In recent times, the role of FINFET devices is increasing in the field of RF-IC design for realizing the high frequency circuits. As a result, high frequency characteristics including RF stability needs to be investigated carefully. Hence this work presents the impact of device geometrical and process parameter variations on the RF stability of 20 nm SELBOX Inverted-T Junctionless FINFET (SELBOX ITJLFET). The stability factor (K), critical frequency (fk) and its dependency on small signal parameters (SSP) are investigated by varying the device and process parameters like Gate Workfunction (GWF), fin height (Hfin), fin width (Wfin), Source underlap spacer length (LUS) and SELBOX length (LG). In addition, the relation between the bias and the stability factor of the device is also studied to identify the bias operating point in a stability perspective. From the simulation results, it is found that larger the SELBOX and LUS, lower the critical frequency and thereby making the device unconditionally stable at lower frequencies. Finally, an optimized design guideline is proposed, which makes the device suitable for high bandwidth applications. © 2020, Springer Nature B.V.