Field-current phase diagrams were measured on in-plane anisotropy Co60Fe20B20 magnetic tunnel junctions to obtain the spin transfer torque (STT) field-current switching window. These measurements were used to characterise junctions with varying free layer thicknesses from 2.5 down to 1.1 nm having a reduced effective demagnetizing field due to the perpendicular magnetic anisotropy at CoFeB/MgO interface. Diagrams were obtained with 100 ns current pulses, of either same or alternating polarity. When consecutive pulses have the same polarity, it is possible to realize the STT switching even for conditions having a low switching probability. This was evidenced in diagrams with consecutive pulses of alternating polarity, with 100% switching obtained at 4.7 MA/cm2, compared to the lower 3.4 MA/cm2 value for same polarity pulses. Although the low level of the current density window is higher in alternating polarity diagrams, the field window in both diagrams is the same and therefore independent of the pulse polarity sequence.