# Compact Design and Impact Ionization: Utilizing Small-Sized Thyristors in a 4-Stage Marx Generator
They are capable of managing significantly higher currents than other semiconductor devices, which makes them well-suited for demanding power-handling applications. Although thyristors offer several advantages, integrating them into an efficient system requires a complex assembly of multiple components, such as fast thyristors, snubbers, and specialized gate drive circuits. While thyristors perform well in numerous applications, standard industrial devices do not meet the necessary criteria to achieve a field rise time below 100 ns, as is necessary for our specific application. Recent studies have introduced the concept of triggering thyristors using overvoltage, or impact-ionization mode. This method allows for rapid switching on a sub-nanosecond scale in high-power applications [11]. Overvoltage refers to a fast-rising pulse applied to the thyristor, characterized by a brief duration and an increase in the applied voltage to at least twice the DC breakdown voltage. Typically, when semiconductors are subjected to high electric fields, an avalanche effect occurs. Under such conditions, carriers within the semiconductor acquire sufficient energy from the electric field to free electrons from the crystal lattice, leading to the generation of electron-hole pairs. This cascading effect, or avalanche effect, leads to a sudden increase in carrier density. In this mode, high electrical fields within the thyristors accelerate charge carriers, leading to collisions with atoms and the generation of additional carriers. This cascade effect facilitates a rapid increase in current flow, allowing the thyristor to transition swiftly from a blocking state to a conducting state [12]. When a sufficiently high and rapidly increasing electric field is applied, ionization takes place throughout the semiconductor material, creating an ionization front [13]. If this front propagates at a velocity exceeding that of the charge carriers, the phenomenon is referred to as impact ionization. This rapid propagation of the ionization front enables the fast triggering capabilities observed in thyristors under overvoltage conditions. This triggering technique involves applying a high rate of voltage change (dU/dt) along with sufficient overvoltage to the thyristor, generating an ionization wavefront within the semiconductor [14]. This method enables ultra-fast switching speeds and can utilize commercially available thyristors, making them not only faster but also capable of handling comparable or higher current densities compared to thyratrons of similar size. According to [14], while typical electron densities in thyristors in the on-state range are in the order of \(10^{16}\) cm\({}^{-3}\) to \(10^{17}\) cm\({}^{-3}\) versus \(10^{16}\) cm\({}^{-3}\) for a thyratron [15], it is the achievable conducted charge that is more critical for practical applications. Thyratrons, with limited gas volume available for conduction, require physically bulky designs to handle high currents and long pulses. In contrast, once triggered, thyristors can conduct continuously, making them more efficient for sustained high-current applications. To initiate impact ionization in four-layer devices like thyristors, the triggering circuit must apply a voltage across the anode-cathode region that exceeds twice the static breakdown voltage, while maintaining a slew rate exceeding 1 kV/ns [16; 17]. Consequently, the circuit must supply sufficient current to charge the target thyristor's parasitic capacitance at the required rate of voltage change (dU/dt). Significant advancements in understanding the conditions necessary for impact ionization in thyristors have emerged. [PERSON]. [11] delved into the behavior of fast, GTO-like thyristors engineered by ABB semiconductors, demonstrating their activation in the impact-ionization wave mode. By leveraging a semiconductor opening switch (SOS) generator, this technique not only accelerated the current rise rate but also effectively minimized turn-on delays. Meanwhile, [PERSON].