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We have studied the device design of 15-nm highspeed n-channel source-heterojunction-MOS transistors (SHOTs) utilizing high-velocity electron injection from the source into the channel region and using the conduction-band-offset energy between the source and the channel regions. The band-offset energy near the source region and the length of the graded heterojunction are key parameters for realizing high-speed operation of SHOTs. A 2D device simulator indicates that the enhancement in transconductance Gm in SHOTs on an SOI substrate over conventional SOI-MOSFETs without source-heterojunction structures strongly depends on the source conduction-band-offset value DeltaEc and the length of graded-heterojunction structures LH in SHOTs. Moreover, the Gm enhancement of SHOTs is affected by the drain and the gate biases. We have shown that, with fully optimized DeltaEc and LH values, the Gm enhancement of SHOT due to high-velocity electron injection can be achieved in a whole range of drain bias. The optimized SHOT is quite promising for high-speed CMOS devices in the 10-nm regime.