Pulsed power is the science and technology of accumulating energy over a period of time and releasing it as a high-power pulse consisting of high voltage and current over a short period^[1]. Applications of pulsed power continue expansion into fields including the microwave, biological cells, and dielectric wall accelerator^[2-4]. In recent years, photoconductive semiconductor switch (PCSS) has been playing an important role in high-frequency, high power and fast pulse applications because of its high breakdown field, trigger isolation, high speed, negligible jitter and high instantaneous power output from a single device^[5-7]. Compared to the first- and second-generation semiconductor materials, silicon carbide has excellent characteristics such as stable chemical properties, high hardness, wide bandgap, high breakdown field, high saturation electron velocity and high thermal conductivity, and the research of PCSS based on silicon carbide has been published^[8-13].

It has been reported that by preparing vanadium-compensation semi-insulating (VCSI) 4H-SiC vertical PCSS and using 1 MHz repetition frequency and 1030 nm tunable pulse width laser, it can generate 1 MHz repetition frequency pulse with a minimum half-width of 365 ps^[5]. The vertical PCSS based on VCSI 6H-SiC can achieve a maximum peak electrical power of up to 1 MW and a pulse width of about 1.1 ns when the peak optical power is several hundreds kW^[12]. Individual VCSI 4H-SiC PCSS often operates at high pulse currents (>50 A) and high pulse voltage (1−20 kV)^[14-16]. Due to its ability to generate a transient pulse with a significant change in voltage, the impact of such pulse signals on the surrounding environment warrants further investigation.

In this paper, vertical PCSS with circular ring electrodes and lateral PCSS are designed based on VCSI 4H-SiC substrate. Through the experimental test and analysis, the single vertical PCSS can generate a peak voltage of approximately 6 800 V at a bias voltage of 13 kV, and the output photocurrent can reach up to 136 A. By constructing a coil module around the PCSS, we observe a distinctive pulse envelope characteristic curve. The coil generates a specific number of pulses (10–13) during the main pulse phase of the PCSS (which is equal to the laser pulsewidth), with each pulse signal exhibiting a very short width (0.9–1.3 ns). The output amplitude of the signal shows a linear correlation with the pulse amplitude of the PCSS, while the number of output pulse is determined by the number of turns in the coil. Experiments show that when the nanosecond laser pulse irradiates the surface of the PCSS, large numbers of photogenerated carriers are generated inside it. Photogenerated carriers form a pulsed current in response to the input bias voltage, which in turn creates a time-varying magnetic field at the boundary. The radiated magnetic fields are associated with their corresponding radiated electric fields through free space resistance. The PCSS radiates an electromagnetic wave, which is received by the metal coil and generates an oscillating signal.

The substrate material utilized in this experiment is VCSI silicon carbide (SiC), with a crystal structure of 4H and a dark resistivity exceeding 10⁹ Ω$ \cdot $ cm. The electrode of the vertical PCSS features a uniform distribution of circular electrodes, with an inner diameter of 7 mm for the upper electrode and 4.5 mm for the lower electrode. The circular electrode can balance the electric field at the edge of the device’s electrode to avoid the disadvantage of being affected by the larger field strength at the corners. In addition, the current is made to pass through the delimited circular electrodes, dispersing the current density and homogenising the field strength inside the device.

The fabrication process of both the vertical PCSS and lateral PCSS in this study involves cleaning the sample surface using the standard Radio Corporation of America (RCA) cleaning. Then, the top and bottom electrodes are deposited using physical vapour deposition (PVD) and metal mask deposition. The electrode metal composition is nickel, with a film thickness of 300 nm. Following this, a rapid thermal annealing process is conducted at 1 000 ℃ for 180 s in a nitrogen atmosphere. Finally, using PVD again, a layer of 120 nm titanium metal and 200 nm gold is deposited on the nickel layer^[17-18].

To further investigate the effect of the PCSS on the surrounding electromagnetic field during operation, a coil module insulated with the PCSS is constructed on the periphery of the PCSS. Then the induced electromotive force can be collected from the closed coil according to the law of electromagnetic induction. In this study, the key parameters manipulated are the number of turns in the coil module and the applied bias voltage, different structures are adopted to study their effect on the oscillations. Fig.1 show the switching structure of vertical 4H-SiC PCSS.

The protective resistance (R_M) of the vertical 4H-SiC PCSS measures 100 MΩ, the load resistance (R_L) is 50 Ω, and the charging capacitance amounts to 8.38 nF. Both ends of the capacitor are charged by the high voltage power supply shown in Fig.2(a), and the pulse generation is controlled by the PCSS. Then the output waveform of the vertical PCSS and coil module under different bias voltages are collected by oscilloscope. Fig.2(b) shows the incidence direction of the PCSS and the overall structure of the circuit. The experiment utilized a high-voltage source with a voltage range of 0−70 kV. The oscilloscope has an analog bandwidth of 2 GHz and a maximum realtime sampling rate of 10 GSa/s. The high-voltage probe used in the experiment has a bandwidth of 220 MHz and an attenuation ratio of 1000∶1. The laser operating at a wavelength of 532 nm and delivering a laser energy of approximately 170 mJ. The incident PCSS has a laser spot diameter of approximately 7 mm. The laser pulse has a pulse width ranging from 12 ns to 13 ns and repetition frequency between 1 Hz and 10 Hz.

Initially, the output performance of the prepared single vertical PCSS is tested. Table 1 presents the measured values of the peak output voltage (V_max) across the load resistance, and the peak output current (I_max) of the PCSS under varying bias voltages ranging from 3 to 13 kV (V_in). The vertical PCSS outputs a peak voltage of 6 800 V at 13 kV bias, a peak current of 136 A, and outputs a maximum peak power of 0.9 MW. Moreover, the designed vertical PCSS can reach 7 kV at 6 ns with a fast rising edge. Fig.3 illustrates the output waveforms of the vertical PCSS at different bias voltages.

$ {{I}}_{\max}=\dfrac{{{V}}_{\max}}{50} $ (1)

$ {{P}}_{\max}={{V}}_{\max} {{I}}_{\max} $ (2)

A single PCSS with a coil is assembled as shown in Fig.1. The circuit diagram is shown in Fig.2. The oscilloscope is connected to both ends of the coil module and the load resistance simultaneously. The bias voltage is gradually increased from 3 kV to 13 kV (V_in), and two output waveforms are obtained for both the coil module and load resistance at the same bias voltage, so that a new electromagnetic oscillation phenomenon based on the PCSS is observed. The phenomenon shows that the coil outputs a high-frequency oscillation during an on-off phase of the PCSS and is related only to the output characteristics of the PCSS.

In Table 2, the peak load resistance output voltage (V_max), peak output current (I_max), maximum output voltage (V_max+c) of the coil module, and the number of coil output pulses (N) are recorded for bias voltages of 5 kV, 10 kV, and coil turns of 1, 2, 6. Fig.4-Fig.6 illustrate the output waveforms of the vertical PCSS and the coil under the aforementioned turn and bias voltage. In these figures, the black line represents the output waveform of the load resistance controlled by the PCSS, while the red line represents the output waveform of the metal coil module. The area delimited by the two vertical lines in the upper left corner of each figure indicates the identification position of the main pulse (which is equal to the laser pulsewidth) of the PCSS, with an average time domain length of 12.5 ns. The high-frequency oscillation studied in this paper precisely originates from this region.

From these figures, it can be observed that during the main pulse stage of the PCSS, the metal coil module generates a voltage pulse envelope curve. By counting the number of pulses output by the coil at different bias voltages, it is observed that the number of pulses does not change significantly with variations in the PCSS bias voltage. And the pulse amplitude within the pulse envelope is positively correlated with the bias voltage. This structure significantly reduces the rise-fall time of the output pulse. Furthermore, the amplitude of the output waveform from the metal coil module can be controlled by regulating the amplitude of the PCSS output waveform.

By altering the number of turns in the metal coil, the coil module is able to produce a stable number of narrow pulses during a primary pulse stage of the PCSS. Using the method of preparing PCSS–metal coils in this paper, it is possible to have the number of coil output pulses stabilised at 10–13 pulses in one main pulse phase of the PCSS output waveform, with the pulse width of one of the coil output pulse envelope at 0.9–1.3 ns, and the maximum magnitude of the coil output up to 2 980 V. This enables the output of pulses with narrower pulses (sub-nanoseconds) without changing the laser pulse width and frequency.

The experimental results above indicate that the generation of such oscillating signals is independent of the bias voltage and the number of coil turns. To further determine the source of the oscillations, the output waveforms of the metal coil and the load resistance are obtained when the switching structure is changed to the lateral PCSS. As shown in Fig.7(a), the oscillatory phenomenon is not related to the switching structure. Fig.7(b)-Fig.7(d) show the two output waveforms of the coil and the load when the laser energy is 40 mJ and the metal coil is connected in series with a 1 Ω resistor for different number of turns. It is verified that the main factor affecting the peak output power of the coil is the output characteristics of the switch.

In this paper, by varying the number of turns and the input bias voltage, it is observed that the coil has time domain synchronization with the switch output pulse, as soon as the switch outputs a main pulse, the coil module outputs a high-frequency pulse envelope. As demonstrated in Fig.8(a), the coil ceases to generate high-frequency oscillations when the switch main pulse has ended.

We further used the RLC series model to fit the second half of the coil output waveform in Fig.8(b) and obtained the equivalent resistance of the coil module as 3.356 Ω, the equivalent inductance as 0.139 µH, and the equivalent capacitance as 0.1 nF. The RLC series oscillation frequency is calculated to be about 23.4 ns, which compares to a high-frequency oscillation pulse width of about 1 ns at the switching main pulse output. This shows that the high frequency oscillations generated by the coil are different from our common RLC oscillations, and the output waveform of the coil module belongs to the superposition of the two. Based on this experimental phenomenon, it is believed that the oscillation is related to the way the carriers move inside the switch itself, rather than being caused by the influence of external disturbances.

The research indicates that this oscillation phenomenon is independent of external factors such as the number of coil turns, input voltage, and switching structure. Combining the current surge model in the theory of photoconducting antennas^[19-20], this oscillation phenomenon can be explained as follows: an internal electric field is formed between the positive and negative poles due to bias voltage. As shown in Fig.8(c), when the sample surface is irradiated with laser pulses on the order of nanoseconds, a large number of photo-generated charge carriers are generated internally. Under the influence of the internal electric field, these charge carriers form transient currents, thereby creating a device similar to an electric dipole antenna. At the interface of the PCSS, transient current generates electric fields radiating inward and outward. According to the current surge model, the electric field simultaneously creates magnetic fields radiating inward and outward. The mutually orthogonal electromagnetic radiation fields propagate from the PCSS surface, creating a fluctuating electromagnetic field. Then, it couples with external inductive elements and the observed oscillation waveform is produced.

In this paper, a vertical type PCSS is prepared, and its output voltage amplitude can reach 7 800 V, the output peak power is up to 0.9 MW, with bias voltage of 13 kV. Then, by using the direction of the current conduction of the vertical PCSS, a periphery coil is constructed. By changing the input bias and the number of turns, a kind of oscillation signal is collected. The presence of this signal demonstrates that semi-insulated VCSI SiC PCSS form an electromagnetic radiation field around the switch under the action of laser pulses and output bias, and the variations in this field can be picked up by external components such as inductors. The phenomenon can be explained by the current surge model^[19], in which the transient circuit changes inside the switch during conduction cause the switch to form inward and outward electromagnetic fields, which in turn radiate electromagnetic waves. Based on the experimental results, it is observed that the output waveform of the coil remains stable when using 1 or 2 turns. In each main pulse of the switch, the coil generates an average of 10−13 pulses. These pulses have a timedomain pulse width of approximately 1ns and a maximum average amplitude close to 3 kV. The study further indicates that the amplitude of the oscillating signal is directly proportional to the input voltage, and altering the number of turns in the coil can also change the amplitude of the output high-frequency pulse envelope as well as the number of pulses. By modifying the above factors, along with the external circuitry, we can control and stabilize the output waveform of the coil, generating a consistent high-frequency pulse envelope.