETs have become increasing important in discrete power device applications due primarily to their high input impedance, rapid switching times, and low resistance. However, the on-resistance of such devices increases with increasing drain-source voltage capability, thereby limiting the practical value of power MOSFETs to applications below a few hundred volts. Here we will describe the fabrication and characteristics of a new vertical power MOSFET structure that provides an on-resistance value about one-tenth of that of conventional power MOSFETs of the same size and voltage capability. In this semiconductor device, the conductivity of the epitaxial drain region of a conventional MOSFET is dramatically increased (modulated) by injected carriers, this mechanism results in a significant reduction in the device on-resistance and leads to the acronym IGBTs.
This device, while similar in structure to the MOS-gated thyristor, is different in a fundamental way; it maintains gate control (does not latch) over a wide range of anode current and voltage. The structure and the equivalent circuit of the IGBT and IGBT schematics is shown in figures respectively. They are similar to those of an MOS-gated thyristor, except for the presence of the shunting resistance RG in each unit cell. The fabrication is like that of a standard N-channel power MOSFET except that the N~ epitaxial silicon layer is grown on a P+ substrate instead of an N+ substrate. The heavily doped P+ region in the center of each unit cell, combined with the sintered aluminium contact shorting the N+ and P+ regions, provides the shunting resistance RS shown in IGBT schematics figure.This has the effect of lowering the current gain of the N-P-N transistor (αN-P-N) so that αN-P-N + αP-N-P < 1- Thus latching is avoided and gate control is maintained within a large operating range of anode voltage and current.
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