{"id":2711,"date":"2011-07-19T15:06:26","date_gmt":"2011-07-19T15:06:26","guid":{"rendered":"https:\/\/mtlsites.mit.edu\/annual_reports\/2011\/?p=2711"},"modified":"2011-08-02T18:28:23","modified_gmt":"2011-08-02T18:28:23","slug":"virtual-source-based-self-consistent-charge-and-transport-models-for-ballistic-mosfets-2","status":"publish","type":"post","link":"https:\/\/mtlsites.mit.edu\/annual_reports\/2011\/virtual-source-based-self-consistent-charge-and-transport-models-for-ballistic-mosfets-2\/","title":{"rendered":"Virtual-source-based Self-consistent Charge and Transport Models for Ballistic MOSFETs"},"content":{"rendered":"

Compact models describing the voltage-dependent terminal current and charges (or equivalently, capacitances) are essential for small-signal and transient circuit simulation.\u00a0 In this work, we extend the virtual-source (VS)-based transport model [1<\/a>] <\/sup> with a self-consistent channel charge model for quasi-ballistic or fully ballistic devices, when the gradual channel approximation (GCA) and the drift transport theory are no longer valid. From a parabolic channel potential profile approximation and current continuity boundary condition, we derive a voltage-dependent charge model that is self-consistent with the transport model in the ballistic regime. The extended VS model has been implemented in Verilog-A language. [2<\/a>] <\/sup><\/p>\n

Devices operating in the ballistic regime in saturation have less channel charge than predicted by the drift-diffusion theories, which is in principle advantageous from the performance point of view.\u00a0 The quasi-ballistic (QB) model predicts 61% and 58% fewer intrinsic channel charges than the saturation velocity model (Vsat) and non-saturation drift velocity model (NVsat), respectively (Figure 1).\u00a0 The difference diminishes in the linear region or because the device essentially operates with low carrier velocity and a lot of scattering with low Vgs<\/sub><\/em> or Vds<\/sub><\/em>.\u00a0\u00a0 It is also shown that the benefits of fast carrier transport in tight-pitch logic circuits diminish due to the presence of extrinsic charges, particularly at higher fan-outs. As shown in Figure 2, the stage delay of a 5-stage ring oscillator predicted by QB model is only 5% and 3% less than that by Vsat and Nsat models, respectively. However, for RF applications the benefit of quasi-ballisticity in Si or near-full ballisticity in III-V HEMTs calculated by the model can be significant.<\/p>\n\n\t\t