High Voltage Photoconductive Semiconductor Switch Based on 4H-SiC

Abstract

Photoconductive semiconductor switch (PCSSs) has been investigated as a promising optoelectronic device for high power applications. Compared to other high power switches, such as the spark gap, insulated gate bipolar transistor (IGBT), and thyristor, PCSSs have unique advantages including ultra-fast switching, reliable operation, and negligible jitter time. The solid-state microwave generation techniques using the PCSSs have attracted interest in applications, such as ultra-wideband (UWB) and high power microwave (HPM) generators. Several semiconductor materials have been considered as candidates for PCSSs, including silicon (Si), gallium arsenide (GaAs), silicon carbide (SiC), gallium nitride (GaN), and diamond. The excellent material properties of SiC, such as wide bandgap energy (3.23 eV), high breakdown field (2–4 MV/cm), high electron saturation velocity (2.0 × 107 cm/s), and high thermal conductivity (4.9 W·cm−1K−1), have driven active studies of SiC-based PCSSs in search of higher voltage, higher current, and more stable operations. High purity semi-insulating (HPSI) 4H-SiC grown without using a deep level dopant became a probable material candidate of PCSSs, due to its long recombination lifetime (6.5 ns) and high electron mobility (μn⊥c of 950, and μn

c of 1141 cm2/V·s). In this thesis, the characterization and fabrication process of the PCSSs based on 4H-SiC were investigated. The output characteristics of PCSSs based on high purity semi-insulating (HPSI) 4H-SiC, one with a lateral structure for front-side and back-side illumination, and the other with a vertical structure (vPCSS) for side-illumination, were investigated. The vPCSS exhibited much earlier turn-on behavior and a larger pulse width than the lateral PCSSs. The side-illuminated vPCSS has a large effective contact electrode area and increased photon absorption efficiency. By arranging the optical path at a right angle with the electronic transport, a minimum on-state resistance of 0.34 Ω was achieved at an optical energy of 8 mJ. This achieved minimum on-state resistance was lower than that of the previously reported vPCSS performances, even with a lower optical illumination energy. The characteristics of output waveforms, including the relation between turn-on time of the PCSS and optical energy, and the relation between pulse width and peak output voltage, was figured out. The operational characteristics of PCSS also have been investigated for high power switching with a load resistance ranging from 0.05 to 50-Ω. To observe maximum current switching characteristics, a current viewing resistor (CVR) has been employed in series with the PCSS. Because the CVR has a resistance in the sub-Ω range, the on-state resistance of the PCSS is much higher than the resistance of the CVR and dominantly drives the output current. The output characteristics, including equivalent resistance and times at peak output, of the PCSS was characterized with 50-Ω and 0.05-Ω load by transient equivalent circuit analysis. The current oscillation was caused by the natural response of the series RLC circuit with RCVR, LCVR, and CPCSS. The PCSS in linear mode achieved a minimum on-state resistance of 0.27 Ω at the optical energy of 8 mJ and a maximum output current of 657 A at the bias voltage of 4.8 kV, where the surface flashover was observed and was identified in the transient current pulse. The randomly occurring secondary pulses, which exhibited similar peak current and FWHM values, was found to be the breakdown mechanism of the PCSS. During fabrication of the SiC-based semiconductor devices, high temperature treatment is indispensable because of the low diffusion coefficients of the dopants (Al, N, B, P) in SiC. For the diffusion and activation of ion-implanted dopants, high temperature annealing processes are typically carried out at high temperatures, above 1500℃. The Z1/2 defect, a deep acceptor level in high purity semi-insulating (HPSI) 4H-SiC, plays an important role in optoelectronic properties, particularly in below bandgap photon absorption and carrier recombination processes, and its concentration is highly dependent upon high temperature annealing. To study the effect of high temperature annealing on the properties of HPSI 4H-SiC, vertical-type photoconductive semiconductor switches were fabricated on two types of substrates, where one was non-annealed and the other was annealed at 1500℃ for 15 min. The high temperature annealing reduced the Z1/2 defect concentration in the PCSS, and its effect on the optoelectronic properties of the PCSS, including the number of photo-generated carriers and falling time, was identified with variation of optical energy and bias voltage. For PCSS devices, the high quality ohmic contact is beneficial because it can reduce the on-state resistance and prevent early breakdown at the edge of the metal-semiconductor interfacial layer. High purity semi-insulating (HPSI) 4H-SiC substrate was doped with nitrogen (N) by ion implantation followed by activation annealing at 1500℃ for 15 min. Ni-based ohmic metallization was deposited on top of the n-doped SiC layer to fabricate photoconductive semiconductor switch. To study the effect of the ohmic contact formation on the performance of the PCSS, two different directions of the bias voltage was applied to the PCSS. When the output characteristics of the PCSSs with a load resistance of 0.1-Ω were compared, PCSS employing ohmic contact on the anode exhibited 1.5 times higher peak output current than the PCSS employing ohmic contact on the cathode under the optical excitation energy of 8 mJ. Linearly graded guard-ring (LG-GR) was introduced to PCSS for operation at high voltage. To optimize the guard-ring profile, the electric field distribution varying the gap of the rings were investigated by TCAD device simulation. Maximum electric field in the PCSS adopting LG-GR was about 1/6 of one in the PCSS without LG-GR. The breakdown voltage, where surface flashover was companied, can be improved by the LG-GR.