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The I-V characteristic of carbon nanotube (CNT) transistors is dictated by the Schottky barrier (SB) at metal-nanotube interface. The SB is defined by the diameter of the CNT along with the source/drain metal and is presumed a device constant for single CNT transistors. Given the wide distribution of diameter of CVD-grown nanotubes, the presumption of single SB, however, is inappropriate for transistors with aligned array of CNTs. Indeed, array transistors with similar geometries and contact material can still exhibit considerable variation in threshold voltage (VT), on current (ION), and device resistance (Rd). We use measured diameter distribution within the framework of detailed numerical simulations to demonstrate that the diameter distribution of CNTs (in CNT FETs) plays a dominant role in defining the fluctuation of array transistors. Besides, it is argued that the large-scale integration of these devices within an IC would be feasible only if the distribution of diameter is considerably narrowed.