MOS Technology Used in Microprocessors

Most of the LSI/VLSI digital memory and microprocessor circuits is based on the MOS Technology. More transistor and circuit functions can be achieved on a single chip with MOS technology, which is the considerable advantage of the same over bipolar circuits. Below given are the reasons for this advantage of MOS technology:

  • Less chip area is demanded by an Individual MOS transistor, which results in more functions in less area.
  • Critical defects per unit chip area is low for a MOS transistor because it involves fewer steps in the fabrication of a MOS transistor.
  • Dynamic circuit techniques  are practical in MOS technology, but not in bipolar technology. A dynamic circuit technique involves use of fewer transistors to realise a circuit function.

So you are already clear that because of above said reasons, its considerably cheap to use MOS technology over Bipolar one.In different generation of microprocessor which MOS technology is used are listed down

1.First Generation microprocessors 

Shortly after 4004 appeared in the commercial marketplace,many electronic companies invested in this field and three microprocessors were soon introduced: Rockwell International 4-bit PPS-4, Intel 8-bit 8008, National Semiconductor 16-bit IMP-16. These microcontrollers were based on PMOS Technology.

What is PMOS ?

PMOS (pMOSFET) is a type of MOSFET. A PMOS transistor is made up of p-type source and drain and a n-type substrate. When a positive voltage is applied between the source and the gate (negative voltage between gate and source), a p-type channel is formed between the source and the drain with opposite polarities. A current is carried by holes from source to the drain through an induced p-type channel. A high voltage on the gate will cause a PMOS not to conduct, while a low voltage on the gate will cause it to conduct. Logic gates and other digital devices implemented using PMOS are said have PMOS logic. PMOS technology is low cost and has a good noise immunity.

2.Second Generation microprocessors

In the early 1970s we see second generation microprocessors in the marketplace,designed using NMOS technology.The shift to NMOS technology resulted in higher execution speeds,as well as higher chip densities. During this time we see 8 bit microprocessors such as Motorola6800,Intel 8080 and 8085,the highly popular,Zilog Z80,Motorola 6800 and 6809.

What is NMOS ?

NMOS (nMOSFET) is a type of MOSFET. A NMOS transistor is made up of n-type source and drain and a p-type substrate. When a voltage is applied to the gate, holes in the body (p-type substrate) are driven away from the gate. This allows forming an n-type channel between the source and the drain and a current is carried by electrons from source to the drain through an induced n-type channel. Logic gates and other digital devices implemented using NMOSs are said to have NMOS logic. There are three modes of operation in a NMOS called the cut-off, triode and saturation. NMOS logic is easy to design and manufacture. But circuits with NMOS logic gates dissipate static power when the circuit is idling, since DC current flows through the logic gate when the output is low.

3.Third Generation microprocessors

The third generation of microprocesosor were based on HMOS technology,which resulted in higher speeds and high chip densities.During  1978 we see 16bit microprocessor such as Intel 8086,Motorola 68000 and Zilog Z8000. 8086 was also used in early PC design called PC XT.

What is HMOS ?

8086 was introduced in 1978. The 8086 was sequenced using a mixture of random logic and microcode and was implemented using depletion-load nMOS circuitry with approximately 20,000 active transistors (29,000 counting all ROM and PLA sites). It was soon moved to a new refined nMOS manufacturing process called HMOS (for High performance MOS) that Intel originally developed for manufacturing of fast static RAM products. This was followed by HMOS-II, HMOS-III.

4.Fourth Generation microprocessors

The fourth generation of microprocessors appeared around the 1980s and the technology was based on HCMOS. During this generation,32 bit devices came into marketplace.Intel introduced highly popular microprocessor 80386,80486 and pentium family. Motorola introduced 68020 family.

What is HCMOS ?

HCMOShigh-speed CMOS, is the set of specifications for electrical ratings and characteristics, forming the 74HC00 family, a part of the 7400 series of integrated circuits.HCMOS also stands for high-density CMOS. The term was used to describe microprocessors and other complex integrated circuits, which use a smaller manufacturing processes, producing more transistors per area. The Freescale 68HC11 is an example of a popular HCMOS microcontroller. The 74HC00 family followed, and improved upon, the 74C00 series (which provided an alternative CMOS logic family to the 4000 series.

5.Fifth Generation microprocessors

With superscalar technology, the Pentium can execute many instructions at a rate of two instructions per cycle. Superscalar architecture usually is associated with high-output RISC chips. The Pentium is one of the first CISC chips to be considered superscalar. The Pentium is almost like having two 486 chips under the hood

First-Generation Pentium Processors

The first-generation design came in 60MHz and 66MHz processor speeds. The first-generation Pentium was created through a 0.8-micron BiCMOS process. Unfortunately, this process, combined with the 3.1 million transistor count, resulted in a die that was overly large and complicated to manufacture

Second-Generation Pentium Processors

Intel announced the second-generation Pentium on March 7, 1994. This processor was introduced in 90MHz and 100MHz versions, with a 75MHz version not far behind. Eventually, 120MHz, 133MHz, 150MHz, 166MHz, and 200MHz versions were also introduced. The second-generation Pentium uses 0.6-micron (75/90/100MHz) BiCMOS technology to shrink the die and reduce power consumption. The newer, faster 120MHz (and higher) second-generation versions incorporate an even smaller die built on a 0.35-micron BiCMOS process.

What is BiCMOS ?

BiCMOS technology is a combination of Bipolar and CMOS technology. CMOS technology offers less power dissipation, smaller noise margins, and higher packing density. Bipolar technology, on the other hand, ensures high switching and I/O speed and good noise performance. It follows that BiCMOS technology accomplishes both – improved speed over CMOS and lower power dissipation than bipolar technology. The main drawback of BiCMOS technology is the higher costs due to the added process complexity. Impurity profiles have to be optimized to both NPN and CMOS issues.

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