We report on the design and demonstration of ${beta}-(Al_{0.18}Ga_{0.82})_2O_3/Ga_2O_3$ modulation doped heterostructures to achieve high sheet charge density. The use of a thin spacer layer between the Si delta-doping and heterojunction interface was investigated in ${beta}-(Al_{0.18}Ga_{0.82})_2O_3/Ga_2O_3$ modulation doped structures. We find that that this strategy enables higher 2DEG sheet charge density up to 6.1x10^12 cm^2 with mobility of 147 cm^2/Vs. The presence of a degenerate 2DEG channel was confirmed by the measurement of low temperature effective mobility of 378 cm^2/V-s and a lack of carrier freeze out from low temperature capacitance voltage measurements. The electron density of 6.1x10^12 cm^2 is the highest reported sheet charge density obtained without parallel conduction channels in an $(Al_{0.18}Ga_{0.82})_2O_3/Ga_2O_3$ heterostructure system.
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