Si(100)/CoFe/AlO(x)/CoFe/FeMn/Cu/Ta magnetic tunnelling transistors (MTTs) with differing base thicknesses (W) were investigated. The magneto-transport properties of the MTTs were measured at 77 K and room temperature (RT). We obtained magneto-current ratios of 48.3% and 55.9% for emitter-base bias voltages of 1.45 and 2.0 V, respectively, at 77 K. The transfer ratios are 2.83 x 10(-5) and 1.52 x 10(-4), respectively, corresponding to bias voltages of 1.45 and 2.0 V. Moreover, the highest tunnel magneto-resistance (TMR) ratios turned out to be 12% and 20% for a base thickness of 30 A at RT and 77 K, respectively. These properties raise not only some fundamental questions regarding the phenomenon of spin-independent tunnelling at low and room temperatures, but also show some promising aspect for magneto-electronic applications. In addition, we attempted to elucidate the reason behind the outstanding TMR effect at low and room temperatures. Finally, the origin of the decrease in the mean free path asymmetry ([Formula: see text]) was clarified by using x-ray photoelectron spectroscopy profile analysis of the elements existing in the interface between Si and the CoFe base (Co, Fe, Al, Si, O).