How the 8008 fits into the history of semiconductor technology
The 4004 and 8008 both used silicon-gate enhancement-mode PMOS, a semiconductor technology that was only used briefly. This puts the chips at an interesting point in chip fabrication technology.
The 8008 (and modern processors) uses MOS transistors. These transistors had a long path to acceptance, being slower and less reliable than the bipolar transistors used in most computers of the 1960s.
By the late 1960s, MOS integrated circuits were becoming more common; the standard technology was PMOS transistors with metal gates.
The gates of the transistor consisted of metal, which was also used to connect components of the chip. Chips essentially had two layers of functionality: the silicon itself, and the metal wiring on top.
This technology was used in many Texas Instruments calculator chips, as well as the TMC 1795 chip (the chip that had the same instruction set as the 8008).
A key innovation that made the 8008 practical was the self-aligned gate—a transistor using a gate of polysilicon rather than metal.
Although this technology was invented by Fairchild and Bell Labs, it was Intel that pushed the technology ahead.
Polysilicon gate transistors had much better performance than metal gate (for complex semiconductor reasons).
In addition, adding a polysilicon layer made routing of signals in the chip much easier, making the chips denser.
The diagram below shows the benefit of self-aligned gates: the metal-gate TMC 1795 is bigger than the 4004 and 8008 chips combined.
Intel’s 4004 and 8008 processors are much denser than Texas Instruments’ TMC 1795 chip, largely due to their use of self-aligned gates.
Shortly afterwards, semiconductor technology improved again with the use of NMOS transistors instead of PMOS transistors.
Although PMOS transistors were easier to manufacture initially, NMOS transistors are faster, so once NMOS could be fabricated reliably, they were a clear win.
NMOS led to more powerful chips such as the Intel 8080 and the Motorola 6800 (both 1974).
Another technology improvement of this time was ion-implantation to change the characteristics of transformers. This allowed the creation of “depletion-mode” transistors for use as pull-up resistors. These transistors improved chip performance and reduced power consumption.
They also allowed the creation of chips that ran on standard five-volt supplies.13
The combination of NMOS transistors and depletion-mode pull-ups was used for most of the microprocessors of the late 1970s and early 1980s, such as the 6502 (1975), Z-80 (1976), 68000 (1979), and Intel chips from the 8085 (1976) to the 80286 (1982).
In the mid 1980s, CMOS took over, using NMOS and PMOS transistors together to dramatically reduce power consumption, with chips such as the 80386 (1986), 68020 (1984) and ARM1 (1985). Now almost all chips are CMOS.14
As you can see, the 1970s were a time of large changes in semiconductor chip technology. The 4004 and 8008 were created when the technological capability intersected with the right market.
