Logic gates can be implemented with transistors and fabricated in silicon in many ways. One simple method is RTL, resistor-transistor logic, which uses transistors which effectively correspond to "normally open" switches. The diagrams given below show how to implement the logical NOT, NAND, and NOR gates using RTL.
The figures and figure elements on this page are courtesy It's Only Logical by Jack W. Crenshaw of embedded.com.
jaa@ccs.neu.edu