Recent Progress in SiC Microwave MESFETs
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preferable because it has a larger bandgap and higher electron mobility than 6H-SiC. It is the
wide bandgap of 3.2 eV, compared to 1.1 eV for Si and 1.4 eV for GaAs, that gives SiC its primary advantage for high-power devices. This wide bandgap gives rise to a breakdown electric field that is 10 times higher than in GaAs or Si. This is illustrated in Figure 1, which shows the measured breakdown voltage of 4H-SiC p-n junction diodes as well as the theoretical curves for Si and GaAs. This high breakdown field has been exploited to fabricate sub-micron SiC MESFET's with gate-to-drain breakdown voltages exceeding 200 V. The one drawback to SiC for use in microwave devices is its poor low-field electron mobility, which is in the range of 300 - 500 cm2 /V-sec. for doping levels of interest for MESFET's, i.e., Ix10"7 cm"3 < ND < 5x10 7 cm-'. This results in a larger source resistance and lower transconductance than is typical of GaAs MESFET's, but is partially offset by the ability to operate the SiC devices under extremely high electric fields. The saturated electron velocity in 6H-SiC is 2x 10' cm/s and has been predicted by Monte Carlo simulations to be 2.7x 10' cm/s in 4H-SiC [2], almost 3 times higher than in GaAs at high fields. Although the knee voltage of SiC MESFET's is relatively high (typically ; 9 V), the drain efficiency of the devices is still high because the breakdown voltage is over 100 V. The channel current density is reasonably large, typically around 300 mA/mm for 0.7 ýtm gate length devices, due to the high saturated velocity. When combined with the high breakdown voltage, this results in the large RF power density of over 4 W/mm that has been measured for SiC MESFET's.
15 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
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Doping Level (cm-3)
Figure 1: Measured breakdown voltage of 4H-SiC p-n junction diodes as a function of doping, and the theoretical maximums for
Si and GaAs. The other property of SiC that gives it a significant advantage over other semiconductors is its very high thermal conductivity. In order to take advantage of the high electrical power densities available in this material, it is also necessary to dissipate very high thermal power densities, making the thermal conductivity an extremely important parameter. Measured thermal conductivity data for SiC is shown in Table I for low doped n- and p-type, doped n-type and semi-insulating material. The thermal conductivity was measured by creating a temperature difference across a piece of SiC. Thermocouples were inserted into holes drilled 1 cm apart and AT and applied power were used to calculate the thermal conductivity at 25°C and I 00°C. Copper and Al were used as calibration standards. Thermal conductivity is the product of a material's density, heat capacity and its thermal diffusivity: the latter being dependent on the doping and quality of a material. The very low doped material exhibits a-axis thermal conductivity roughly the same
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