This is Glew Engineering’s 12th installment in the blog series discussing Integrated Circuits (IC)s and Semicondcutor Processing; today’s blog will be addressing integrated circuit types. These articles have been written for those that are not technical specialists in the semiconductor field. While the majority of this series has focused on semiconductor processing, herein we highlight general circuit families and functions of ICs.
A solid-state integrated circuit is comprised of a number of separate functional areas. Each chip, regardless of its intended function, has an input and encode section where the incoming signals are coded into a form the circuit can understand. The majority of the circuit area contains the circuitry required to perform the circuit function, either memory or logic. Once the data is manipulated by the circuit, it goes to a decode section where it is changed into a form that is usable by the machine’s output mechanism. The output section transmits the data to the outside word.
Three Circuit Types
Circuit types fall into three broad categories:
- Logic – Logic circuits perform a specified logical operation on the incoming data. For example, pushing the key for the letter “B” on a keyboard will result in the letter B appearing on the computer screen.
- Memory – Memory circuits are designed to store and give back data in the same form in which it is entered. Selecting the (pi) key on a calculator activates the memory portion of the circuit where the value of pi (3.14159…) is stored. The remembered value, 3.14159, is then displayed on the screen.
- Logic and Memory (microprocessors) – The third type of circuit combines both logic and memory in a microprocessor circuit. The world’s first single chip microprocessor was introduced to the public by Intel in 1971. 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.
Van Zant, P. (2000). Microchip fabrication, a practical guid to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill