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IGBT (Insulated Gate Bipolar Transistor) 1 Differences ...

Semiconductor Group1 IGBT ( insulated gate Bipolar transistor ) 1 differences between mosfet and StructureThe IGBT combines in it all the advantages of the Bipolar and MOS field effect transistor . Ascan be seen from the structures shown below, the only difference lies in the additionalp-zone of the IGBT. Due to the presence of this layer, holes are injected into the highlyresistive n-layer and a carrier overflow is created. This increase in conductivity of the n-layerallows to reduce the on-state voltage of the IGBT. Figure 1 SIPMOS- transistor (MOSFET) SourceGate(n-Poly-Silicon)Drainn-pn+Al+n +SiO2 IGBT FundamentalsSemiconductor Group2 Figure 2 IGBT ( insulated gate Bipolar transistor ) Comparison of the Output Characteristics0102030012345 VDS[V]ID [A]20V10V9V8 VVGE mitterGate(n-Poly-Silicon)Collectorn-ppn +Al+SiO2 Figure 3 Output Characteristics of an IGBTBUP 314 (1000 V) Figure 4 Output Characteristics of a MOSFETBUZ 312 (1000 V) IGBT FundamentalsSemiconductor Group 3 The RDSon of the MOSFET in on-state

Semiconductor Group 1 IGBT (Insulated Gate Bipolar Transistor) 1 Differences Between MOSFET and IGBT 1.1 Structure The IGBT combines in it all the advantages of the bipolar and MOS field effect transistor.

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Transcription of IGBT (Insulated Gate Bipolar Transistor) 1 Differences ...

1 Semiconductor Group1 IGBT ( insulated gate Bipolar transistor ) 1 differences between mosfet and StructureThe IGBT combines in it all the advantages of the Bipolar and MOS field effect transistor . Ascan be seen from the structures shown below, the only difference lies in the additionalp-zone of the IGBT. Due to the presence of this layer, holes are injected into the highlyresistive n-layer and a carrier overflow is created. This increase in conductivity of the n-layerallows to reduce the on-state voltage of the IGBT. Figure 1 SIPMOS- transistor (MOSFET) SourceGate(n-Poly-Silicon)Drainn-pn+Al+n +SiO2 IGBT FundamentalsSemiconductor Group2 Figure 2 IGBT ( insulated gate Bipolar transistor ) Comparison of the Output Characteristics0102030012345 VDS[V]ID [A]20V10V9V8 VVGE mitterGate(n-Poly-Silicon)Collectorn-ppn +Al+SiO2 Figure 3 Output Characteristics of an IGBTBUP 314 (1000 V) Figure 4 Output Characteristics of a MOSFETBUZ 312 (1000 V) IGBT FundamentalsSemiconductor Group 3 The RDSon of the MOSFET in on-state is mainly influenced by a low doped center region,which is essential for the voltage blocking additional p-layer of the IGBT causes a carrier overflow in the center region.

2 In spite ofthe threshold voltage, which is created by the pn-junction at the collector side, a1000 V-IGBT has an on-state resistance , which is reduced by a factor of 5 compared to aMOSFET with similar blocking characterstics and identical chip The Equivalent Circuit of the IGBT Figure 5 Equivalent Circuit of the IGBTThe equivalent circuit of the IGBT can be depicted quite accurately by a pnp- transistor ,where the base current is controlled by a MOS conductivity of the resistor on the base branch is increased (modulated) when the IGBTis turned-on. This way, the greater part of the load current is flowing over the base effects only show for the user by a turn-on delay time and a tail current at turn-off.

3 Forthis reason, the device can be simply considered as a MOS transistor with the correspondingcapacities (see Figure 5).GECRModGCECGCCGECCEIGBT FundamentalsSemiconductor Group 42 IGBT-StructuresToday, two different solutions are known for realizing IGBT that are suitable for existingapplications: The PT-structure and the NPT-structure, which has been developed PT (punch through) structure shows its characteristic epitaxial layers with an N+-dopedregion (buffer layer) and an N -region on a p-doped substrate wafer. The carrier life time isminimized by heavy metal diffusion or highly energetic base material of the NPT (non punch through) structure is a homogeneous N -dopedwafer.

4 On the backside, a specially formed p-layer is created during wafer processing. It isnot necessary to limit the carrier life both cases a typical IGBT cell structure is formed on the front side. Figure 6PT-IGBT ( epitaxial-structure ) Figure 7 NPT-IGBT ( homogeneous structure )EmitterGate(n-Poly-Silicon)Collectorn-p pn+Al+EmitterCollectorn-pn+SiO2 Aln-Buffer+p IGBT FundamentalsSemiconductor Group 53 Switching Switching Behavior in GeneralIGBTs are mainly used as switches, in chopper and frequency converter applications. Inthese applications the adaptation of a (freewheeling) diode is essential, because afterswitching off the IGBT the current is driven on by the load, which is inductive in most attaching suitable diodes, this current flow is the IGBT is turned on again, the current flown diode (flooded by charge carriers) atfirst works like a short.

5 The stored charge Qrr has to be removed first for the diode to blockvoltage. This appears as a surplus current additional to the load current which is called thereverse recovery current of the diode Irr. The maximum of Irr occurs (di/dt = 0) when the sumof the instantaneous voltages across the IGBT and the diode equals the supply voltage(Figure 8).Switching-off the IGBT results in a current change and this makes an overvoltage spike bythe current change in the parasitic inductances according to VCE = L di/dt (Figure 9). Figure 8 Figure 9 IGBT FundamentalsSemiconductor Group 6 Miller-EffectThe Miller-effect is nothing else than the feedback of the collector-emitter voltage VCE via thegate-collector capacitance CGC on the gate .

6 This means a change of VCE has the same effectas an internal current source into the bias circuit, where the current is given by theexpression ig = CGC (VCE) dVCE/dt. Unfortunately CGC is not constant, but it changes its valuewith VCE. The strongest change of CGC results at small explains that:During turning-on (starting with: VCE high, VGE zero or negative) with constant gate chargingcurrent a linear increase of the gate voltage results. With falling collector-emitter voltage VCEthe gate bias current is used for changing the charge of CGC (CGC dVCE/dt) and the gatevoltage remains constant. Later, when the collector-emitter voltage has come down CGCbecomes larger as much that also at reduced slope of VCE still all the bias supplied gatecurrent is used up.

7 Only when finally the current needed for charging becomes smaller thanthe bias supplied current the gate voltage is to rise again (Figure 10).00,20,40,60,811,21,4000,0E+01,0E-62, 0E-63,0E-64,0E-6t [s]IC/IC0 VCE/VCE0 VGE/VGE0 ICVCEVGE Figure 10 Switch-on with Current Commutating from a Freewheeling CircuitIGBT FundamentalsSemiconductor Group 7At turning-off: (starting with: VCE low, VGE positive or greater than threshold voltage Vth) thegate voltage first decreases nearly linearly (at constant gate discharge current). With still lowcollector-emitter voltage VCE and with only moderate increase there is the strongest change(decrease) of CGC. Decrease of a capacitance at constant charge increases the voltage.

8 Asthere is a bias source which is drawing current out of the gate , the gate -emitter voltageremains constant. Subsequently VCE increases and most of the gate discharge current isused up for CGC dVCE/dt; the gate voltage further remains constant. The charge over processfinally is finished when VCE roughly reaches the operating voltage. Now a further decrease ofthe gate voltage is possible (Figure 11).00,20,40,60,811,2000,0E+01,0E-62,0E- 63,0E-64,0E-6t [s]IC/IC0 VCE/VCE0 VGE/VGE0 VCEVGEIC Figure 11 Turn-off of an Inductive Load into a Freewheeling CircuitBy the Miller-effect the gate current during turn-on or turn-off first of all is used for changingthe charge of CGC.

9 This is why charging up or down the gate is slowed down. It should bementioned that CGC-change and VCE-change regulate itself in a way that the available gatecurrent is used up and not more. This means that with a larger gate series resistor all eventstake a longer time, turning-on or turning-off last FundamentalsSemiconductor Group Turn-Off Behavior of PT/NPT-IGBTThe turn-off behavior of both IGBT-types is different with respect to the Figure 12 Strong Increase of Tail-Current With (left 25 C, right 125 C)NPT-IGBT Figure 13 The Tail-Current is Nearly Independent of Temperature; the Tail Starting Level isLower but it Fades Out Slower (left 25 C, right 125 C)IGBT FundamentalsSemiconductor Group Influence of the gate -Series-Resistor on the Switching LossesSwitching losses have their origin in the overlap of current and voltage waveforms duringturn-on and turn-off.

10 They depend on the magnitude of current and speed and by that also turn-on losses can be influenced very easily by the gateseries resistor. In turn-off only the current fall-time can be influenced by the gate resistor but,not the tail 14 Figure 15 IGBT FundamentalsSemiconductor Group104 Short-circuit Behavior of the GeneralThe negative temperature coefficient of the short-circuit current causes a negative thermalfeedback in the device. This is the most important condition for easy paralleling Short-circuit Type IThis case of short-circuit describes the turn-on of an IGBT during an existing short-circuit inthe output circuit (see Figure 16).In short-circuit mode the IGBT limits the maximum collector current according to its outputcharacteristics.


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