Papers by Keyword: Impact Ionization

Abstract: The effects of positive and negative high voltage gate stress on the interface trap density and channel conductivity of lateral, Si-face 4H-SiC MOSFETs is studied, and the possible physical mechanisms for interface trap generation are discussed. Charge pumping and I_D-V_GS measurements are used to measure the trap density and field-effect mobility, respectively. The time dependence of the channel degradation is evaluated for different stress voltages with oxide electric field exceeding 8 MV/cm. Positive stress is shown to generate acceptor traps which degrade the field-effect mobility, and the density of traps follows a universal dependence on the injected electron fluence into the gate oxide. In contrast, negative stress resulted in no degradation of the field-effect mobility even as interface trap density increased, indicating that only donor interface traps are created. Furthermore, the trap density during negative stress does not follow a universal dependence on the injected hole fluence, indicating that other mechanisms are responsible for the trap creation.

Abstract: We determine the effective critical breakdown field for 4H-SiC superjunction (SJ) devices and compare it to their conventional counterparts. Also, we investigate its dependence on SJ device structural parameters, such as drift layer thickness (t) and pillar width (W). In 4H-SiC SJ devices, the effective critical breakdown field was found to be around 30% lower than that of conventional devices owing to their longer ionization paths. In particular, the effective critical electric field varies as ξ_cr α t^-1/10 and ξ_cr α t^-1/6 for 4H-SiC SJ and conventional devices respectively but independent of pillar width and doping concentration for high aspect ratio devices (t/W > 10).

Abstract: Modeling and simulation of 3C-SiC power devices such as MOSFETs and diodes requires a model for the breakdown field that is consistent with the Monte-Carlo-simulated ionization rates of electron and holes and supported by experimental results. The challenge one faces is the limited number of publications reporting such calculations and the limited availability of high-quality ionization breakdown data for 3C-SiC diodes. We therefore performed a series of 2D simulations of both n-type and p-type Schottky diodes and p⁺-n diodes that confirms the general breakdown field trend with doping density obtained from experiments. We uncovered a difference between n-type and p-type diode breakdown behavior, identified the discrepancy between the calculations and the experimental data, and extracted a simple breakdown field model, useful for further 3C-SiC device design and simulation.

Abstract: Recently the intense terahertz electroluminescence from monopolar n⁺⁺–n^– –n⁺ structures of 6H- and 8H-SiC of natural superlattices at helium temperatures due to Bloch oscillations was discovered. In the present work we present the THz emission spectra of bipolar n⁺⁺–π–n⁺ structures (π is a high-resistance layer of p-type conductivity) of natural superlattices 4H-, 8H- and 15R-SiC at 7 K. The bipolar n⁺⁺–π–n⁺ structures of 4H- and 8H-SiC were analogous to those of structures for which the negative differential conductivity effect was observed earlier for three polytypes (4H, 6H and 8H) at T=300 K. We demonstrate resemblance and differences of the spontaneous THz emission spectra for the monopolar and bipolar 4H-, 6H- 8H- and 15R-SiC natural superlattices caused by Bloch oscillations of electrons in the SiC natural superlattice.

Abstract: The charge enhancement in SiC-Schottky Barrier Didoes (SBDs) with different epi-layer thicknesses under the condition of the single-species ion irradiation was simulated to find out the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. The value of ion induced charge depended on the thickness of epitaxial-layer in the SBDs. The simulation result suggests that the impact ionization is one of the key effects to lead ion induced charge enhancement.

Abstract: Optimum n-drift region of a 4H-SiC Junction Barrier Schottky Diode (JBS) was analyzed by simulation with consideration of the anisotropic impact ionization. According to the detailed simulations using SRIM and Sentaurus, model parameters of empirical equations were obtained through fitting, which showed that the anisotropic avalanche model (2D-ANISO) differs significantly from the 1-dimensional empirical model (1D-Cooper) and the old isotropic avalanche model (2D-ISO). These initial results suggested that the JFET resistance and anisotropic impact ionization should be taken into account during the optimization of a 4H-SiC JBS in which field crowding at the corner of p-grid causes higher reverse leakage current.

Abstract: The charge induced in SiC-SBDs with different epi-layer thicknesses by ion incidence was measured to understand the mechanism of heavy-ion-induced anomalous charge collection in SiC-SBDs. SiC SBD of which epitaxial-layer thicknesses is close to ion range show larger anomalous charge collection than SBD with thicker epi-layer although the former one has lower electric field than the later one. The gains of collected charge from the SBDs suggest that the impact ionization under 0.16 - 0.18 MV/cm of the static electric field in depletion layer is not dominant mechanisms for the anomalous charge collection. It is suggested that the epitaxial-layer thickness and ion-induced transient high electric field are key to understand the anomalous charge collection mechanisms in SBDs.

Abstract: A new power law is approximated for effective impact ionization in 4H-SiC, which is then used to generate one-dimensional equations for critical electric field, avalanche breakdown voltage, and depletion layer width that match both simulation and published device results better than previous published equations.

Abstract: Avalanche multiplication of the 0.4-μm-thick a-Se HARP (High-gain Avalanche Rushing amorphous Photoconductor) target was obtained at a high electric field. To study the drift velocity of hot carriers in the a-Se layer, the energy-and field-dependent energy relaxation length was considered in the lucky-drift model. The impact ionization energy E_I of 2.0 eV and the optical phonon energy ћω of 31 meV for a-Se were used to obtain the impact ionization parameters in the a-Se layer. The drift velocity of hot carriers at 1×10⁸ V/m in the 0.4-μm-thick a-Se HARP target was obtained as 1.87×10⁶ cm/s. The drift velocity of hot carriers saturates as the electric field and the avalanche multiplication factor increase. In the 0.4-μm-thick a-Se HARP target, the relaxation length ratio λ_E/λ and the relaxation time ratio τ_E/τ saturate as the avalanche multiplication factor increases. In addition, the relaxation length ratio λ_E/λ and the relaxation time ratio τ_E/τ at 1×10⁸ V/m were 2.75 and 14.66, respectively.

Abstract: The a-Se HARP (High-gain Avalanche Rushing amorphous Photoconductor) target was prepared, and the avalanche characteristics were investigated. In this paper, to study avalanche multiplication at a high electric field, the lucky-drift model was used. In addition, the energy-and field-dependent energy relaxation length was considered. The avalanche multiplication factor was obtained from the current-voltage characteristic of the a-Se HARP target. The threshold field of the 0.4-μm-thick a-Se HARP target was 0.88×10⁸ V/m. The hot carrier energy at the threshold field for the avalanche multiplication in the 0.4-μm-thick a-Se HARP target was 0.21 eV. The hot carrier energy in the a-Se layer increases linearly as the electric field increases. The hot carrier energy also saturates as the avalanche multiplication factor increases. In addition, the energy relaxation length between the inelastic scattering events in the a-Se layer saturates as the hot carrier energy and the avalanche multiplication factor increase.