# Updates on Impact Ionisation Triggering of Thyristors
## 1 Introduction
Additionally, scaling the Marx Generator to achieve higher voltages by adding more stages can lead to increased complexity, larger physical size, and synchronization challenges between the stages, potentially complicating the system's overall operation. Solid-state thyristor switches have shown reliable performance in accelerator environments [8]. These devices offer a balance of precise control, moderate switching speed, high reliability, and low conduction losses. They are solid-state devices with long operational lifetimes, available in a wide range of voltage and current ratings. They can handle significantly higher currents compared to other semiconductor devices, making them suitable for applications that require robust power handling capabilities. However, designing a system that effectively uses thyristors involves complex assembly with multiple components like fast thyristors, snubbers, and custom gate drive circuits. Regular industrial devices do not meet the requirements to generate a field rise time of less than 100 ns as mandated by our application. Recent investigations [9] have demonstrated that thyristors triggered in overvoltage mode, typically called impact ionization (II) mode, offer a new approach to fast triggering (sub-nanosecond scale) for high-power applications. In general, an avalanche is a process that occurs in semiconductors under high electric fields. When carriers gain enough energy from the electric field, they can free electrons in the crystal lattice, creating electron-hole pairs. This process can lead to a rapid increase in the carrier density, resulting in an avalanche effect. When the applied electric field is sufficiently high and rapidly increased, ionization occurs throughout the semiconductor, forming an ionization front [10]. If the ionisation front propagates at a speed higher than the carrier speed, the process is called impact ionization. This triggering method requires the application of high dU/dt and sufficient over-voltage to the thyristor, resulting in the creation of an ionization wavefront within the semiconductor structure [11]. This technique enables ultra-fast switching and can even be implemented using commercially available thyristors, rendering them faster and capable of accommodating higher current density compared to similarly sized thyratrons. To induce impact ionization in four-layered devices, like thyristors, the triggering circuit necessitates a voltage applied across the anode-cathode region exceeding twice the static breakdown voltage, alongside a slew rate surpassing 1 kV/ns ([12; 13]). Consequently, the triggering circuit must deliver sufficient current to charge the thyristor's parasitic capacitance with the required dU/dt. Previous documentation provides in-depth insights into the conditions necessary for inducing impact ionization in thyristors. Researchers in [9] explored the operation of fast, GTO-like thyristors developed by ABB Semiconductors, which were triggered in the impact-ionization wave mode. This innovative triggering method, employing a Semiconductor Opening Switch (SOS) generator, significantly improved the current rise rate and reduced turn-on delays. Subsequently, in a study by [12], researchers investigated the capability of triggering commercial low-frequency thyristors within sub-nanosecond timescales using a fast, sub-nanosecond pulse across the thyristors' main electrodes from a compact solid-state generator. In [13], researchers investigated the parallel switching of high-voltage thyristors triggered in the impact-ionization wave mode, leveraging a Marx generator with a peaking module to achieve the necessary fast voltage rise time. This method effectively handled high current pulses while reducing current imbalance among the devices. Once the feasibility of inducing impact ionization mode in commercial thyristors has been demonstrated, the question arises regarding the selection of the triggering method. In [14], researchers explore the application of a Marx generator using thyristors triggered in shock ionization wave mode.