What the Numbers Mean

You often hear commercial chips described in terms of transistor sizes and number of metal layers, e.g., ``the SuperGizmo 5000 CPU is built in a 0.6~mm, 3-metal CMOS process.'' This description means that the minimum transistor length is 0.6~mm, and the process has three layers of metal wiring. (The meaning of ``transistor length'' is explained in the design example.) Why use these two numbers as metrics? The smaller the transistor's minimum length, the more of them can be put on a chip, and the faster it--and thus, the chip--can operate. More levels of metal enable devices to connect more efficiently. To give you an idea of what's used in today's commercial microprocessors, the Pentium Pro uses a 0.35~mm, 4-metal BiCMOS process (a combination of bipolar and CMOS technologies), while the PowerPC 604 is made in a 0.5~mm, 4-metal CMOS process.