Silicon nanowire (SiNW) field effect transistors (FETs) have been widely investigated as biological sensors for their remarkable sensitivity due to their large surface to volume ratio (S/V) and high selectivity towards a myriad of analytes through functionalization. In this work, we propose a long channel (L > 500 nm) junctionless nanowire transistor (JNT) SiNW sensor based on a highly doped, ultrathin body field-effect transistor with an organic gate dielectric epsilon(r) = 1.7. The operation regime (threshold voltage V-th) and electrical characteristics of JNTs can be directly tuned by the careful design of the NW/Fin FET. JNTs are investigated through 3D Technology Computer Aided Design (TCAD) simulations performed as a function of geometrical dimensions and channel doping concentration N-d for a p-type tri-gated structure. Two different materials, namely, an oxide and an organic monolayer, with varying dielectric constants er provide surface passivation. Mildly doped N-d = 1 x 10(19) cm(-3), thin bodied structures (fin width F-w < 20 nm) with an organic dielectric (epsilon(r) = 1.7) were found to have promising electrical characteristics for FET sensor structures such as V-th similar to 0 V, high relative sensitivities in the subthreshold regime S > 95%, high transconductance values at threshold g(m),(Vfg=0V) > 10 nS, low subthreshold slopes SS similar to 60 mV/dec, high saturation currents I-d,I-max similar to 1-10 mu A and high I-on/I-off > 10(4)-10(10) ratios. Our results provide useful guidelines for the design of junctionless FET nanowire sensors that can be integrated into miniaturized, low power biosensing systems. (c) 2013 Elsevier B.V. All rights reserved.