In this work, the subthreshold characteristics of a junctionless multigate transistor (JMT) with a trapezoidal fin cross section are studied by three-dimensional simulations. The effects of sidewall angle (Θ) on subthreshold swing (SS) and drain-induced barrier lowering (DIBL) are evaluated and compared with the effects of doping concentration. The results show that SS and DIBL are strongly dependent on Θ and more seriously affected by Θ variation. Meanwhile, as compared with those observed in inversion-mode multigate MOSFETs (IM-MuGFETs), the variations in SS and DIBL in JMTs with sidewall angle are better suppressed. A design guideline is finally proposed to define the optimal parameters of JMTs for a given technology.