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The Top 5 Reasons to be Thankful for CMOS
Tis the season to be thankful… November 22nd marks Thanks Giving Day in the US where people gather around a table to consume a feast and give thanks for friends and family. The engineering community has much to be thankful for as well. The number “zero” for instance… without zero, the ability to represent large numbers would take a great deal of beans and things would be very difficult to calculate. The semiconductor industry has many things to be thankful for as well… the transistor is a good example. Without it, building active circuitry would require tubes… tough to put 2 billion of them in anything let alone a small tablet you carry under your arm. The evolution of the transistor led to the invention of the integrated circuit patented by Jack Kilby of Texas Instruments in 1959. What followed was an explosion of invention including the work done by C.T. Sah and Frank Wanlass of Fairchild which led to the development of the Complementary Metal Oxide Semiconductor (CMOS) process in 1963. This process has enabled countless technologies… so in my typical “count down” style, here’s my Top 5 Reasons we love CMOS.
Reason #5 : CMOS has extremely low power. The CMOS process when used in a digital fashion (on or off) uses extremely low power since it only uses current during the state transition. This is called the dynamic power and has a direct correlation to the frequency of operation and the square of the supply voltage. It does however “leak” a bit which adds“static power consumption” to the equation. However, modern low-power processes do an excellent job minimizing this leakage and improving on the dynamic power by lowering the supply voltage through smaller geometries.
Reason #4 : CMOS can be fabricated in extremely small geometries. It was once thought in the recent past that CMOS would “run out of steam” at about the 32 nm geometry node due to the thinning oxide of silicon… well, not so. Give humans a problem and they will find a solution. Enter “Hafnium” oxides which electrically appear “thinner” but are mechanically thicker to allow continued shrinking of the conduction channel. Say hello to sub 32 nm CMOS! Oh, and ask me how they make an optical mask with lines that close that doesn’t diffract 193 nm wavelengths of deep ultraviolet light… that’s another blog topic! Let’s just say constructive and destructive interference with a dash of phase shifting…
Reason #3 : Digital logic loves CMOS. One of the driving issues and possibly the basis for the invention of CMOS is the power dissipation in bipolar logic in the static state. CMOS is recognized as having extremely low power allowing high levels of integration (you can make small transistors close together and still get the heat out). The concept has evolved to even smaller non-planar 3D structures such as TriGate or Multigate FINFETs used in devices such as the Ivy Bridge processors from Intel. These transistors stack the gate vertically to provide more surface area allowing for better conduction when “on” and near zero conduction when “off”. This combats the short channel effects and other issues that lead to leakage in the static state… it continues to get better and better!
Reason #2 : Analog loves CMOS too. So you think digital logic is the only application for CMOS… not so! Analog has embraced CMOS for years for applications in low power amplifiers and comparators, voltage regulators, ADCs and DACs as well as other mixed-signal devices. The design methodology is a bit different in that the power calculations of the analog section must include the desired signal-to-noise ratio (e.g. Pmin = 8kT * f * (S/N) where the signal Vpp = Vsupply). However, lower bias currents combined with efficient logic provides the ability to create overall lower power analog systems. A classic application for this type of design is in class-D amplifiers found in most mobile devices such as smart-phones.
…and (wait for it…)
Reason #1 : You can make very cool things with CMOS. This is where the fun begins (for everyone – not only the engineers!). The fact that you can place billions of transistors on a single die, mix in analog functionality and lower power consumption to almost nothing (relatively), you can create some extremely neat things – and with the holiday season approaching, I’m sure you will be buying many of these amazing devices!
So enjoy your time this holiday with your family and all your cool gadgets which – fundamentally without the CMOS semiconductors would not exist… or at least would not fit in your pocket! Till next time…
What are you thankful for this year?
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