In this article, we have demonstrated 125-nm- gate InGaAs-on-insulator (InGaAs-OI) high-electron-mobility transistors (HEMTs) on Si substrates via wafer bonding. The highlights of this device were the i-In0.53Ga0.47As/i-InAs/ i-In0.53Ga0.47As quantum-well channel for high RF performance by improving electron transport properties and the low processing temperature of 250 degrees C or less for monolithic 3-D integration. As a result, we obtained a current gain cutoff frequency (f(T)) of 227 GHz and a maximum oscillation frequency (f(MAX)) of 187 GHz. These results are the highest ever reported in monolithic 3-D RF transistors above L-G = 100 nm. In addition, through the small-signal modeling, we have found that the impact of the back-gate (BG) on RF performances of top devices is a critical issue in M3D RF systems.