Temperature Dependent Resistivity of 4H-SiC Substrate
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With the rapid development of microelectronics technology and power device, higher requirements have been put forward for the service performance of semiconductor materials under temperature action. Therefore, it is of great significance to conduct temperature dependent research on the electrical properties of typical semiconductor materials. Researchers have studied the temperature dependent resistance characteristics of N-type and semi insulating 4H-SiC, as follows:
1. Temperature Dependent Resistance Characteristics of N-Type 4H SiC Substrate
Researchers used double-sided polished 150mm SiC substrates with different N doping (S1: N content 5.8 x 1018 cm-3; S2: N content 4.5 x 1018 cm-3), and cut 150mm SiC wafers into 1cm x 1cm samples for experimentation. The SiC property is expected to be isotropic in this plane. Evaporate the Ni contact with a thin Au cap layer (40nm) onto the corner and anneal at 950 ° C for 30 seconds. Measure the electrical resistivity of SiC substrates at 4 points using the Vanderbilt structure. The relationship between resistivity and temperature of N doped SiC substrates measured are shown as in Fig. 1:
Fig. 1 Relationship between resistivity and temperature of 4H-SiC substrate with different N doping levels
Both SiC substrates indicate that as the chip temperature rises from room temperature to 175 ° C (448K), the substrate portion of the device’s resistance will decrease; 175 ° C is the typical maximum rated temperature for packaged power devices. This is contrary to the behavior of lower doped epitaxial drift regions, but is expected for these very high SiC doping levels, similar to reports of epitaxial growth or injection of high N-doped 4H-SiC. When considering the resistance temperature coefficient of vertical power devices on SiC, it is necessary to consider these substrate characteristics. Please also note that when the temperature drops below room temperature, the electrical resistivity will also increase.
2. Temperature Dependent Resistance Characteristics of Semi-Insulating 4H–SiC Substrate
The experiment was conducted using semi-insulating 4H-SiC wafers doped with vanadium (V) by PVT method. The wafers were cut into square samples with dimensions of 1cm x 1cm (Sample 1: impurity concentration of 5.11e17; Sample 2: impurity concentration of 4.13e18). Ta-Au ohmic contacts were then made on the semi insulating SiC substrates and measured by Hall testing.
The relationship between resistivity and temperature is shown in Fig. 2. It can be seen that the resistivity of semi-insulating SiC sample 2 is higher than that of semi-insulating SiC Sample 1 at room temperature, and both gradually decrease with increasing temperature. There is a turning point temperature, which shows a significant change around 500K. When the turning point temperature is lower, the resistivity changes rapidly with temperature, while when it is higher, the resistivity changes slowly with temperature. This is because as the temperature increases, the carrier concentration and mobility increase, and the resistivity correspondingly decreases. After entering 500K, the resistivity drops to below 5000 Ω. cm. At this time, the high resistance semi insulating SiC substrate basically becomes a conductor and is no longer suitable as an insulating substrate for devices.
Fig. 2 Temperature dependent resistivity of semi-insulating SiC：a) sample 1; b) sample 2
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