A novel assisted-depletion NJFET was designed in BiCMOS technology. The N-Well layer that was located at the region between gate and drain of the MOS transistor was implanted with boron, which had formed a buried P-BOT layer in the P-Sub. The P-BOT layer’s assisted-depletion effect was used to raise the gate-drain breakdown voltage of NJFET by preventing the premature transverse breakdown. The breakdown voltage of the BiCMOS NJFET was simulated by Sentaurus TCAD tools. The simulation results showed that the breakdown voltage of the proposed NJFET structure had been improved to 104 V, which was enhanced up by 57.6% compared with that of the traditional NJFET at the same doping concentration in the N-Well layer.