Materials Science Forum Vols. 483-485

Abstract: Lateral inversion channel metal-oxide-semiconductor field-effect transistors (MOSFETs) were manufactured on 6H-SiC and two gate oxidation recipes were compared. In one case the gate oxide was grown in N2O using quartz environment. The resulting peak field-effect mobility was µFE=43 cm2/Vs. In the other case the gate oxide was grown in oxygen using alumina environment and the resulting peak field-effect mobility was µFE=130 cm2/Vs. Oxidizing in an environment made from sintered alumina introduces contaminants into the oxide that effect the oxidation in several^ways. The oxidation rate is increased and the resulting SiC/SiO2 interface allows higher inversion channel mobility.

Abstract: We report on fabrication and characterization of n-channel Si face 4H-SiC MOSFETs made using sublimation grown epitaxial material. Transistors made on this material exhibit record-high peak field effect mobility of 208 cm2/Vs while reference transistors made on a commercial epitaxial material grown by chemical vapor deposition (CVD) show field effect mobility of 125 cm2/Vs. The mobility enhancement is attributed to better surface morphology of the sublimation grown epitaxial layer.

Abstract: The drain-induced barrier lowering (DIBL) effect in 4H-SiC MESFETs has been studied using the physical drift and diffusion model. Our simulation results showed that the high drain voltage typically applied in short-channel 4H-SiC MESFETs could substantially reduce the channel barrier and result in large threshold voltage shift. It is also found that the DIBL effect is more dependent on the ratio of the gate length to channel thickness (Lg/a), rather than the channel thickness itself. In order to minimize the DIBL effect, the ratio of Lg/a should be kept greater than 3 for practical 4H-SiC MESFETs.

Abstract: High impedance silicon carbide power RF transistors are reported, which use the technology of Lateral Epitaxy Metal-Semiconductor FET (LEMES). The LEMES transistor utilizes a heavily doped buried depletion stopper to increase output impedance and breakdown voltage and to eliminate undesirable hot-carrier trapping effects. A power density of 2-3 W/mm at 2 GHz is routinely achieved resulting in a total output power of 10W for packaged components. The value of input and output impedance is around 50 Ohms for a frequency of around 2 GHz.

Abstract: We have designed and characterized preliminary versions of two wideband SiC-based RF power amplifiers using SiC MESFETs from Chalmers University and Lateral Epitaxy SiC MESFETs fabricated at AMDS AB. When optimized transistors are available they will be used in the design of amplifiers for a 100 – 500 MHz multifunction EW system.

Abstract: Because SiC does not have velocity overshooting behaviour, the current density of SiC metal-semiconductor field-effect transistors (MESFETs) is restricted by low drift velocity in the parasitic region between source and gate where the applied electric field is low. In addition, the extension of the depletion region toward the drain side at high drain voltages increases the effective channel length and, as a result, lowers the cut-off frequency due to the increased transit time.

Abstract: Conductance DLTS measurements have been performed on 4H-SiC MESFETs. A broad band due to electron emission by different levels is observed. An additional “hole-like” level with activation energy of 0.9 eV is obtained in linear regime but not in saturation regime. From the results, it is proposed that this “hole-like” signal is due to capture of electron present at a conductive SiC/SiO2 interfacial layer.

Abstract: Quantitative assessment of the influence of deep traps in semiinsulating (SI) SiC substrates on transient behavior and substrate leakage current of SiC MESFET is reported. Twodimensional device simulation confirmed that favorable reduction of the current-collapse happens when a fully depleted buffer is used. Simultaneously, the high-purity buffer causes an undesirable increase of the current bypassing the physical channel. A similar and even more pronounced effect is observed when very high purity SI substrates are used. The deep level-induced transient behavior disappears for the concentration of deep acceptor traps below the order of 1x1015 cm-3. However, this low trap concentration results in a virtual inability to pinch-off the channel even at very high gate biases. It has been demonstrated that the electric field preventing electron injection from thechannel into the substrate is very sensitive to the initial charge state of the traps prior to device biasing, which in turn is determined by the energy position of the deep levels in the substrate.

Abstract: The dependence of short-circuit photo-current on the voltage applied to the source-gate (or drain-gate) junction Vg was studied. One should consider the fact that the photo-current begins to drop with voltage exceeding certain threshold value. Thus the 6H-SiC JFET behaves uncommonly. Moreover Isd ~Vg dependence shows a drop Isd to zero at a condition that there is a non-pinched-off part of the channel. For a comparison parallel investigations were conducted with two industrial Si JFETs with a structure similar to that of 6H-SiC JFET under study. The results obtained for Si JFETs are trivial for photocurrent and Isd. We believe that the effects observed in 6H-SiC JFET should be attributed to the Wannier-Stark localization regime.