2. Fundamentals of CMOS Devices

SWITCHES are the main building blocks of any hardware logic implementation. Computers in today's meaning^2.1 have been realized using mechanical and later electromechanical switches. The main shortcomings of such components are their low speed and their high power consumption. Vacuum tubes, which are switches without moving parts, have been used as replacements, but suffered from poor reliability. The invention of semiconductor switches gave a fast and reliable alternative. Bipolar transistors allow a high switching speed and a large amplification, however, current flow into the base contact must be maintained to keep the switch open. In metal-oxide-semiconductor field-effect transistors the current flow is controlled by a voltage. Ideally, no power is needed to control the on- and off-state. Complementary MOS technology is based on complementary-type transistors where current flows only during the switching process. These devices allow hardware logic implementations with extremely low standby power, high speed, and small footprint. Fig. 2.1 shows a schematic and a simplified layout of a CMOS inverter, the workhorse of all modern computers. An n-type MOS (nMOS) and a p-type MOS (pMOS) device are fabricated on the same p-doped wafer, with the pMOS device embedded in an n-doped well. The footprint of these structures is very small and allows high integration densities.

Figure 2.1: Schematics of a CMOS inverter and its layout.

Subsections

• 2.1 Historical Overview
• 2.2 Obstacles to Device Miniaturization
  • 2.2.1 Channel
  • 2.2.2 Gate Stack
  • 2.2.3 Source and Drain
  • 2.2.4 Gate Dielectric
  
  
• 2.3 Novel Device Concepts
  • 2.3.1 Strained-Silicon Devices
  • 2.3.2 Depleted-Substrate Devices
  • 2.3.3 Vertical Transistors
  • 2.3.4 Carbon Nanotube FET
  
  
• 2.4 Semiconductor Device Simulation
  • 2.4.1 Hierarchy of Semiconductor Device Simulation Models
  • 2.4.2 Classical Device Simulation
    • 2.4.2.1 The Drift-Diffusion Model
    • 2.4.2.2 The Energy-Transport Model
    • 2.4.2.3 Device Simulation
  • 2.4.3 Quantum Device Simulation
    • 2.4.3.1 SCHRÖDINGER Equation and SCHRÖDINGER-POISSON Solvers
    • 2.4.3.2 WIGNER Equation and the Density-Gradient Model
    • 2.4.3.3 Quantum Device Simulation
    • 2.4.3.4 Non-Equilibrium GREEN's Function Device Simulation

A. Gehring: Simulation of Tunneling in Semiconductor Devices