. This microprocessor allowed for the design of powerful personal electronics such as computers, digital watches, and one-chip calculators to name a few. While microprocessors have been nick named a “computer on a chip”, they are not truly a computer. Even the simplest computers require large amounts of memory, which microprocessors don’t have.
- Analog Cicuits – Analog circuits were the earliest logic circuits developed. Process engineer Dave Talbert and designer Robert Widlar created the first commercially successful analog IC in 1964 . Analog circuits have an output that is proportional to the input, where digital circuits have a predetermined output in response to a variety of inputs. A dimmer light switch is an example of an analog device. Turing the control varies the voltage, which varies the brightness of the light.
- Digital Circuits – A traditional on-off switch is a digital device where only two levels of brightness are available: on or off.
Memory Circuits in More Detail
In the early 1970s industry forecasters saw their predictions become a reality when solid-state memories finally surpassed core memory. While core memory offered lower costs, solid circuits were smaller, more reliable, and faster.
A nonvolatile memory device is one that doesn’t lose its stored information when power is lost. Nonvolatile memory circuits include ROM, PROM, EPROM, and EEPROM. In integrated circuits, the ROM (Read Only Memory) design is the principal nonvolatile circuit. The function of this circuit is to give back precoded information that is designed into the chip memory array section during fabrication. PROM stands for programmable read only memory. Each memory cell is connected into the circuit through a fuse. Users program the PROM into their memory circuit requirements by blowing fuses at the unwanted memory cell locations. Once programming is completed, the PROM is changed to a ROM and the information is permanently coded into the chip. EPROM (erasable programmable ROM) allows for information stored in the ROM to be changed without having to replace the whole chip. The erasable feature is built in by using MMOS (memory MOS) transistors. EEPROM (electronically erasable PROM) allows for memory to be programed and reprogramed while the chip is in the socket of the machine. Programming and erasing takes place by pulses from the outside that place charges in selected memory cells or drain the charges away.
While nonvolatile memory provides protection against power loss, it is often slow, not very dense, and does not have write capability. Volatile memory is used to produce fast and high-density memory circuits. RAM (random-access memory) is one type of circuit used for high-density memory storage. “Random” refers to the ability of the computer to directly retrieve any information stored in the circuit. DRMAs (dynamic random access memory) come in two designs: dynamic and static. Information is stored on DRAMs by a charge built up in the capacitor. However, this charge drains rapidly and the memory information must be re-inputted, or refreshed, on a constant basis. The goal of DRAM IC design is small-cell design for high-density and closely spaced components with small and thin parts for speed. SRAM (static random-access ram) memories are based on a cell design that does not need to be refreshed. Once the information is on the chip, it will stay as long as the power remains on. This is done with a cell containing several transistors and capacitors. Information can be read and written with a SRAM cell much faster than with a RAM design since transistors can be switched faster than capacitors can be charged and drained. The downside to SRAM is that they are less dense than DRAMs.
We hope you enjoyed this brief review of various integrated circuit types. Check back next week to see our next installment of Semiconductor Processing and Integrated Circuits. If you would like to read more on this topic feel free to click the links below.
Semiconductor Processing and Integrated Circuits Part 11 Semiconductor Processing and Integrated Circuits Part 10  http://inventors.about.com/od/mstartinventions/a/microprocessor.htm
Van Zant, P. (2000). Microchip fabrication, a practical guid to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill