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AlGaN/GaN high electron mobility transistors (HEMTs) were grown on Si substrates by MOCVD. In the HEMT structure, the 1 μm GaN buffer layer was partially doped with Mg in an attempt to increase the resistivity and minimize the buffer leakage current. Afterwards, an AlN spacer layer was inserted between the AlGaN barrier layer and the GaN channel layer to effectively reduce impurity scattering and improve the mobility and sheet carrier density of two-dimensional electron gas (2DEG). Devices of AlGaN/GaN HEMTs with 1 nm AlN spacer layer grown on undoped and partially Mg-doped GaN buffer layers were processed and characterized for comparison. For the DC characteristics, a low drain leakage current density of 55.8 nA/mm, a low gate leakage current density of 2.73 μA/mm, and a high off-state breakdown voltage of 104 V were achieved with device dimensions Lg/Wg/Lgs/Lgd = 1/10/1/1 μm, using the sample with partially Mg-doped GaN buffer layer. For the small signal RF characteristics, the device with partially Mg-doped has lower current gain cutoff frequency (fT) and lower power gain cutoff frequency (fmax) than that of the device without Mg-doped, indicating that doping Mg in GaN buffer layer should introduce the potential parasitic capacitance, which is not beneficial to RF performance of HEMT devices.