IGBT Basic (2)

2. Device structure and operation
2-1. Structure
The IGBT combines the advantages of a power MOSFET and a bipolar power transistor. Similarly its structure is a combination of the two devices.

As shown in Fig. 1, the input has a MOS gate structure, and the output is a wide base PNP transistor. The base drive current for the PNP transistor is fed through the input channel. Besides the PNP transistor, there is an NPN transistor, which is designed to be inactivated by shorting the base and the emitter to the MOSFET source metal. The 4 layers of PNPN, which comprises the PNP transistor and the NPN transistor form a thyristor structure, which causes the possibility of a latch-up. Unlike the power MOSFET, it does not have an integral reverse diode that exists parasitically, and because of this it needs to be connected with the appropriate fast recovery diode when needed.

PT & NPT
An IGBT is called a PT (punch-through) or asymmetrical when there is an N+ buffer layer between the P+ substrate and N- drift region. Otherwise, it is called an NPT (non-punchthrough) IGBT or symmetrical IGBT. The N+ buffer layer improves turn-off speed by reducing the minority carrier injection quantity and by raising the recombination rate during the switching transition. In addition, latch-up characteristics are also improved by reducing the current gain of the PNP transistor. The problem is that the on-state voltage drop increases. However, the thickness of the N- drift region can be reduced with the same forward voltage blocking capability because the N+ buffer layer improves the forward voltage blocking capability. As a result, on-state voltage drop can be lowered. Hence, the PT-IGBT has superior trade-off characteristics as compared to the NPT-IGBT in switching speed and forward voltage drop. Currently, most commercialized IGBTs (Fairchild IGBTs) are PT-IGBTs. Section (3-3) about static blocking characteristics illustrates that IGBT forward and reverse blocking capability are approximately equal because both are determined by the same N- drift layer thickness and resistance. The reverse-blocking voltage of PT-IGBTs that contain N+ buffer layer between P+ substrate and N- drift region is lowered to tens of volts due to the existence of a heavy doping region on both sides of J1.

2-2. Operation

Turn-on

When the device is in the forward blocking mode, and if the positive gate bias (threshold voltage), which is enough to invert the surface of P-base region under the gate, is applied, then an n-type channel forms and current begins to flow. At this time the anode-cathode voltage must be above 0.7V (potential barrier) so that it can forward bias the P+ substrate / N- drift junction (J1). The electron current, which flows from the N+ emitter via the channel to the N- drift region, is the base drive current of the vertical PNP transistor. It induces the injection of hole current from the P+ region to the N- base region. The conductivity modulation improves because of this high level injection of the minority carrier (hole). This increases the conductivity of the drift region by a factor varying from ten to hundred. This conductivity modulation enables IGBTs to be used in high voltage applications by significantly reducing the drift region resistance. There are two kinds of currents flowing into the emitter electrode. One is the electron current (MOS current) flowing through the channel, and the other is the hole current (bipolar current) flowing through the P+ body / N- drift junction (J2).

Turn-off

The gate must be shorted to the emitter or a negative bias must be applied to the gate. When
the gate voltage falls below the threshold voltage, the inversion layer cannot be maintained,
and the supply of electrons into the N- drift region is blocked, at which point, the turn-off process
begins. However, the turn-off cannot be quickly completed due to the high concentration
minority carrier injected into the N- drift region during forward conduction. First, the collector
current rapidly decreases due to the termination of the electron current through the channel,
and then the collector current gradually reduces, as the minority carrier density decays due to
recombination.

to be continued……….

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