## 3 Overview
### Classification of Available In-House Built Generators for nsPEF Electroporation  
It was identified, that the traditional transmission line (including Blumlein-type) pulse forming circuits are no longer dominating in the nsPEF electroporation applications. The progress of semiconductor technologies enabled the development of cost effective, small-size and flexible sub-microsecond generators based on direct capacitor discharge topology. The Marx generator topology with new ultra-fast semiconductor switches forms a new leading technology trend now. In addition, there were few successful attempts to develop resonant-circuit nsPEF generators, which at the end were not followed by other researchers. Unpopularity of this topology is driven by the limited pulse duration flexibility and resulting Gaussian pulse shape. All topologies demonstrate the possibility to produce high-voltage pulses, however, the transmission line (including Blumlein-type) topologies, which use spark gap switches, demonstrate a possibility to produce very high amplitude (peak voltages exceeding 10 kV) nsPEF rectangular wave pulses. Yet there is complexity associated with these designs and recent developments of Marx-bank [69; 70] and other direct capacitor discharge circuits [71] indicate the growing availability of the off-the-shelf components fulfilling the high-voltage pulse delivery requirements. It is expected that the transmission line topologies will be pushed out of the sub-microsecond range due to a lack of flexibility, requirement of impedance matching and distorted waveforms. These drawbacks are not associated with the reported direct capacitor discharge circuits (including Marx-bank). However, the sub-100 ns pulses in the range of tens of kV are still yet hardly achievable by the direct capacitor discharge technology. Indeed, for the high-voltage sub nsPEF pulse delivery, the switch performance and characteristics are crucial. The spark gap switches are fast and cost-effective for nsPEF generation, but these switches have a short lifetime due to electrode erosion, poor pulse duration control and can be frequently associated with turn-on jitter [36]. In contrast, the semiconductor switches offer high flexibility of pulse control, but are more constrained by high-voltage or current withstand limits as well as switch opening and closing times. A series connected switches can solve high-voltage limitation issues, but this increases the stray inductance, which is making the circuit slower. One of the latest comparison between typical MOSFET, IGBT and bipolar junction transistor (BJT) switches was made in 2019 [72]. It was demonstrated that all three could ensure the breakdown voltage (collector-emitter or drain-source voltage) higher than 1 kV. However, the BJT switches, which among all would be the most cost-effective option, are slower than MOSFETs or IGBTs and cannot fulfill the nsPEF pulse forming requirements. It was demonstrated that only the high power MOSFETs can form pulses within the 100-300 ns range with the transition times (rise and fall time) faster than 100 ns [72]. More than a half of reported nsPEF electroporators (Table 2) use the MOSFET switch as a main pulse forming component. The review indicates that the direct capacitor discharge topology is taking a lead in the nsPEF applications when the sub-microsecond pulse is required. In addition, the MOSFET switches are now the main technology to produce high-voltage nsPEF pulses in various circuit topologies. The spark gap switches are still applicable in case of a very high-voltage amplitude or ultra-short pulse duration (a few nanoseconds) [64]. Other types of switches (like photoconductive semiconductor, optoelectronic switch or even IGBT) are rarely applicable even if they also provide a capability to deliver ultra-short pulses. In all cases, the transient processes are triggered during the switch turn-on and turn-off, which negatively influence the pulse parameters.