Symmetry breaking in the drain current of multi-finger transistors

The drain current of a multi-finger MOSFET is typically calculated as the product of that of a single-finger MOSFET and the number of fingers. Careful investigation of currents in different fingers of a multi-finger transistor in the presence of parasitic effects shows differences between the per-finger drain current of the multi-finger transistor and the drain current of a corresponding single-finger transistor. We show that each of the following factors alone causes the drain current in one or more fingers of a multi-finger transistor to be different from that in other fingers of the transistor and the per-finger drain current of the multi-finger transistor to be different from the drain current of a corresponding single-finger transistor: (a) the resistance of wires that connect multiple fingers together, (b) the contact resistance, (c) the diffusion resistance, and (d) self heating. Excluding all of the above factors, the uncorrelated variations among the sub-threshold drain currents of different finger cause the per-finger median sub-threshold drain current of the multi-finger transistor to be different from the median sub-threshold drain current of the single-finger transistor.