Wikipedia

- Mosfet applet - Applet describing MOSFET operation
- Mosfet applet 2
Another applet treating the MOS transistor as a digital switch rather than
an analog device. Also demonstrates the operating principles of basic digital gates.
- Principles of Semiconductor Devices - Web book, by Bart van Zeghbroek at University of Colorado.
- Design of VLSI Systems -
Web course by Daniel Mlynek and Yusuf Leblebici at EPFL. The 'links'
section at this site contains an excelent set of links.
- Queens University ELEC-853 Course Online learning material for RFIC design. Contains many useful pages also for digital VLSI.