Measurements of Band to Band Tunneling in Bulk Silicon as a function of Crystal Orientation

As technology is scaled down, band to band tunneling becomes an increasingly important source of leakage [1] as is manifested in GIDL (gate induced drain leakage currents). A transistor under conditions favorable to GIDL (high drain bias and low gate bias) is depicted in Fig. 1 where it is seen that the gate field induces tunneling in a path more or less parallel to the surface. This makes GIDL sensitive to the surface orientation of the FET. For instance in IBM’s 11LP technology [2] and others, the FETs are purposely oriented in the 100 direction on a (100) wafer (45 deg. off flat) to reduce GIDL rather than in the conventional 011 direction. Easy tunneling directions are expected to lie along paths of lowest effective mass, thus one, in contradiction to the above experience, would expect 001 to be the easiest tunneling direction for silicon since four of the low transverse conduction-band effective mass (0.19) valleys are available in contrast to 011 where only two valleys are available. 111 is thus expected to be a difficult tunneling direction since the effective mass of all six valleys, projected along the tunneling direction, are large (0.41). In contrast to this, the valence split-off band in silicon (see Fig. 2 and Table 1) have low effective masses for tunneling of holes, especially in the 111 (Γ-L) direction (0.083). Systematic experimental data and reliable theoretical data are lacking, therefore we present here a timely and important experimental study of the directional dependence of band-to-band tunneling in silicon.