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 June1996·31TECHNICAL REPORTS The World’s 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 six￾inch 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 mitigat￾ing 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 power￾control systems that are simpler, more compact and more reliable. However, increasing thyris￾tor blocking voltage and capacity generally in￾volves sacrificing on-state voltage (VTM), reverse recovered charge (Qrr), critical rate-of-rise of on￾state current (di/dt), leakage current and other operating 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 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 semi￾conductor element. The element consists of small light-sensitive pilot thyristors at the cen￾ter that are triggered by an optical signal, and the main thyristor, which turns on in response to a gate current supplied by the pilot thyris￾tors. The package is designed to ensure uniform mounting pressure, and is constructed to pre￾vent alloying between the element and the heat￾absorber disc. The ceramic seal has a clear window to admit light. The photocurrent induced in the pilot thyris￾tor and the displacement current generated by dv/dt turn on the pilot thyristor, forcing a trade￾off 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 mak￾ing better use of the large photocurrent near the wafer surface (Fig. 2). To improve the dv/dt capac￾ity, we surrounded the main thyristor with pilot thyristors, and developed a gate-emitter structure that prevents dv/dt mistriggering. These struc￾tural enhancements, combined with design op￾timizations, 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 thyris￾tors (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