For electron microscopy, ultrafast detectors with high sensitivity are intrinsically demanded, especially for biomedical applications, where probing electrons can damage biological samples under investigation. Addressing this, a novel ultrafast electron detector with field-effect transistor (FET) enhanced in situ signal amplification is proposed and verified in this study. Through Silvaco-based simulation, the detector demonstrates a high gain of over sevenfold of the current generated in conventional p-i-n-based detectors and an ultrafast response with a cutoff frequency of over 500 MHz. Furthermore, a proposed nonuniform gating oxide design shows an improvement in the detector output current from 61 to $87 \mu \text{A}/\mu \text{m}$ . Besides, a nonlinear doping profile of the electron absorption layer is also proven effective in enhancing the in situ signal amplification effects. Particularly, a detector with a thick electron absorption layer of $50 \mu \text{m}$ exhibits excellent performances with an output current over $16 \mu \text{A}/\mu \text{m}$ and a cutoff frequency of approximately 200 MHz, making it possible to detect high-energy electrons.