Photocurrent efficiency and limitations in 4H-SiC photoconductive semiconductor switches

Silicon Carbide (SiC) is a semiconductor material well suited for applications requiring high power, high temperature, or high frequency electronic devices. A specific application SiC is well suited for is photoconductive semiconductor switches (PCSSs) for pulsed power applications. PCSSs are advantageous in applications where high voltage, low jitter, highly controllable pulses are desired. A variety of PCSS designs have been previously investigated by a number of research groups with the primary focus of this previous research being vertical or lateral, bulk, PCSSs. This prior research yielded a thorough understanding of the high electric field behavior (0-250~kV/cm) of bulk SiC PCSSs, demonstrated high power operation ($>$1~MW) of these devices, and demonstrated the high-repetition rate potential (65~MHz) of these devices. However, this previous work found bulk SiC PCSSs to exhibit a device lifetimes on the order of $103$ high power switching cycles, and photocurrent efficiency in these devices was observed to be on the order of $1-10 \%$ in the tested parameter space (5,000-100,000~$\mu$J/cm$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ / 350-380~nm / R=50~$\Omega$ & 1.2~k$\Omega$).

This dissertation focuses on three primary topics: understanding the primary processes leading to the limited device lifetime exhibited by bulk-lateral, SiC PCSSs, characterizing and analyzing the photocurrent efficiency of bulk-lateral SiC PCSSs over a large parameter space including ranges previously untested, and lastly, the design, fabrication, characterization, and analysis of a PIN-based SiC PCSS. Experimental and simulation results are presented demonstrating that transient electric fields and high current densities both located at the SiC/metal interface are the primary cause of the observed failure mode in bulk-lateral PCSSs. Experimental results and analysis detailing the effects of optical wavelength (295-375~nm), optical fluence (30-30,000~$\mu$J/cm$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$), DC bias voltage (0-4~kV) and load resistance (10-300~$\Omega$) on photocurrent efficiency in bulk-lateral SiC PCSSs are presented. Photocurrent efficiency in bulk-lateral PCSSs was found to be directly correlated to the on-state electric field of the PCSS. Furthermore, photocurrent efficiency in bulk lateral PCSSs was revealed to depend on the absorption length in the bulk material producing a maximum photocurrent efficiency in the range of 315-355~nm. Finally, the design, fabrication, characterization, and analysis of a PIN SiC PCSS are presented. The developed PIN SiC PCSS was tested over the parameter space of E=0.5-900~$\mu$J/cm$2$, V=100-1500~V, $\lambda laser$=275-355~nm, and R=0-300~$\Omega$. The PIN PCSS demonstrated a greater than 6x improvement in photocurrent efficiency relative to a comparable lateral-bulk PCSSs, photocurrent efficiency was observed to be maximum over the range of 305-340~$nm$, carrier gain was observed, and the device lifetime was found to be greater than 10$6$ switching cycles at $\approx$~200~$kW$ into a 5.8~$\Omega$ load.

Embargo status: Restricted until 09/2022. To request the author grant access, click on the PDF link to the left.