Compact MOSFET Models for VLSI Design

8

Non-classical MOSFET Structures

8.1 Introduction

In spite of decades of lithography driven classical scaling of bulk MOSFET device structure to meet higher packing density and superior performance, the basic structure remained invariant except for innovations mostly in process technology. The modifications in mainstream bulk MOSFETs revolve around drain and substrate engineering leading to the following illustrative structures [1–3]:

1. Lightly Doped Drain (LDD) structure. This architecture requires drain engineering for a creation of a lightly doped drain by ion implantation. This reduces the electric field near the drain, reducing hot electron related reliability problems in MOS devices.
2. Retrograde doping profile in the substrate. This architecture needs channel engineering to reduce bulk punchthrough and resulting leakage current.
3. Ground plane structure with a thin layer of silicon film. This structure realizes the property of a retrograde substrate structure by substitution of implant technology through CVD based atomic layer technology of silicon on the bulk substrate.
4. Pocket or Halo implant. The DIBL effect is minimized through creation of highly doped pockets near the source and drain in the channel. This creates inhomogeneities in the channel adjacent to the source and drain, which are responsible for an increased leakage current due to band-to-band tunneling between the source and pocket regions.

The performance of a nanoscale MOS device, bordering near the atomic limit, ...