AlGaN/GaN heterostructure field-effect transistors (HFETs) with different floating gate lengths and floating gates annealed at different temperatures,are fabricated.Using the measured capacitance-voltage curves of the gate Shottky contacts for the AlGaN/GaN HFETs,we find that after floating gate experiences 600 ℃ rapid thermal annealing,the larger the floating gate length,the larger the two-dimensional electron gas electron density under the gate region is.Based on the measured capacitance-voltage and current-voltage curves,the strain of the A1GaN barrier layer in the gate region is calculated,which proves that the increased electron density originates from the increased strain of the A1GaN barrier layer.