* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download NTD_Final_Ch3-3_3-4 DOWNLOAD
Wireless power transfer wikipedia , lookup
Thermal runaway wikipedia , lookup
Three-phase electric power wikipedia , lookup
Electrification wikipedia , lookup
Power over Ethernet wikipedia , lookup
Electrical ballast wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Electric power system wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Power inverter wikipedia , lookup
Current source wikipedia , lookup
Electrical substation wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Mercury-arc valve wikipedia , lookup
Stray voltage wikipedia , lookup
History of electric power transmission wikipedia , lookup
Power engineering wikipedia , lookup
Voltage optimisation wikipedia , lookup
Distribution management system wikipedia , lookup
Surge protector wikipedia , lookup
Opto-isolator wikipedia , lookup
History of the transistor wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Mains electricity wikipedia , lookup
Power electronics wikipedia , lookup
Alternating current wikipedia , lookup
Buck converter wikipedia , lookup
3.3. POWER SEMICONDUCTOR DEVICES 3.3.1. Semiconductor devices [23,24,25,26,27] (1) Diode Most modern diodes are based on semiconductor p-n junctions. In the p-n diode, current can flow from the p-type to the n-type side, but cannot flow in the opposite direction. The Schottky diode, is formed by a contact between a metal and a semiconductor rather than by a p-n junction. The basic application of the diode is as a rectifier that converts alternating current (AC) into pulsating direct current (DC). Fig. 5 shows a full-wave bridge rectifier which consists of four diodes and a resistor. From the aspect of switching, the diodes can be called as uncontrolled switches. No signals turn the diode on or off. M D4 AC D1 B A RL D2 D3 N During +ve half cycle M is +ve and N is -ve D1, D3 : forward D2, D4 : reverse During -ve half cycle M is -ve and N is +ve D2, D4 : forward D1, D3 : reverse Fig. 5. A full-wave bridge rectifier with four diodes and a resistor. (2) Bipolar Junction Transistor (BJT) The bipolar junction transistor is a three-terminal component composed of emitter (E), collector (C), and base (B). The emitter and collector terminals are made of the same type of semiconductor material, either p-type or n-type, while the base is made of the other. There are two types of bipolar junction transistors, npn and pnp transistors, and the npn transistor is shown in Fig. 6. (heavily doped) (lightly doped) E N P C N C (moderately doped) NPN-type E B Fig. 6. Schematic diagram and symbol of an npn transistor. The output current, voltage, and/or power of a transistor are controlled by its input current. Thus, it is used as the primary component in the amplifier, a circuit that is used 1 to increase the strength of an ac signal. Also, the transistor is used as a high-speed electronic switch that is capable of switching between two operating states (open and closed) at a rate of several billions of times per second. A small change in the base current causes a large change in both the emitter and collector currents. Operation characteristics of a bipolar junction transistor are presented in Fig. 7, and it shows the relationship between the emitter-collector voltage and the collector current which is the output of a transistor. At a constant emitter-collector voltage, the collector current is proportional to the base current which is the input current. The maximum power is represented by a dashed parabola. IC(mA) Saturation region 8 90 A 80 A 7 70 A 6 60 A 50 A 5 40 A 4 Pcmax=VCEIC 3 30 A Active region 20 A 2 10 A 1 IB=0 A 5 0 VCES 10 15 Cutoff region ICEO ~ = ICBO 20 VCE(V) Fig. 7. Operation characteristics of a bipolar junction transistor. As shown in the figure, the BJT has three operation modes, linear (active), cutoff, and saturation regions, respectively. In the linear region, as the base current increases, the collector current increases linearly. Thus, the amplification is possible. When the transistor is used as a switch, it uses cutoff and saturation regions. Ideally, the collector current (IC) is zero in the cutoff condition, but if the base current IB=0, then IC=ICEO (collector emitter leakage current) and ICEO is very small. The collector current in the saturation condition is set at the limited value. Therefore, transition between cutoff and saturation is a switching of a transistor in a circuit. The modern transistor is usually used as a switch. (3) Silicon Controlled Rectifier (SCR) The BJT transistor is widely used as a switch, but, its power rating is not so big. To overcome the disadvantage of the BJT, 4-layer devices are used, and their family is called thyristor. The thyristor is a four-layer, three-terminal semiconductor switch. A representative device in this family is Silicon Controlled Rectifier (SCR). Only silicon is used for the SCR, and rectification is its main duty. 3rd silicon layer of the SCR is used for controlling the circuit opening or shorting. The configuration of SCR is the same as a pnp transistor connected to an npn transistor as shown in Fig. 8. The anode 2 has (+) polarity for cathode in the forward conduction state. But, unless the sufficient current supplies to the gate terminal, the device is maintained as an off state. An equivalent circuit diagram is shown in Fig. 9. Anode + IA p IGT n p Cathode Anode -+ Anode + n p p n p n IGT n Cathode -- IGT Gate Gate -Cathode Fig. 8. Two transistor equivalent SCR, symbol and basic construction. V IA=ICO VGT VG E1 E1 IB1 Q1 0V t1 V t2 t3 t4 t VGT=0V IB2=0 IC2=ICO Q2 E2 VBE2 (a) High Z open circuit E2 V IA V E1 Q1 IC1 IB2 + VGT=VG -- E1 IB1 VBE2 IC2 Low Z short circuit Q2 E2 IK=IA E2 (b) Fig. 9. An equivalent circuit diagram of the SCR and its operation. When the gate signal is applied to the gate terminal of SCR, VGT is 0 before t1, thus, IB2= 0. IC2, the base current of Q1 is about ICO. This current is too small to make Q1 an on state. Therefore, two transistors are all off state, and status between E1 and E2 is an open circuit. When VG is applied to the base terminal of Q2, VBE2 is equal to VG. So, the enough current IB2 to make Q2 on state can flow. Then, IC2, the collector current of Q2, increases, which can make Q1 on state. When Q1 becomes on state, IC1 is added to IB2, then IC2 increases more. Finally, the circuit between emitters of two transistors becomes short state. Although VGT becomes 0 at t2 as shown in Fig. 9, SCR is not turned off because, once SCR turns on, all 4-layers of SCR are full of charge carriers. Therefore, once SCR turns on, the gate does not function. Fig. 10 shows an example of SCR applications. It is a circuit for half wave variable resistance phase control. The gate current to SCR is limited by R and variable resistor R1. Gate input current is the sinusoidal wave with the same phase as IL. As R1 decreases and reaches a point, the gate current increases and reaches a value to trigger the SCR. At this moment, the SCR turns on. By setting the proper resistor value, the phase when the SCR is triggered can be adjusted. By this, electric power on a load can be controlled so as required root-mean square current can be achieved. 3 RL IL R A + R1 IG Trigger level G K -90 Fig. 10. A circuit for half wave variable resistance phase control. (4) Gate Turn-Off Thyristor (GTO) The gate turn-off thyristor (GTO) is a power semiconductor device like the thyristor. It has an added advantage that a negative gate-current pulse can turn off the device. In fact, the GTO is a turn-off semiconductor switch with the highest current and voltage ratings presently available. An electric-circuit symbol for the GTO is similar to that of the thyristor except an addition to the gate electrode to represent the property of gate turn-off as shown in Fig. 11. The structure of the GTO is essentially the same as the conventional thyristor. As shown in Fig. 11, there are four silicon layers (pnpn), three junctions, and three terminals (anode, cathode and gate). Anode + Anode + p n Gate p Gate n -Cathode -Cathode Fig. 11. GTO composition and its electric-circuit symbol. The most important characteristic of the GTO is its turn-off ability. If a negative gate signal is injected while the GTO is in the on-state, the device will switch off. It will remain off until the conditions of a positive anode and a positive gate with respect to the cathode are met. The useful and economic power range of the GTO lies between the bipolar power transistor (or IGBT) and the thyristor (SCR). It does not switch as fast as the transistor but can handle higher voltage and current levels. It is faster than the phasecontrolled thyristor and can be turned off with a gate signal, but is not capable of modulating such high powers as the thyristor. Accordingly by cost and ratings, thyristors will not be replaced by GTOs at the highest power levels. The overall switching speed of the GTO is faster than the thyristor with comparable voltage and 4 current, but the GTO has a higher on-state voltage drop. Since the GTO has unidirectional current conduction and can be turned off at any time, its applications are mainly in chopper circuits (dc-dc conversion) and in inverter circuits (dc-ac conversion) at power levels above which MOSFETs, BJTs and IGBTs cannot be used. (5) Field Effect Transistor (FET) The most significant difference between BJT and FET is that the BJT is a current controlled device in which output current is controlled by the current between base and emitter, while FET is a voltage controlled device in which output current is controlled by the voltage between gate and source (VGS). There are n- or p-channel FETs like the npn or pnp types in BJTs. While the BJT is a bipolar device in which the conduction state is dependent on the two charge carriers (electron, hole), the FET is an unipolar device using one type of charge carrier. In the FET, the electric field is formed by the charge carriers that control the conduction path. There are several types of FET including: JFET (Junction Field Effect Transistor) MOSFET (Metal Oxide Semiconductor Field Effect Transistor): Depletion-type MOSFET Enhancement-type MOSFET CMOS (Complementary MOSFET) Fig. 12 shows a basic n-channel depletion type MOSFET. Three terminals in the FET, source (S), gate (G) and drain (D) have metal contacts, and S and D are connected to the n-channel. The gate is separated from the n-channel by a very thin SiO2 layer. Since the SiO2 is a dielectric material, when electric field is applied to this layer, opposite directional electric field is generated inside it. Since there is no electric contact between gate and other terminals due to the SiO2 layer, input impedance of the MOSFET is very large. D-Drain SiO2 D n-channel n n e Metal contact e e G-Gate n p Substrate Substrate SS e n e G p SS VDD e e e n n n-doped region S-Source (a) S ID=IS=IDSS (b) Fig. 12. An n-channel depletion type MOSFET and its operation with VGS=0 V and applied voltage VDD. 5 When VGS=0 V, and a VDD is applied between source and drain, the current in nchannel can flow. If a negative voltage is applied to the gate, positive voltage is induced at the gate side in the SiO2 layer and a negative voltage is induced at the n-channel side. This negative voltage pushes the electrons in the n-channel, and attracts holes in the psubstrate. As a result, the recombination between electron and hole increases, and the conduction current decreases. As the negative voltage at the gate increases, the pinchoff occurs. This mode is called a cutoff region. If a positive voltage is applied to the gate, electrons which are the minority carriers in the p-substrate are accelerated to the nchannel. These electrons make other electrons become ionized, and make ID abruptly increase. This mode is called an enhancement region. Thus, the current flow between source and drain can be controlled by the gate signal The basic structure of an enhancement-type MOSFET is shown in Fig. 13. A SiO2 layer separates the gate and the p-type substrate, therefore, in the case of VGS=0 V, there is no current between D and S even though the voltage is applied. Fig. 14 shows the basic operation of the enhancement-type MOSFET. As shown in Fig. 14(a), when a positive voltage is applied to the VDS and VGS, holes in the p-substrate are repelled by this positive voltage, and electrons which are the minority carriers in the p-substrate are attracted by the voltage. Since SiO2 is an insulator, electrons are not absorbed. As VGS increases, electrons form a channel, that makes the current between D and S flow. According to the V GS, electron channel is formed, so it is called the enhancement MOSFET. After fixing the VGS, increasing VDS makes the gate less positive after pinch-off as shown in Fig. 14(b). D-Drain SiO2 No channel n Metal contact G-Gate p Substrate Substrate SS n n-doped region S-Source Fig. 13. The basic structure of enhancement-type MOSFET. 6 Electrons attracted to + Gate Region depleted of p-type carriers(holes) ID n D G VGS S Pinch-off(beginning) + + + + + + n D + + + + p + + + + e e e e e e e e Depletion region ID e SS VDS n p e VGS S SS VDS e e nn Holes repelled by + Gate IS=ID (a) IS=ID (b) Fig. 14. The basic operation of enhancement-type MOSFET, (a) Channel formation in the n-channel enhancement type MOSFET, (b) Change in channel and depletion region. (6) Insulated Gate Bipolar Transistor (IGBT) GTO is available for high voltage and current applications but limited to a relatively low frequency. Also it requires a relatively high power for gate control. MOSFET can be applied to a very high frequency, and its control is very easy. But, it is limited to relatively low power applications. IGBTs are used in the power and frequency ranges between GTOs and MOSFETs. The IGBT is a three-terminal power semiconductor device that combines the simple gate-drive characteristic of the MOSFET with the high current and low saturation voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. The IGBT is used in medium- to high-power applications such as switched-mode power supply, traction motor control and induction heating, and its application field is being extended. There are two types of IGBT, PT (punch through) and NPT (non-punch through) types. The structures of the NPT and PT IGBTs are shown in Fig. 15. In an actual case, many IGBT unit elements are connected in parallel, and it is called as IGBT device or module. 7 SiO2 n+ Polysilicon Gate SiO2 p+ + + + p- n+ n-ch. formation ee e ee p+n- junction R-modulation + + + Emitter + + + ee e ee n+ p+ n- p+ Polysilicon Gate Collctor + + + ee ee e + + + ee ee e n+ pp+ (a) NPT IGBT Emitter p- n+ buffer n- Collctor p+ pp+ (b) PT IGBT Fig. 15. The structures and operations of NPT and PT IGBTs. The NPT IGBT consists of 4 layers of n+pn-p+, which comprises two transistors like the thyristor. However, unlike the thyristor, the gate is separated by SiO2 oxide layer. The gate terminal is connected to a polysilicon layer which covers n+ and nregions. The metal contact for the emitter terminal covers n+ and p- regions. If the collector of an NPT IGBT is positively biased for the emitter, and if the gate potential is 0, there is no current flow from the collector to the emitter. If the gate is positively biased against the emitter as shown in Fig. 15(a), the electrons in the p- region are attracted and collected between n+ and n- regions due to the role of the SiO2 oxide layer. Therefore, an n-channel between the emitter and the n+-p--n- is formed. Then, electrons are entering from the emitter to the n- region and holes are injected from the collector to the n- region. These excess electrons and holes reduce the resistivity of n- region. This is called an R-modulation or a conductivity modulation, which reduces the on-state resistance of IGBT. This allows the IGBT to be used in a higher power than the MOSFET that has no R-modulation. The PT type IGBT has an n+ buffer layer between the n- and p+ regions. Some of holes injected from the collector to the p+ region are lost in this region due to the recombination. This layer improves turn-off speed by reducing the minority carrier injection quantity and by raising the recombination rate during the switching transition. However, on-state voltage drop increases due to this layer. Therefore, PT IGBT has trade-off characteristics when compared to the NPT type for the switching speed and forward voltage drop or power dissipation. In the PT IGBT fabrication procedure, an n- layer is grown by the epitaxial method after growing an n+ buffer layer on the p+ substrate. But, the thickness of the epitaxial layer that can be grown is limited. Therefore, the limited thickness of the n- layer makes the blocking voltage of PT IGBT lower than the NPT type. In fabricating the NPT IGBT, the base material is an n- type silicon wafer with an extremely uniform dopant concentration, and NTD silicon can be applied for it. The p+ region in the collector side is formed by an implantation method. The emitter side and MOSFET structure are achieved by a diffusion method. IGBTs have the trade-off characteristics between a low conduction loss due to the bipolar current and a high switching loss due to the tail current at turn-off. So, 8 technology for the minority carrier lifetime control has been necessarily developed. The switching features were significantly improved by using the punch-through epitaxial structure in comparison with NPT IGBT initially fabricated in the early 1980s. However, since the thickness of the drift region should be large for a higher blocking voltage, NPT structure fabricated using a bulk starting material has been preferred for high voltage applications [28,29]. The short circuit capability of NPT type IGBT with homogeneous base material is much more robust than the PT type based on the epitaxial technology. In the late 2000s, the competition of the IGBT device market from 600 to 3500V has been between PT and NPT IGBT devices. PT IGBTs are dominant in the lowvoltage region less than 1000 V, whereas NPT-IGBTs are dominant in the region higher than 2500 V. However, PT and NPT IGBTs have competed for some years in the midvoltage region [30]. 3.3.2. NTD technology and power semiconductor devices Power semiconductor devices or power devices are used to control the electricity over several watts. They play a crucial role in regulation, distribution, transformation and adaptation of electrical energy between suppliers and users. Consequently, the performance of the power devices has a significant impact on the efficient use of electricity. Power devices are usually used as switches or rectifiers in power electronic circuits (switch mode power supplies for example). Accommodation of higher current density, higher power dissipation and/or higher reverse breakdown voltage is a basic property required for the power semiconductor devices. Also in order to prevent the device disorder due to the heat, a power device is usually attached to a heat sink to remove the heat generated by operation loss. Power semiconductor devices have following typical characteristics: (1) High breakdown voltage The high breakdown voltage is required for their applications to high voltage. For this, the drift region of the charge carriers must be thick with low doping density. However, these make the on-resistance of this region in the state of current flowing high, then a high power loss accompanied with heat cannot be avoided. So, there is a trade-off between breakdown voltage rating and on-resistance. (2) Low on-resistance If a large number of parallel cells are used, the on-resistance can be lowered enough for the circuit to be available for the high current rating. But this leads to a higher overall capacitance and after all, the switching speed may be slowed down. (3) Fast switching In order for a device to be switched between on- and off-state as fast as possible, rise and fall times must be short. It is practically very important in the aspect of the heat treatment of a device since the switching loss is mainly responsible for the heat generation. (4) Wide safe operation area 9 In order for a device to avoid being out of ordered or destroyed, the safe operation area from thermal dissipation must be as wide as possible. After the invention of the transistor in the late 1940s, semiconductor devices were first used in low power applications for communications, information processing, and computers. In 1958, the first thyristor called SCR was developed [31], which is the era of power electronics [32]. Since then, there has been a steady growth in the use of power devices, and ratings of these components are also steadily increased owing to a demand for them in various applications. Nowadays, various devices such as rectifier diodes, bipolar transistor, thyristor, GTO, Diac, Triac, power MOSFET, IGBT, IPM(Intelligent Power Module), etc. are widely used. Fig. 16 shows the current/voltage/switching frequency of the main power electronic devices [33]. It is found that the thyristor is the best for power applications, and the MOSFET is proper for high frequency applications. Fig. 16. The current/voltage/switching frequency of the main power electronic devices. The power semiconductor diode operates on similar principles as its low power counterpart. But, it is able to carry a high current and to support a high reverse bias voltage in the off state. NTD technology can be applied to P-i-N rectifiers with n- drift region of a low doping concentration. Since the introduction of the metal-oxide-semiconductor (MOS) technology, the power MOS-gated field effect transistors (MOSFETs) were developed in the 1970s. Their performance was found to be powerful in the aspect of the operating frequency but unsatisfactory for medium-power applications where the operating voltages exceed 300 V. Today, power MOSFETs are the most commonly used power switches below 200 V [34]. Thus, their utilities are limited to the low-voltage, high-frequency applications. NTD technology is hardly applied to MOSFETs with p-type substrate. The thyristor is a representative rectifier in high power region. Fig. 17 shows the typical doping profile of a thyristor. The structure is usually constructed by starting with 10 a lightly doped n-type silicon wafer whose resistivity is chosen based upon the blocking voltage rating for the device [35]. The n- drift layer is much wider than other regions, and it has the lowest doping level about 1014 cm-3 for high breakdown voltage [36]. NTD technology is capable of making this low doping concentration and uniformity throughout the whole region. -3 Impurity concentration profile [cm ] 21 10 n+ diffusion 20 10 p+ diffusion p diffusion 19 10 18 10 17 10 16 10 15 10 n- drift 14 10 13 10 Anode p+ n- p n+ Cathode Gate Fig. 17. Typical doping profile of a thyristor. Fig. 18 shows a typical thyristor fabrication process [37]. The anode p+ region is formed by the diffusion of dopants from the backside of the wafer to a junction depth. The p- base and n+ cathode regions are formed by the diffusion of dopants from the front of the wafer, respectively. Electrodes are formed on the front and back sides of the wafer. The electron or proton irradiation may be performed to reduce the carrier lifetime as the final fabrication step. 11 n Starting NTD-silicon p n p p n p p n p p-type diffusion n+ n-type emitter diffusion n+ n-type emitter structure Metal n+ p n Metal deposition p Metal n+ Edge contour and passivation Passivation Electron or proton irradiation Fig. 18. Typical thyristor fabrication process [37]. The growth of the power rating for the thyristors is shown in Fig. 19, and Professor B. J. Baliga pointed out that it is much indebted to the development of NTD technology [35]. 12 5 Advent of NTD 8 4 6 3 4 2 2 0 1950 Current Voltage 1960 1970 1980 1 1990 Thyristor current rating [kA] Thyristor voltage rating [kV] 10 0 2000 Year Fig. 19. Variation of applicable power to a thyristor device [35]. "Starting with a modest current of 100 A in the 1950s, the current rating has been scaled to approach 5,000 A for a single device. These high-current levels are required for power distribution systems such as high-voltage DC (HVDC) transmission networks. From the figure, it can be observed that the most rapid increase in the current-handling capability took place at the end of the 1970s. This outcome can be traced to the development of the neutron transmutation doping (NTD) process in the mid-1970 time frame. Using the NTD process, it became possible to obtain larger diameter silicon wafers with uniform properties enabling the observed scaling of the current-handling capability of thyristors. There was a concomitant increase in the voltage-blocking capability for the thyristor as illustrated in the chart. Beginning with devices capable of operating up to a few hundred volts in the 1950s, the voltage rating for thyristors has been escalated to 8,000 V. The increase in the voltage rating had to be accomplished by the availability of higher resistivity silicon wafers. This was initially achieved by the development of the float-zone process. However, the resistivity variation produced by this process was inadequate for utilization in the large diameter wafers desired to increase the current ratings. The NTD process was instrumental in providing the breakthrough required to create large diameter silicon wafers with low n-type doping concentration and high uniformity in the resistivity across the wafers. Consequently, a substantial gain in the voltage rating occurred in the late 1970s after the commercial availability of NTD silicon as indicated in the chart." The gate turn-off thyristor (GTO) was developed from a significant interest in the control of DC-motors. It needs a large reverse gate drive current. In spite of the bulky and expensive gate control circuits, GTO was widely used for the control of traction motors so far. Like the thyristor, the GTO has been one of main parts of NTD wafer consumption. Since the power-handling capability of the insulated gate bipolar transistor (IGBT) becomes very high in the 2000s, the GTO has been replaced by the IGBT. Since the introduction of thyristors in the 1950s, many bipolar power devices were developed. Among them, the power bipolar transistor extended the operating frequency 13 of power systems. However, since it was a current-controlled device using a small current gain, the control circuits should be bulky and expensive. For this reason, the power bipolar transistor has been replaced in the 1990s by the insulated gate bipolar transistor (IGBT) which is a voltage-controlled device. The IGBT was introduced in the 1980s to provide superior characteristics in the medium-power applications. It combined the best features of the bipolar power transistor and the power MOSFET. Nowadays, it has become the most powerful power device in the medium-power range, and it has been widely accepted for various applications including motor control systems. As shown in Fig. 15, the p+ anode (collector) in PT IGBT is the low-resistivity substrate material, and the n- layer is an epitaxial layer about 50 μm thick with a doping concentration below 1014 cm-3 [38]. In the fabrication process of a punch through (PT) IGBT, the starting material is high-purity, dislocation-free Czochralski (CZ) or float zone (FZ) silicon having orientation <100>. They are usually heavily doped with p-type dopant to form a p+ substrate. Thus, NTD technology is not capable of the fabrication of the PT IGBT. The non-punch through (NPT) IGBT has a large n- substrate layer, and neutron-transmutation doped (NTD) n-type float zone silicon is used for this layer because the NTD provides tight tolerances for the dopant density fluctuations required to achieve high manufacturing yields for large-area IGBT chips. As shown in the above, the floating-zone NTD silicon wafer which has the best quality has been mainly applied to the extremely high power range such as thyristors, GTOs and a part of the IGBTs. Recently, the application of the NTD wafer is being extended to the medium power range like IGBT modules for traction control of the hybrid electric vehicle (HEV). A HEV equipped with an electric power source like electric motors and batteries in addition to the conventional propulsion system became available to the public from late 1990s. The inverter is the most important component for controlling the electricity flow between the motor, the generator and the battery for the traction of an HEV, and the power devices are key components for their performance. Power devices used in the HEV should be small to be equipped in a limited space, and they should have additional features like ruggedness under the rather severe environment. Most HEVs adopt IGBTs as power devices for controlling the traction. A great improvement in the IGBT properties for the HEV has been brought by the field-stop (FS) IGBT fabricated by a thin wafer processing technology using FZ-wafers [39]. The switching loss of FS-IGBT is much lower than typical NPT IGBT. FS IGBT has greatly contributed to the shrink of the chip size and the thickness [40]. Table 2 compares three different IGBTs [41,42]. Prius II by Toyota, which is the most popular HEV model nowadays, adopted the conventional planar FS IGBT concept for the main inverter. For the better performance of the inverter, the use of the NTD wafer with the most uniform dopant distribution seems to be crucial. The HEV market has been growing very fast, and it can be easily expected that the demand for NTD wafer would be growing according to the development of the HEV. Infineon, a power device company, estimated that a future hybrid electric vehicle would consume approximately one 6 inch wafer of semiconductor [43]. Table 2. Comparison of the different IGBTs. 14 PT-IGBT NPT-IGBT FS-IGBT Structure n+ buffer layer between p+ substrate and n- drift region n- drift region on implanted p+ layer (no buffer layer) Fabrication Epitaxial growth FZ wafer process Doping concentration of additional n+ layer Paralleling for chips or modules NTD wafer applicability More than 1017 cm-3 n+ field stop layer between implanted p+ layer and n- drift region FZ thin wafer process 1015 to 1016 cm-3 Difficult Easy Easy No Yes Yes 3.4. RESISTIVITY PROFILE As described above, the neutron transmutation doping (NTD) for silicon is a method for producing an n-type silicon with highly uniform phosphorous distribution. In general, the uniform p-type doping with a p-type material such as boron and aluminium is easily achieved by non-nuclear technologies because dopants are relatively stable in the molten silicon. However, there are several problems in conventional technologies for the n-type doping due to the volatility of the group 15 elements, such as small distribution coefficient, etc. Since silicon is very transparent to neutrons, if an irradiation device is carefully designed for the uniform and accurate neutron irradiation, the NTD can give very high doping uniformity compared to other technologies. Fig. 20 compares the radial resistivity profiles of an NTD silicon with another one doped by conventional method [44]. Local resistivity depends on the charge carrier density at that position which is determined by the dopant concentration. As shown in Fig. 20, the NTD method results in an extremely uniform concentration of dopant. The local resistivity variation of the NTD silicon is usually less than 5 %. But, for the conventional method, the variation is over 80 ~ 90 %. In addition to larger resistivity variation, the concentric distribution pattern like an annual ring is represented in the radial resistivity profile due to the striation of the dopant distribution. Table 3 compares typical radial resistivity uniformity of the floating zone (FZ) silicon crystals doped by gas doping and NTD. The FZ silicons are classified, according to the uniformity of the resistivity, into premium class, super-premium class and NTD FZs. Premium class and super-premium class FZ silicon crystals are obtained by the gas doping method. The FZ silicon doped by the NTD method shows a much more uniform resistivity than those by gas doping. 15 Fig. 20. Radial resistivity profiles of the neutron transmutation doped silicon and that by conventional doping method. Table 3. Resistivity variations of floating zone (FZ) silicon crystals doped by gas doping and NTD methods. Material Doping method Premium FZ Super premium FZ NTD FZ Gas doping Gas doping NTD Radial Resistivity Gradient (RRG) 8 ~ 25 % 5 ~ 15 % 1.5 ~ 5 % Local uniformity in very small range 3% 2.5 % 0.8 % 16