On-resistance versus Breakdown Voltage Capabilities of Emerging Semiconductors for Opto- and Power-Electronics
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Compared to silicon, wide-bandgap (WBG) and ultrawide-bandgap (UWBG) semiconductors exhibit high internal electric critical fields, low intrinsic carrier concentration, and higher saturation velocities. Those properties make them suitable for electric vehicles, radio frequency electronics, solar-blind ultraviolet photodetectors, and power conditioning to name a few. As those new materials emerge for modern electronics, an accurate assessment of their capabilities is necessary for informed material selection and device designs. For power electronics, the Baliga figure-of-merit (BFoM) is the metric commonly used to assess the trade-offs between the on-resistance and the breakdown voltage; however, the effects of underlying physics on material parameters are often not accounted for, leading to unrealistic performance predictions. This work improves devices’ performance predictions by investigating dopants’ incomplete activation and ionization and using impact ionization coefficients for breakdown voltage calculations. In addition to silicon, this study is carried out on WBG (4H-SiC and GaN) and UWBG (AlxGa1-xN, β-Ga2O3, and Diamond) semiconductors.
The work first investigates the on-resistance part of the BFoM. Poisson’s equation is used to evaluate the steady-state concentration of ionized dopants. Challenges related to dopant activation and ionization are addressed. Capacitance-voltage-temperature characteristics measured on 4H-SiC p-i-n diodes provide a baseline for validating the model of incomplete ionization on WBG semiconductors. The work shows that for UWBG semiconductors with high ionization energy, failure to account for incomplete ionization may result in an estimated on-resistance 1000 times lower than their practical values. In the second part of the dissertation, the breakdown voltage is evaluated using the reduced ionization integral. A modified Thornber expression, calibrated with impact ionization coefficients surveyed from the literature, measured breakdown voltages, and measured multiplication, is proposed for a temperature-dependent model of impact ionization coefficients. Results show that an emphasis should be placed on the minimum doping (background doping) concentration of materials as it determines their maximum blocking capabilities. Impact ionization coefficients are essential for modeling power- and opto-electronic devices; they need to be quantified more accurately, as processed data suggest that they are doping-dependent.
4H-SiC p-i-n diodes are used to measure the photo-multiplication and for the experimental extraction of impact ionization coefficients. For a better understanding of the diode performance, deep-level transient spectroscopy (DLTS) characterization is used to extract the density of generation-recombination centers and address the source of the dark current. Because the diodes are opto-electronics devices, insight is gained into the carrier multiplication process by measuring their responsivity, quantum efficiency, dark count, and single-photon detection efficiency. The calculations of this work help understand the operation of avalanche photodiodes by establishing how calculated field profiles drive multiplication processes. Calculated breakdown voltages and on-resistances allow the assessment of the efficiency and power density of the investigated materials through modeling of conduction and switching losses. This work led to the development of a 1D-simulator that predicts the performance of power electronic devices based on their geometry, the doping concentration of their constituent layers, and the material selected for their fabrication. The results of this work provide parameters essential for technology computer-aided design (TCAD) modeling of power electronic devices, including vertical power devices.