MOTOROLA Order this document SEMICONDUCTOR APPLICATION NOTE by AN1541/D AN1541 Introduction to Insulated Gate Bipolar Transistors Prepared by:Jack Takesuye and Scott Deuty Motorola Inc. INTRODUCTION As power conversion relies more on switched applications The IGBT is,in fact,a spin-off from power MOSFET semiconductor manufacturers need to create products that technology and the structure of an IGBT closely resembles approach the ideal switch.The ideal switch would have: that of a power MOSFET.The IGBT has high input impedance 1)zero resistance or forward voltage drop in the on-state and fast turn-on speed like a MOSFET.IGBTs exhibit an 2)infinite resistance in the off-state,3)switch with infinite on-voltage and current density comparable to a bipolar speed,and 4)would not require any input power to make it transistor while switching much faster.IGBTs are replacing switch. MOSFETs in high voltage applications where conduction When using existing solid-state switch technologies,the losses must be kept low.With zero current switching or designer must deviate from the ideal switch and choose a resonant switching techniques,the IGBT can be operated in device that best suits the application with a minimal loss of the hundreds of kilohertz range [1]. efficiency.The choice involves considerations such as Although turn-on speeds are very fast,turn-off of the IGBT voltage,current,switching speed,drive circuitry,load,and is slower than a MOSFET.The IGBT exhibits a current fall time temperature effects.There are a variety of solid state switch or "tailing."The tailing restricts the devices to operating at technologies available to perform switching functions; moderate frequencies(less than 50 kHz)in traditional"square however,all have strong and weak points. waveform"PWM,switching applications. At operating frequencies between 1 and 50 kHz,IGBTs offer HIGH VOLTAGE POWER MOSFETs an attractive solution over the traditional bipolar transistors The primary characteristics that are most desirable in a MOSFETs and thyristors.Compared to thyristors,the IGBT is solid-state switch are fast switching speed,simple drive faster,has better dv/dtimmunity and,above all,has better gate requirements and low conduction loss.For low voltage turn-off capability.While some thyristors such as GTOs are applications,power MOSFETs offer extremely low capable of being turned off at the gate,substantial reverse on-resistance.RDS(on).and approach the desired ideal gate current is required,whereas turning off an IGBT only switch.In high voltage applications,MOSFETs exhibit requires that the gate capacitance be discharged.A thyristor increased RDS(on)resulting in lower efficiency due to has a slightly lower forward-on voltage and higher surge increased conduction losses.In a power MOSFET,the capability than an IGBT. on-resistance is proportional to the breakdown voltage raised MOSFETs are often used because of their simple gate drive to approximately the 2.7 power(1). requirements.Since the structure of both devices are so MOSFET technology has advanced to a point where cell similar,the change to IGBTs can be made without having to densities are limited by manufacturing equipment capabilities redesign the gate drive circuit.IGBTs,like MOSFETs,are and geometries have been optimized to a point where the transconductance devices and can remain fully on by keeping RDS(on)is near the predicted theoretical limit.Since the cell the gate voltage above a certain threshold. density,geometry and the resistivity of the device structure As shown in Figure 1a,using an IGBT in place of a power play a major role,no significant reduction in the RDS(on)is MOSFET dramatically reduces the forward voltage drop at foreseen.New technologies are needed to circumvent the current levels above 12 amps.By reducing the forward drop problem of increased on-resistance without sacrificing the conduction loss of the device is decreased.The gradual switching speed. rising slope of the MOSFET in Figure 1a can be attributed to the relationship of VDS to RDS(on).The IGBT curve has an RDs(on)Vpss offset due to an internal forward biased p-n junction and a fast DSS (1) rising slope typical of a minority carrier device. It is possible to replace the MOSFET with an IGBT and improve the efficiency and/or reduce the cost.As shown in ENTER THE IGBT Figure 1b,an IGBT has considerably less silicon area than a similarly rated MOSFET.Device cost is related to silicon area; By combining the low conduction loss of a BJT with the therefore,the reduced silicon area makes the IGBT the lower switching speed of a power MOSFET an optimal solid state cost solution.Figure 1c shows the resulting package area switch would exist.The Insulated-Gate Bipolar Transistor reduction realized by using the IGBT.The IGBT is more space (IGBT)technology offers a combination of these attributes. efficient than an equivalently rated MOSFET which makes it perfect for space conscious designs. MOTOROLA Motorola,Inc.1995
MOTOROLA 1 Prepared by: Jack Takesuye and Scott Deuty Motorola Inc. INTRODUCTION As power conversion relies more on switched applications, semiconductor manufacturers need to create products that approach the ideal switch. The ideal switch would have: 1) zero resistance or forward voltage drop in the on–state, 2) infinite resistance in the off–state, 3) switch with infinite speed, and 4) would not require any input power to make it switch. When using existing solid–state switch technologies, the designer must deviate from the ideal switch and choose a device that best suits the application with a minimal loss of efficiency. The choice involves considerations such as voltage, current, switching speed, drive circuitry, load, and temperature effects. There are a variety of solid state switch technologies available to perform switching functions; however, all have strong and weak points. HIGH VOLTAGE POWER MOSFETs The primary characteristics that are most desirable in a solid–state switch are fast switching speed, simple drive requirements and low conduction loss. For low voltage applications, power MOSFETs offer extremely low on–resistance, RDS(on), and approach the desired ideal switch. In high voltage applications, MOSFETs exhibit increased RDS(on) resulting in lower efficiency due to increased conduction losses. In a power MOSFET, the on–resistance is proportional to the breakdown voltage raised to approximately the 2.7 power (1). MOSFET technology has advanced to a point where cell densities are limited by manufacturing equipment capabilities and geometries have been optimized to a point where the RDS(on) is near the predicted theoretical limit. Since the cell density, geometry and the resistivity of the device structure play a major role, no significant reduction in the RDS(on) is foreseen. New technologies are needed to circumvent the problem of increased on–resistance without sacrificing switching speed. RDS(on) V 2.7 DSS (1) ENTER THE IGBT By combining the low conduction loss of a BJT with the switching speed of a power MOSFET an optimal solid state switch would exist. The Insulated–Gate Bipolar Transistor (IGBT) technology offers a combination of these attributes. The IGBT is, in fact, a spin–off from power MOSFET technology and the structure of an IGBT closely resembles that of a power MOSFET. The IGBT has high input impedance and fast turn–on speed like a MOSFET. IGBTs exhibit an on–voltage and current density comparable to a bipolar transistor while switching much faster. IGBTs are replacing MOSFETs in high voltage applications where conduction losses must be kept low. With zero current switching or resonant switching techniques, the IGBT can be operated in the hundreds of kilohertz range [1]. Although turn–on speeds are very fast, turn–off of the IGBT is slower than a MOSFET. The IGBT exhibits a current fall time or “tailing.” The tailing restricts the devices to operating at moderate frequencies (less than 50 kHz) in traditional “square waveform” PWM, switching applications. At operating frequencies between 1 and 50 kHz, IGBTs offer an attractive solution over the traditional bipolar transistors, MOSFETs and thyristors. Compared to thyristors, the IGBT is faster, has better dv/dt immunity and, above all, has better gate turn–off capability. While some thyristors such as GTOs are capable of being turned off at the gate, substantial reverse gate current is required, whereas turning off an IGBT only requires that the gate capacitance be discharged. A thyristor has a slightly lower forward–on voltage and higher surge capability than an IGBT. MOSFETs are often used because of their simple gate drive requirements. Since the structure of both devices are so similar, the change to IGBTs can be made without having to redesign the gate drive circuit. IGBTs, like MOSFETs, are transconductance devices and can remain fully on by keeping the gate voltage above a certain threshold. As shown in Figure 1a, using an IGBT in place of a power MOSFET dramatically reduces the forward voltage drop at current levels above 12 amps. By reducing the forward drop, the conduction loss of the device is decreased. The gradual rising slope of the MOSFET in Figure 1a can be attributed to the relationship of VDS to RDS(on). The IGBT curve has an offset due to an internal forward biased p–n junction and a fast rising slope typical of a minority carrier device. It is possible to replace the MOSFET with an IGBT and improve the efficiency and/or reduce the cost. As shown in Figure 1b, an IGBT has considerably less silicon area than a similarly rated MOSFET. Device cost is related to silicon area; therefore, the reduced silicon area makes the IGBT the lower cost solution. Figure 1c shows the resulting package area reduction realized by using the IGBT. The IGBT is more space efficient than an equivalently rated MOSFET which makes it perfect for space conscious designs. Order this document by AN1541/D SEMICONDUCTOR APPLICATION NOTE Motorola, Inc. 1995
AN1541 40 0.10 35 VCE(sat) 囚 IGBT DIE SIZE MGW20N60D IGBT (0.17X0.227 30 VDS MTW20N50E (S3HONI ☑ MOSFET DIE SIZE 25 (0.35X0.26) MOSFET 0.05 20 15 10 0 5 Figure 1b.Reduced Die Size of IGBT Realized 0 When Compared to a MOSFET with Similar Ratings 6 0 FORWARD DROP (VOLTS) 0.60 Figure 1a.Reduced Forward Voltage Drop of IGBT Realized When Compared to a MOSFET 囚 IGBT PACKAGE with Similar Ratings S1zE(T0-220) 0.40 ☑ MOSFET PACKAGE When compared to BJTs,IGBTs have similar ratings in HONI SIZE(TO-247) terms of voltage and current.However,the presence of an isolated gate in an IGBT makes it simpler to drive than a BJT. 0.20 BJTs require that base current be continuously supplied in a quantity sufficient enough to maintain saturation.Base currents of one-tenth of the collector current are typical to keep a BJT in saturation.BJT drive circuits must be sensitive to variable load conditions.The base current of a BJT must be kept proportional to the collector current to prevent Figure 1c.Reduced Package Size of IGBT Realized desaturation under high-current loads and excessive base When Compared to a MOSFET with Similar Ratings drive under low-load conditions.This additional base current increases the power dissipation of the drive circuit.BJTs are Because the loss period is a small percentage of the total on minority carrier devices and charge storage effects including time,slower switching is traded for lower conduction loss.In recombination slow the performance when compared to a higher frequency application,just the opposite would be true majority carrier devices such as MOSFETs.IGBTs also and the device would be made faster and have greater experience recombination that accounts for the current conduction losses.Notice that the curves in Figure 2 show "tailing"yet IGBTs have been observed to switch faster than reductions inboth the forward drop(VCE(sat))and thefall time. BJTs. tf of newer generation devices.These capabilities make the Thus far.the IGBT has demonstrated certain advantages IGBT the device of choice for applications such as motor over power MOSFETs with the exception of switching speed. drives.power supplies and inverters that require devices rated Since the initial introduction of IGBTs in the early 1980s for 600 to 1200 volts. semiconductor manufacturers have leamned how to make the devices faster.As illustrated in Figure 2,some trade-offs in conduction loss versus switching speed exist.Lower frequency applications can tolerate slower switching devices 3.5 3.0 1ST GENERATION COMPETITOR 1985 2.5 2ND GENERATION COMPETITOR 1989 2.0 1ST GENERATION MOTOROLA 1993 1.5 3RD GENERATION COMPETITOR 1993 1.0 HIGH SPEED LOW SATURATION 2ND GENERATION MOTOROLA DEMONSTRATED 0.5 SERIES SERIES 00 0.1 02 0.30.40.50.60.70.80.91.0 斯us) Figure 2.Advanced Features Offered by the Latest Motorola IGBT Technologies for Forward Voltage Drop(VCE(sat))and Fall Time(tf) 2 MOTOROLA
2 MOTOROLA FORWARD DROP (VOLTS) Figure 1a. Reduced Forward Voltage Drop of IGBT Realized When Compared to a MOSFET with Similar Ratings 40 35 30 25 20 15 10 5 0 0 2 4 6 8 10 PEAK CURRENT THROUGH DEVICE (AMPS) VCE(sat) MGW20N60D IGBT VDS MTW20N50E MOSFET When compared to BJTs, IGBTs have similar ratings in terms of voltage and current. However, the presence of an isolated gate in an IGBT makes it simpler to drive than a BJT. BJTs require that base current be continuously supplied in a quantity sufficient enough to maintain saturation. Base currents of one–tenth of the collector current are typical to keep a BJT in saturation. BJT drive circuits must be sensitive to variable load conditions. The base current of a BJT must be kept proportional to the collector current to prevent desaturation under high–current loads and excessive base drive under low–load conditions. This additional base current increases the power dissipation of the drive circuit. BJTs are minority carrier devices and charge storage effects including recombination slow the performance when compared to majority carrier devices such as MOSFETs. IGBTs also experience recombination that accounts for the current “tailing” yet IGBTs have been observed to switch faster than BJTs. Thus far, the IGBT has demonstrated certain advantages over power MOSFETs with the exception of switching speed. Since the initial introduction of IGBTs in the early 1980s, semiconductor manufacturers have learned how to make the devices faster. As illustrated in Figure 2, some trade–offs in conduction loss versus switching speed exist. Lower frequency applications can tolerate slower switching devices. ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÇÇÇÇÇÇ ÇÇÇÇÇÇ ÇÇÇÇÇÇ ÇÇÇÇÇÇ Figure 1b. Reduced Die Size of IGBT Realized When Compared to a MOSFET with Similar Ratings 0.10 0.05 0 1 AREA (SQ. INCHES) IGBT DIE SIZE (0.17 X 0.227) MOSFET DIE SIZE (0.35 X 0.26) Ç Ç É ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÉÉÉÉÉÉ ÇÇÇÇÇÇ ÇÇÇÇÇÇ ÇÇÇÇÇÇ ÇÇÇÇÇÇ Figure 1c. Reduced Package Size of IGBT Realized When Compared to a MOSFET with Similar Ratings 0.60 0.20 0 1 AREA (SQ. INCHES) IGBT PACKAGE SIZE (TO–220) MOSFET PACKAGE SIZE (T0–247) Ç Ç É 0.40 Because the loss period is a small percentage of the total on time, slower switching is traded for lower conduction loss. In a higher frequency application, just the opposite would be true and the device would be made faster and have greater conduction losses. Notice that the curves in Figure 2 show reductions in both the forward drop (VCE(sat)) and the fall time, tf of newer generation devices. These capabilities make the IGBT the device of choice for applications such as motor drives, power supplies and inverters that require devices rated for 600 to 1200 volts. Figure 2. Advanced Features Offered by the Latest Motorola IGBT Technologies for Forward Voltage Drop (VCE(sat)) and Fall Time (tf) tf (µs) 3.5 3.0 2.5 2.0 1.5 1.0 0.5 00 0.9 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1.0 1ST GENERATION COMPETITOR 1985 2ND GENERATION COMPETITOR 1989 1ST GENERATION MOTOROLA 1993 3RD GENERATION COMPETITOR 1993 2ND GENERATION MOTOROLA DEMONSTRATED VCE(sat) (VOLTS) HIGH SPEED SERIES LOW SATURATION SERIES 0.1
AN1541 CHARACTERISTICS OF IGBTs: DEVICE STRUCTURE The structure of an IGBT is similar to that of a double The n-epi resistivity determines the breakdown voltage of diffused(DMOS)power MOSFET.One difference between a a MOSFET as mentioned earlier using relationship(1). MOSFET and an IGBT is the substrate of the starting material ,2.7 By varying the starting material and altering certain process RpS(on) DSS (1) steps,an IGBT may be produced from a power MOSFET mask;however,at Motorola mask sets are designed To increase the breakdown voltage of the MOSFET,the specifically for IGBTs.In a MOSFET the substrate is N+as n-epi region thickness (vertical direction in figure)is shown in Figure 3b.The substrate for an IGBT is P+as shown increased.As depicted in the classical resistance relationship in Figure 3a. (2),reducing the RDS(on)of a high voltage device requires greater silicon area A to make up for the increased n-epi region. GATE (2) EMITTER KEY Rx ■METAL 口si02 Device designers were challenged to overcome the effects POLYSILICON GATE of the high resistive n-epi region.The solution to this came in the form of conductivity modulation.The n-epi region to this N+ was placed on the P+substrate forming a p-n junction where conductivity modulation takes place.Because of conductivity P+ modulation,the IGBT has a much greater current density than MOSFET Rmod a power MOSFET and the forward voltage drop is reduced. Nowthe P+substrate,n-epilayer and P+"emitter"forma BJT transistor and the n-epi acts as a wide base region N-EPI The subject of current tailing has been mentioned several N+BUFFER times.Thus far,the device structure as shown in Figure 3 provides insight as to what causes the tailing.Minority carriers P+SUBSTRATE build up to form the basis for conductivity modulation.When the device turns off,these carriers do not have a current path to exit the device.Recombination is the only way to eliminate the stored charge resulting from the build-up of excess COLLECTOR carriers.Additional recombination centers are formed by placing an N+buffer layer between the n-epi and P+ Figure 3a.Cross Section and Equivalent Schematic substrate of an Insulated Gate Bipolar Transistor(IGBT)Cell While the N+buffer layer may speed up the recombination, it also increases the forward drop of the device.Hence the tradeoff between switching speed and conduction loss becomes a factor in optimizing device performance GATE Additional benefits of the N+buffer layer include preventing SOURCE KEY thermal runaway and punch-through of the depletion region. ■METAL This allows a thinner n-epi to be used which somewhat 口Si02 POLYSILICON GATE decreases forward voltage drop. JFET N COLLECTOR channel GATE Drain-to-Source Body Diode (Created when NPN base-emitter is properly N-EPI EMITTER shorted by source metal) Figure 4a.IGBT Schematic Symbol N+SUBSTRATE DRAIN DRAIN GATE Figure 3b.Cross Section and Equivalent SOURCE Schematic of an Metal-Oxide-Semiconductor Field-Effect Transistor(MOSFET)Cell Figure 4b.MOSFET Schematic Symbol MOTOROLA
MOTOROLA 3 CHARACTERISTICS OF IGBTs: DEVICE STRUCTURE The structure of an IGBT is similar to that of a double diffused (DMOS) power MOSFET. One difference between a MOSFET and an IGBT is the substrate of the starting material. By varying the starting material and altering certain process steps, an IGBT may be produced from a power MOSFET mask; however, at Motorola mask sets are designed specifically for IGBTs. In a MOSFET the substrate is N+ as shown in Figure 3b. The substrate for an IGBT is P+ as shown in Figure 3a. Figure 3a. Cross Section and Equivalent Schematic of an Insulated Gate Bipolar Transistor (IGBT) Cell POLYSILICON GATE N+ P+ N– EPI N+ BUFFER P+ SUBSTRATE P– Rmod EMITTER N+ P+ P– COLLECTOR GATE NPN MOSFET PNP KEY METAL SiO2 Rshorting Figure 3b. Cross Section and Equivalent Schematic of an Metal–Oxide–Semiconductor Field–Effect Transistor (MOSFET) Cell JFET channel Drain–to–Source Body Diode (Created when NPN base–emitter is properly shorted by source metal) POLYSILICON GATE N+ P+ N– EPI N+ SUBSTRATE SOURCE N+ P+ DRAIN NPN KEY METAL SiO2 GATE P– The n– epi resistivity determines the breakdown voltage of a MOSFET as mentioned earlier using relationship (1). RDS(on) V 2.7 DSS (1) To increase the breakdown voltage of the MOSFET, the n– epi region thickness (vertical direction in figure) is increased. As depicted in the classical resistance relationship (2), reducing the RDS(on) of a high voltage device requires greater silicon area A to make up for the increased n– epi region. R 1 A (2) Device designers were challenged to overcome the effects of the high resistive n– epi region. The solution to this came in the form of conductivity modulation. The n– epi region to this was placed on the P+ substrate forming a p–n junction where conductivity modulation takes place. Because of conductivity modulation, the IGBT has a much greater current density than a power MOSFET and the forward voltage drop is reduced. Now the P+ substrate, n– epi layer and P+ “emitter” form a BJT transistor and the n– epi acts as a wide base region. The subject of current tailing has been mentioned several times. Thus far, the device structure as shown in Figure 3 provides insight as to what causes the tailing. Minority carriers build up to form the basis for conductivity modulation. When the device turns off, these carriers do not have a current path to exit the device. Recombination is the only way to eliminate the stored charge resulting from the build–up of excess carriers. Additional recombination centers are formed by placing an N+ buffer layer between the n– epi and P+ substrate. While the N+ buffer layer may speed up the recombination, it also increases the forward drop of the device. Hence the tradeoff between switching speed and conduction loss becomes a factor in optimizing device performance. Additional benefits of the N+ buffer layer include preventing thermal runaway and punch–through of the depletion region. This allows a thinner n– epi to be used which somewhat decreases forward voltage drop. Figure 4b. MOSFET Schematic Symbol Figure 4a. IGBT Schematic Symbol COLLECTOR EMITTER GATE GATE SOURCE DRAIN
AN1541 The IGBT has a four layer (P-N-P-N)structure.This The IGBT's on-voltage is represented by sum of the offset structure resembles that of a thyristor device known as a voltage of the collector to base junction of the PNP transistor, Silicon Controlled Rectifier(SCR).Unlike the SCR where the the voltage drop across the modulated resistance Rmod and device latches and gate control is lost,an IGBT is designed the channel resistance of the internal MOSFET.Unlike the so that it does not latch on.Full control of the device can be MOSFET where increased temperature results in increased maintained through the gate drive. RDS(on)and increased forward voltage drop,the forward To maximize the performance of the IGBT,process steps drop of an IGBT stays relatively unchanged at increased are optimized to control the geometry,doping and lifetime. temperatures. The possibility of latching is also reduced by strategic processing of the device.Geometry and doping levels are Switching Speed optimized to minimize the on-voltage,switching speed and Until recently,the feature that limited the IGBT from achieve other key parametric variations.Because the IGBT is serving a wide variety of applications was its relatively slow a four-layer structure,it does not have the inverse parallel turn-off speed when compared to a power MOSFET.While diode inherent to power MOSFETs.This is a disadvantage to tur-on is fairly rapid,initial IGBTs had current fall times of motor control designers who use the anti-parallel diode to around three microseconds recover energy from the motor. The turn-off time of an IGBT is slow because many minority Like a power MOSFET,the gate of the IGBT is electrically carriers are stored in the n-epi region.When the gate is isolated from the rest of the chip by a thin layer of silicon initially brought below the threshold voltage,the n-epi dioxide,SiO2.The IGBT has a high input impedance due to contains a very large concentration of electrons and there will the isolated gate and it exhibits the accompanying be significant injection into the p+substrate and a advantages of modest gate drive requirements and excellent corresponding hole injection into the n-epi.As the electron gate drive efficiency. concentration in the n-region decreases,the electron injection decreases,leaving the rest of the electrons to Equivalent Circuit of IGBT recombine.Therefore.the turn-off of an IGBT has two Figure 4b shows the terminals of the IGBT as determined by phases:an injection phase where the collector current falls JEDEC.Notice that the IGBT has a gate like a MOSFET yet very quickly,and a recombination phase in which the collector it has an emitter and a collector like a BJT. current decrease more slowly.Figure 5 shows the switching The operation of the IGBT is best understood by again waveform and the tail time contributing factors of a"fast"IGBT referring to the cross section of the device and its equivalent designed for PWM motor control service. circuit as shown in Figure 3a.Current flowing from collector to emitter must pass through a p-n junction formed by the P+ substrate and n-epi layer.This drop is similar to that seen in TAIL TIME of MOTOROLA GEN.2 IGBT #2 in 1.0 hp MOTOR DRIVE at 1750 RPM a forward biased p-n junction diode and results in an offset voltage in the output characteristic.Current flow contributions are shown in Figure 3a using varying line thickness with the TAIL TIME thicker lines indicating a high current path.For a fast device, the N+buffer layer is highly doped for recombination and MOSFET TURN-OFF speedy turn off.The additional doping keeps the gain of the PORTION PNP low and allows two-thirds of the current to flow through the base of the PNP(electron current)while one-third passes PNP TURN-OFF PORTION through the collector (hole current). Rshorting is the parasitic resistance ofthe P+emitter region. Current flowing through Rshorting can result in a voltage across the base-emitter junction of the NPN.If the base-emitter voltage is above a certain threshold level,the 200 400 600 800 1000 NPN will begin to conduct causing the NPN and PNP to enhance each other's current flow and both devices can become saturated.This results in the device latching in a Figure 5.IGBT Current Turn-off Waveform fashion similar to an SCR.Device processing directs currents within the device and keeps the voltage across Rshorting lowto In power MOSFETs,the switching speed can be greatly avoid latching.The IGBT can be gated off unlike the SCR which has to wait for the current to cease allowing affected by the impedance in the gate drive circuit.Efforts to minimize gate drive impedance for IGBTs are also recombination to take place in order to turn off.IGBTs offer an recommended.Also,choose an optimal device based on advantage over the SCR by controlling the current with the switching speed or use a slower device with lower forward device,not the device with the current.The interal MOSFET of the IGBT when gated off will stop current flow and at that drop and employ external circuitry to enhance turn off.A turn-off mechanism is suggested in a paper by Baliga et al [2]. point,the stored charges can only be dissipated through recombination. MOTOROLA
4 MOTOROLA The IGBT has a four layer (P–N–P–N) structure. This structure resembles that of a thyristor device known as a Silicon Controlled Rectifier (SCR). Unlike the SCR where the device latches and gate control is lost, an IGBT is designed so that it does not latch on. Full control of the device can be maintained through the gate drive. To maximize the performance of the IGBT, process steps are optimized to control the geometry, doping and lifetime. The possibility of latching is also reduced by strategic processing of the device. Geometry and doping levels are optimized to minimize the on–voltage, switching speed and achieve other key parametric variations. Because the IGBT is a four–layer structure, it does not have the inverse parallel diode inherent to power MOSFETs. This is a disadvantage to motor control designers who use the anti–parallel diode to recover energy from the motor. Like a power MOSFET, the gate of the IGBT is electrically isolated from the rest of the chip by a thin layer of silicon dioxide, SiO2. The IGBT has a high input impedance due to the isolated gate and it exhibits the accompanying advantages of modest gate drive requirements and excellent gate drive efficiency. Equivalent Circuit of IGBT Figure 4b shows the terminals of the IGBT as determined by JEDEC. Notice that the IGBT has a gate like a MOSFET yet it has an emitter and a collector like a BJT. The operation of the IGBT is best understood by again referring to the cross section of the device and its equivalent circuit as shown in Figure 3a. Current flowing from collector to emitter must pass through a p–n junction formed by the P+ substrate and n– epi layer. This drop is similar to that seen in a forward biased p–n junction diode and results in an offset voltage in the output characteristic. Current flow contributions are shown in Figure 3a using varying line thickness with the thicker lines indicating a high current path. For a fast device, the N+ buffer layer is highly doped for recombination and speedy turn off. The additional doping keeps the gain of the PNP low and allows two–thirds of the current to flow through the base of the PNP (electron current) while one–third passes through the collector (hole current). Rshorting is the parasitic resistance of the P+ emitter region. Current flowing through Rshorting can result in a voltage across the base–emitter junction of the NPN. If the base–emitter voltage is above a certain threshold level, the NPN will begin to conduct causing the NPN and PNP to enhance each other’s current flow and both devices can become saturated. This results in the device latching in a fashion similar to an SCR. Device processing directs currents within the device and keeps the voltage across Rshorting low to avoid latching. The IGBT can be gated off unlike the SCR which has to wait for the current to cease allowing recombination to take place in order to turn off. IGBTs offer an advantage over the SCR by controlling the current with the device, not the device with the current. The internal MOSFET of the IGBT when gated off will stop current flow and at that point, the stored charges can only be dissipated through recombination. The IGBT’s on–voltage is represented by sum of the offset voltage of the collector to base junction of the PNP transistor, the voltage drop across the modulated resistance Rmod and the channel resistance of the internal MOSFET. Unlike the MOSFET where increased temperature results in increased RDS(on) and increased forward voltage drop, the forward drop of an IGBT stays relatively unchanged at increased temperatures. Switching Speed Until recently, the feature that limited the IGBT from serving a wide variety of applications was its relatively slow turn–off speed when compared to a power MOSFET. While turn–on is fairly rapid, initial IGBTs had current fall times of around three microseconds. The turn–off time of an IGBT is slow because many minority carriers are stored in the n– epi region. When the gate is initially brought below the threshold voltage, the n– epi contains a very large concentration of electrons and there will be significant injection into the P+ substrate and a corresponding hole injection into the n– epi. As the electron concentration in the n–region decreases, the electron injection decreases, leaving the rest of the electrons to recombine. Therefore, the turn–off of an IGBT has two phases: an injection phase where the collector current falls very quickly, and a recombination phase in which the collector current decrease more slowly. Figure 5 shows the switching waveform and the tail time contributing factors of a “fast” IGBT designed for PWM motor control service. ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ ÉÉÉÉ ÇÇÇÇ ÇÇ ÇÇ ÇÇ ÇÇ Figure 5. IGBT Current Turn–off Waveform 6 5 4 3 2 1 0 –1 0 200 400 600 800 1000 I C (AMPS) TAIL TIME of MOTOROLA GEN. 2 IGBT #2 in 1.0 hp MOTOR DRIVE at 1750 RPM PNP TURN–OFF PORTION TAIL TIME MOSFET TURN–OFF PORTION In power MOSFETs, the switching speed can be greatly affected by the impedance in the gate drive circuit. Efforts to minimize gate drive impedance for IGBTs are also recommended. Also, choose an optimal device based on switching speed or use a slower device with lower forward drop and employ external circuitry to enhance turn off. A turn–off mechanism is suggested in a paper by Baliga et al [2].
AN1541 A FINAL COMPARISON OF IGBTs,BJTs AND Short Circuit Rated Devices POWER MOSFETs Using IGBTs in motor control environments requires the device to withstand short circuit current for a given period The conduction losses of BJTs and IGBTs is related to the forward voltage drop of the device while MOSFETs determine Although this period varies with the application,a typical value of ten microseconds is used for designing these conduction loss based on RDS(on).To get a relative comparison of turn-off time and conduction associated specialized IGBT's.Notice that this is only a typical value and it is suggested that the reader confirm the value given on the losses,data is presented in Table 1 where the on-resistances data sheet.IGBTs can be made to withstand short circuit of a power MOSFET,an IGBT and a BJT at junction temperatures of 25C and 150C are shown. conditions by altering the device structure to include an additional resistance(Re,in Figure 6)in the main current path. Note that the devices in Table 1 have approximately the The benefits associated with the additional series resistance same ratings.However,to achieve these ratings the chip size are twofold. ofthe devices vary significantly.The bipolar transistor requires 1.2 times more silicon area than the IGBT and the MOSFET requires 2.2 times the area of the IGBT to achieve the same GATE ratings.This differences in die area directly impacts the cost EMITTER KEY of the product.At higher currents and at elevated ■METAL temperatures,the IGBT offers low forward drop and a POLYSILICON GATE ▣Si02 switching time similar to the BJT without the drive difficulties. Table 1 confirms the findings offered earlier in Figure 1a and N+ elaborates further to include a BJT comparison and temperature effects.The reduced power conduction losses offered by the IGBT lower power dissipation and heat sink Rshorting MOSFET m00 size. Thermal Resistance N-EPI An IGBT and power MOSFET produced from the same size N+BUFFER die have similar junction-to-case thermal resistance because of their similar structures.The thermal resistance of a power P+SUBSTRATE MOSFET can be determined by testing for variations in temperature sensitive parameters(TSPs).These parameters are the source-to-drain diode on-voltage,the gate-to-source threshold voltage,and the drain-to-source COLLECTOR on-resistance.All previous measurements of thermal resistance of power MOSFETs at Motorola were performed Figure 6.Cross Section and Equivalent Schematic of a Short Circuit Rated Insulated Gate using the source-to-drain diode as the TSP.Since an IGBT does not have an inverse parallel diode,another TSP had to Bipolar Transistor Cell be used to determine the thermal resistance.The gate-to-emitter threshold voltage was used as the TSP to First,the voltage created across Re,by the large current measure the junction temperature of an IGBT to determine its passing through Re,increases the percentage of the gate thermal resistance.However before testing IGBTs,a voltage across Re.by the classic voltage divider equation. correlation between the two test methods was established by Assuming the drive voltage applied to the gate-to-emitter comparing the test results of MOSFETs using both TSPs.By remains the same,the voltage actually applied across the testing for variations in threshold voltage,it was determined gate-to-source portion of the device is now lower,and the that the thermal resistance of MOSFETs and IGBTs are device is operating in an area of the transconductance curve essentially the same for devices with equivalent die size. that reduces the gain and it will pass less current. Table 1.Advantages Offered by the IGBT When Comparing the MOSFET,IGBT and Bipolar Transistor On-Resistances (Over Junction Temperature)and Fall Times(Resistance Values at 10 Amps of Current) Characteristic TMOS IGBT Bipolar Current Rating 20A 20A 20A Voltage Rating 500V 600V 500V R(on)@TJ=25C 0.22 0.242 0.182 R(on)@TJ=150C 0.62 0.232 0.242 Fall Time (Typical) 40ns 200ns 200ns Indicates VCEO Rating #BJT TJj=100℃ MOTOROLA 5
MOTOROLA 5 A FINAL COMPARISON OF IGBTs, BJTs AND POWER MOSFETs The conduction losses of BJTs and IGBTs is related to the forward voltage drop of the device while MOSFETs determine conduction loss based on RDS(on). To get a relative comparison of turn–off time and conduction associated losses, data is presented in Table 1 where the on–resistances of a power MOSFET, an IGBT and a BJT at junction temperatures of 25°C and 150°C are shown. Note that the devices in Table 1 have approximately the same ratings. However, to achieve these ratings the chip size of the devices vary significantly. The bipolar transistor requires 1.2 times more silicon area than the IGBT and the MOSFET requires 2.2 times the area of the IGBT to achieve the same ratings. This differences in die area directly impacts the cost of the product. At higher currents and at elevated temperatures, the IGBT offers low forward drop and a switching time similar to the BJT without the drive difficulties. Table 1 confirms the findings offered earlier in Figure 1a and elaborates further to include a BJT comparison and temperature effects. The reduced power conduction losses offered by the IGBT lower power dissipation and heat sink size. Thermal Resistance An IGBT and power MOSFET produced from the same size die have similar junction–to–case thermal resistance because of their similar structures. The thermal resistance of a power MOSFET can be determined by testing for variations in temperature sensitive parameters (TSPs). These parameters are the source–to–drain diode on–voltage, the gate–to–source threshold voltage, and the drain–to–source on–resistance. All previous measurements of thermal resistance of power MOSFETs at Motorola were performed using the source–to–drain diode as the TSP. Since an IGBT does not have an inverse parallel diode, another TSP had to be used to determine the thermal resistance. The gate–to–emitter threshold voltage was used as the TSP to measure the junction temperature of an IGBT to determine its thermal resistance. However before testing IGBTs, a correlation between the two test methods was established by comparing the test results of MOSFETs using both TSPs. By testing for variations in threshold voltage, it was determined that the thermal resistance of MOSFETs and IGBTs are essentially the same for devices with equivalent die size . Short Circuit Rated Devices Using IGBTs in motor control environments requires the device to withstand short circuit current for a given period. Although this period varies with the application, a typical value of ten microseconds is used for designing these specialized IGBT’s. Notice that this is only a typical value and it is suggested that the reader confirm the value given on the data sheet. IGBTs can be made to withstand short circuit conditions by altering the device structure to include an additional resistance (Re, in Figure 6) in the main current path. The benefits associated with the additional series resistance are twofold. Figure 6. Cross Section and Equivalent Schematic of a Short Circuit Rated Insulated Gate Bipolar Transistor Cell POLYSILICON GATE N+ P+ N– EPI N+ BUFFER P+ SUBSTRATE P– Rmod EMITTER N+ P+ P– COLLECTOR GATE NPN MOSFET PNP KEY METAL SiO2 Rshorting Re First, the voltage created across Re, by the large current passing through Re, increases the percentage of the gate voltage across Re, by the classic voltage divider equation. Assuming the drive voltage applied to the gate–to–emitter remains the same, the voltage actually applied across the gate–to–source portion of the device is now lower, and the device is operating in an area of the transconductance curve that reduces the gain and it will pass less current. Table 1. Advantages Offered by the IGBT When Comparing the MOSFET, IGBT and Bipolar Transistor On–Resistances (Over Junction Temperature) and Fall Times (Resistance Values at 10 Amps of Current) ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Characteristic ÁÁÁÁÁÁÁÁ TMOS ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ IGBT ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ Bipolar ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Current Rating ÁÁÁÁÁÁÁÁ 20 A ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 20 A ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 20 A ÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Voltage Rating Á ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁ 500 V Á ÁÁÁÁÁÁÁ ÁÁÁÁÁ 600 V Á ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁ 500 V* Á ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ R(on) @ TJ = 25°C ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 0.2 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 0.24 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 0.18 Ω ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ R(on) @ TJ = 150°C ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁ 0.6 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 0.23 Ω ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 0.24 Ω** ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ Fall Time (Typical) ÁÁÁÁÁÁÁÁ 40 ns ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 200 ns ÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁ 200 ns * Indicates VCEO Rating ** BJT TJ = 100°C