Basic Transistor Physics
David Giessel
University of Alaska Fairbanks
Physics 212 Web Project, Spring 2003

   Gate count is increasing at rates almost as mind boggling as the rate of gate size decrease. Modern CPUs have as many as 55 million transistors (Pentium 4). PowerPC processors are a bit more efficient on gates than their x86 counterparts (x86 being intel and intel compatible chips such as athlons, crusoes, cyrix processors...etc). Older G3 chips only had 6.35 million gates. Current G3s have 29 million gates (most of which are used for on die cache) and the current G4 processor (PPC7455) has approximately 33 million gates. The older G4 (7400/7410) had 10.5 million gates (no on die level 2 cache). The decreases in gate size have allowed these massive increases without large increases in power or chip size however. Pentium 4 uses approximately 50-60 watts of power at 2.5-3.0 GHz. PPC7455 (G4) uses approximately 20 watts at 1.42 GHz. And the current G3 (PPC750FX) uses about 3.6 watts at 1.0 GHz. Something to note is that the frequencies of these chips are greatly varied. This is due to inherent design differences. In reality a dual 1.42 GHz 7455 system is a fair amount more powerful than a 3.0 GHz Pentium 4 system while still maintaining a 20 watt power consumption advantage.

   A main reason why PowerPC chips are generally more efficient in their use of transistors is the primary application for these processors is in embedded applications such as high end routers. Conversely Intel targets the desktop market directly with the Pentium 4. Intel also sought to gain a marketing advantage with Pentium 4 by ramping clock speed higher. By using more transistors and taking smaller bites at the data they were able to increase clock speed greatly while only seeing marginal gains in performance. An AMD processor for example can compete with a 3 GHz P4 even if it is clocked at about 2.2 GHz. In other words, intel was not concerned about efficiency in their design. Hence the 60 watt power usage vs 20 for the G4 PowerPC.

   IBM is targeting the desktop and low end server market with their PowerPC 970 chip (pictured above). It is designed for higher clock speed operation than the G4 processor and will include all the latest fabrication developments when released. Processor core voltage for this chip will be as low as 1.2v allowing a higher transistor count with very low power dissipation. Pentium 4 core voltage is in the range of 1.55v.

   Smaller, faster, cheaper...transistors aren't quite as boring as you thought huh?