TECHNICAL REPORTS The World's Largest-Capacity 8kV/3.6kA Light-Triggered Thyristor by Katsumi sato* Mitsubishi Electric has developed an 8kV/3.6kA light-triggered thyristor (LTT) based on a six inch wafer for power-converter applications in igh-voltage DC transmission and back-to-back systems. New design features give the device double the power-control capacity of previous LTTs based on four-inch wafers while mitigat ing the problems that occur with large-capacity evices Development Objectives There is a market for high-capacity LTTs with a high blocking voltage, since they will enable equipment manufacturers to build power- Fig. 1 The package and semiconductor element of the 8kV/3. 6kA LTT and more reliable. However, increasing thyris tor blocking voltage and capacI ity generally in- 」 Antireflection coating volves sacrificing on-state voltage (VTM), reverse recovered charge(@rr), critical rate-of-rise of on- state current(di/dt) leakage current and other perating characteristics. In developing the new LTT, we needed to improve these characteris- tics, reduce the minimum light triggering power (PLT)(which helps extends the lifetime of the light source), reduce the loss in the snubber cir cuit, and forreasons of equipment size, improve critical rate-of-rise of off-state voltage(dv/dt) capability Anode contact Description Fig. 1 shows the 8kv/3.6kA LTT and its semi conductor element. The element consists of small light-sensitive pilot thyristors at the cen- ter that are triggered by an optical signal, and Fig. 2 A cross section of the light-sensitive area ain thyristor, which turns on in response to a gate current supplied by the pilot thyris ing better use of the large photocurrent near the rs.The package is designed to ensure uniform wafer surface(Fig. 2). To improve the dv/dt capac mounting pressure, and is constructed to pre- ity, we surrounded the main thyristor with pilot vent alloying between the element and the heat thyristors, and developed a gate-emitterstructure absorber disc. The ceramic seal has a clear that prevents dv/dt mistriggering. These struc window to admit light tural enhancements, combined with design op- The photocurrent induced in the pilot th timizations, yield a dramatic improvement in tor and the displacement current generated by the trade-off relationship between PLT and dv/dt dv/dt turn on the pilot thyristor, forcing a trade capability(Fig 3) off between PLT and dv/dt capacity. We reduced Large-capacity thyristors typically employ a PLT without sacrificing dv/dt capability by use dynamic gate structure, with the pilot thyris f a channeled cross section for the junction tors(which supply the trigger current) arranged between the p-base and n-base layers, thus mak in concentric circles. however in order to turn *Katsumi sato is with the power device division June1996·31
TECHNICAL REPORTS The Worlds Largest-Capacity 8kV/3.6kA Light-Triggered Thyristor by Katsumi Sato* *Katsumi Sato is with the Power Device Division. Mitsubishi Electric has developed an 8kV/3.6kA light-triggered thyristor (LTT) based on a sixinch wafer for power-converter applications in high-voltage DC transmission and back-to-back systems. New design features give the device double the power-control capacity of previous LTTs based on four-inch wafers while mitigating the problems that occur with large-capacity devices. Development Objectives There is a market for high-capacity LTTs with a high blocking voltage, since they will enable equipment manufacturers to build powercontrol systems that are simpler, more compact and more reliable. However, increasing thyristor blocking voltage and capacity generally involves sacrificing on-state voltage (VTM), reverse recovered charge (Qrr), critical rate-of-rise of onstate current (di/dt), leakage current and other operating characteristics. In developing the new LTT, we needed to improve these characteristics, reduce the minimum light triggering power (PLT) (which helps extends the lifetime of the light source), reduce the loss in the snubber circuit, and for reasons of equipment size, improve critical rate-of-rise of off-state voltage (dv/dt) capability. Description Fig. 1 shows the 8kV/3.6kA LTT and its semiconductor element. The element consists of small light-sensitive pilot thyristors at the center that are triggered by an optical signal, and the main thyristor, which turns on in response to a gate current supplied by the pilot thyristors. The package is designed to ensure uniform mounting pressure, and is constructed to prevent alloying between the element and the heatabsorber disc. The ceramic seal has a clear window to admit light. The photocurrent induced in the pilot thyristor and the displacement current generated by dv/dt turn on the pilot thyristor, forcing a tradeoff between PLT and dv/dt capacity. We reduced PLT without sacrificing dv/dt capability by use of a channeled cross section for the junction between the p-base and n-base layers, thus making better use of the large photocurrent near the wafer surface (Fig. 2). To improve the dv/dt capacity, we surrounded the main thyristor with pilot thyristors, and developed a gate-emitter structure that prevents dv/dt mistriggering. These structural enhancements, combined with design optimizations, yield a dramatic improvement in the trade-off relationship between PLT and dv/dt capability (Fig. 3). Large-capacity thyristors typically employ a dynamic gate structure, with the pilot thyristors (which supply the trigger current) arranged in concentric circles. However, in order to turn Fig. 1 The package and semiconductor element of the 8kV/3.6kA LTT. Fig. 2 A cross section of the light-sensitive area. Cathode contact (pilot thyristor) Light irradiation Antireflection coating Anode contact J1 J2 J3 NE PE NB PB Key J1~J3 Semiconductor junctions NB n base NE n emitter PB p base PE p emitter June 1996 · 31
TECHNICAL R Table 1 major Ratings and Characteristics Item Symbo Value epetitive peak off-state VDRM 8000V Average current 3600A di/dt ● ConventionaILTT On-state voltage Critical rate-of-rise of PLT(arbitrary units) off-state voltage ig. 3 The tradeoff between Pr and dv/ Minimum light triggering P 8.0mW power Turn-off time Thermal resistance 0.004°c tors. This arrangement has dramatically im- proved the turn-on characteristics: With an applied voltage of 8.8kV, a di/dt of 200A/us is achieved. Fig. 4 shows a typical turn-on wave- form at 8.8kV Units Lifetime control technology helps optimize the trade-off between VTm and Qrr, but conven- Time 1us/div tional techniques such as heavy metal diffusion and electron-beam irradiation affect the entire wafer. We chose instead to use proton irradia- Fig 4 The turn-on waveforms at 8.8kv. tion, which allowed us to control the lifetime profile in the thickness direction. This local- d lifetime control prevents a rise in VTM while reducing Qrr and the subsequent continuous o New LTT leakage currents. (There is no reduction of carrier ● ConventionalLY lifetime of the n-base. Fig 5 shows the trade- off relationship between VTM and Qrr of a con ventional LTT compared with that of this device indicating a 50% improvement. Table 1 lists the major specifications of the new device vement The unprecedented voltage and current ratings of this light-triggered thyristor will make it VrM(arbitrary units) possible to manufacture high-capacity power- control and power-conversion equipment that Fig. 5 The trade-off between VIM and Qrr is compact and reliable. O on the main thyristor more quickly, we devel oped a new dynamic gate structure in which the main thyristor area is dotted with pilot thyri 2. Mitsubishi Electric ADVaNce
TECHNICAL REPORTS on the main thyristor more quickly, we developed a new dynamic gate structure in which the main thyristor area is dotted with pilot thyrisFig. 5 The trade-off between VTM and Qrr. Qrr (arbitrary units) VTM (arbitrary units) Key New LTT ● Conventional LTT 50% improvement 0.6 0.7 0.8 0.9 1.0 1.1 0.2 0.4 0.6 0.8 1.0 1.2 Light pulse Units VD 2,000V/div IT 250A/div Time 1µs/div IT VD Fig. 4 The turn-on waveforms at 8.8kV. Fig. 3 The tradeoff between PLT and dv/dt capability. PLT (arbitrary units) 70% improvement 0.01 0.1 1 Key New LTT ● Conventional LTT dv/dt capability (V/µs) 100 1,000 10,000 Table 1 Major Ratings and Characteristics Item Symbol Value Repetitive peak off-state VDRM 8,000V voltage Repetitive peak reverse VRRM 8,000V voltage Average current IT(AV) 3,600A Critical rate-of-rise of di/dt 200A/µs on-state current On-state voltage VTM 2.8V Critical rate-of-rise of dv/dt 2,300V/µs off-state voltage Minimum light triggering PLT 8.0mW power Turn-off time tq 400µs Thermal resistance Rth(j-f) 0.004°C/W Table 1 Major Ratings and Characteristics 32 · Mitsubishi Electric ADVANCE tors. This arrangement has dramatically improved the turn-on characteristics: With an applied voltage of 8.8kV, a di/dt of 200A/µs is achieved. Fig. 4 shows a typical turn-on waveform at 8.8kV. Lifetime control technology helps optimize the trade-off between VTM and Qrr, but conventional techniques such as heavy metal diffusion and electron-beam irradiation affect the entire wafer. We chose instead to use proton irradiation, which allowed us to control the lifetime profile in the thickness direction. This localized lifetime control prevents a rise in VTM while reducing Qrr and the subsequent continuous leakage currents. (There is no reduction of carrier lifetime of the n-base.) Fig. 5 shows the tradeoff relationship between VTM and Qrr of a conventional LTT compared with that of this device, indicating a 50% improvement. Table 1 lists the major specifications of the new device. The unprecedented voltage and current ratings of this light-triggered thyristor will make it possible to manufacture high-capacity powercontrol and power-conversion equipment that is compact and reliable. ❑