IGBT Fundamentals – Introduction
The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with high input impedance and large bipolar current-carrying capability. Many designers view IGBT as a device with MOS input characteristics and bipolar output characteristic that is a voltage-controlled bipolar device to make use of the advantages of both Power MOSFET and BJT devices in monolithic form.
It is evident that the silicon cross-section of an IGBT is almost identical to that of a vertical Power MOSFET except for the P+ injecting layer. It shares similar MOS gate structure and P wells with N+ source regions. The N+ layer at the top is the source or emitter and the P+ layer at the bottom is the drain or collector. It is also feasible to make P- channel IGBTs and for which the doping profile in each layer will be reversed
Some IGBT, without the N+ buffer layer, are called non-punch through NPT) IGBTs whereas those with this layer are called punch-through (PT) IGBTs. The presence of this buffer layer can significantly improve the performance of the device. This Device along with The MOSFET
Comparison of NPT & PT IBGTS
For a given switching speed, NPT technology generally has a higher VCE(on) than PT technology.This difference is magnified further by fact that VCE(on) increases with temperature for NPT (positive temperature coefficient), whereas VCE(on) decreases with temperature for PT (negative temperature coefficient). However, for any IGBT, whether PT or NPT, switching loss is traded off against VCE(on). Higher speed IGBTs have a higher VCE(on); lower speed IGBTs have a lower VCE(on). In fact, it is possible that a very fast PT device can have a higher VCE(on) than a NPT device of slower switching speed.
For a given VCE(on), PT IGBTs have a higher speed switching capability with lower total switching energy. This is due to higher gain and minority carrier lifetime reduction, which quenches the tail current.
NPT IGBTs are typically short circuit rated while PT devices often are not, and NPT IGBTs can absorb more avalanche energy than PT IGBTs. NPT technology is more rugged due to the wider base and lower gain of the PNP bipolar transistor. This is the main advantage gained by trading off switching speed with NPT technology. It is difficult to make a PT IGBT with greater than 600 Volt VCES whereas it is easily done with NPT technology.
For both PT and NPT IGBTs, turn-on switching speed and loss are practically unaffected by temperature Reverse recovery current in a diode however increases with temperature, so temperature effects of an external diode in the power circuit affect IGBT turn-on loss.
Series & parallel combination;
NPT IGBTs typically have a positive temperature coefficient, which makes them well suited for paralleling . A positive temperature coefficient is desirable for paralleling devices because a hot device will conduct less current than a cooler device, so all the parallel devices tend to naturally share current however that PT
IGBTs cannot be paralleled because of their negative temperature coefficient
Construction of IGBT
FIG (1) Shows the Vertical of N-CHANNEL IGBT. PCHANNEL IGBT IS ALSO POSSIBLE.BUT N CHANNEL IGBTS ARE MORE COMMON.
The major difference with the corresponding MOSFET cell structure lies in the addition of a p+INJECTING LAYER. This layer forms a pn junction with the drain layer and injects minority carriers into it. The n type drain layer itself may have two different doping levels. The lightly doped n- IS CALLED DRAIN DRIFT REGION.DOPING LEVEL & WIDTH OF THIS LAYER SETS THE FORWARD BLOCKING VOLTAGE (determined by the reverse break down voltage of J2)of the device.However it does not affect the on state voltage drop of the device due to conductivity modulation in connection with the power module.This Construction of the device is called Punch Through (PT) Design. The non – punch through Construction does not have this added n+ buffer layer.The ‘PT’ construction does offer lower on state voltage as compared to the NPT particularly for the lower voltage rated devices.However it does so at the cost of lower reverse breakdown voltage for the device,since the reverse breakdown voltage of the junction J1 is small.The rest of the conStruction is similar to that of Vertical MOSFETincluding the insulated gate structure & the shorted body (p type) – Emitter (n+type) structure.The doping level & physical geometry of the p type body region is considerably different from of that MOSFET.
THE IGBT cell has a parasitic p-n-p-n thyristor structure as shown in fig 2 (a). The constituent p-n-p Transistor ,n-p-n transistor & driver MOSFET are shown by dotted lines . Important resistances in the Current glow path are indicated.
FIG 2(B) the exact static equi. Of the the IGBT CELL Structure. The tpo n-p-n transistor is formed by the p+ injecting layer as the emitter , the n type drain drift layer as the base & the p type body layer as the collector. The lower n-p-n transistor has the n+ type source , the p type body & the n type drain as the Emitter .base & collector res.The base if the lower n-p-n transistor is shorted to the emitter by emitter Metallization. However due to imperfect shorting the IGBT includes the body spreading resistance Between the base & the emitter of the lower the lower . transistor. If the output current is large enough
The voltage drop across this resistance may forward bias the lower n-p-n transistor & inititate the latch up process of the the p-n-p-n transistor structure. Once this structure latches up the gate control of The IGBT is lost & the device is destroyed due to excessive power loss.
A major effort in the development of the IGBT has been towards prevention of the latch up of the Parasitic thyristor . This has been achieved by modifying the dopping level & physical geometry of the Body region. The modern IGBT is latch up proof for all practical purprose.
Operating principle of an IGBT can be explained in terms of the schematic cell structure and equivalent circuit of Fig 7.2(a) and (c). From the input side the IGBT behaves essentially as a
MOSFET. Therefore, when the gate emitter voltage is less then the threshold voltage no
inversion layer is formed in the p type body region and the device is in the off state. The forward
voltage applied between the collector & the emitter drops almost entirely across the junctionJ2.
Very small leakage current flows through the device under this condition. When the gate emitter
Voltage is lower than the threshold voltage the driving MOSFET of the Darlington configuration
remains off. & hence putput p-n-p transistor also remains off.
When the gate Emitter Voltage exceeds the threshold .an inversion layer forms in the p type body region under the gate.This Inversion layer (channel) shorts the emitter & the drain drift layer & the Electron current flows from the emitter through this channel to the drift region.This in turns cuauses substantial hole injuction from the p+ type collector to the drift drift region.
A portion of this holes recombine with electrons arriving at the drain drift region through the channel. The rest of the holes cross the drift region to reach the p type body where they are collected by the source metallization.
It is clear that the n type drain drift region acts as the base of the output p-n-p- Transistor.The doping level & the thickness of this layer determines the current gain of the p-n-p transistor. This is Internationally kept low so that most of the device current flow through THE MOSFET & not through the output p-n-p transistor collector ,this helps to reduce the voltage drop across the body Spreading resistance as shown in fig 2 & eliminate the possibility of the statc latch up of the IGBT.
The total on state voltage drop across a conducting IGBT has three components. The voltage drop across J1 follows the usual exponential law of PN Junction. The next component of the voltage drop Is due to drain drift rgion resistance.This component in IGBT is lower as compared to compared to a MOSFET due to strong conducitivity modulation by the injected minority carriers from the collector This is the main reason for the reduced voltage drop across an IGBT compared to an equi. MOSFET.
The last component of the voltage drop across the IGBT is due to the channel resistance & its magnitudeis equal to that of the Comparable MOSFET..