Deep – really deep – inside Tri-Gate
As we mentioned earlier, Tri-Gate is a variation of the FinFET concept, in which instead of lying below the gate as in a traditional planar transistor design, the charge-carrying silicon pokes up into the gate itself.
In a planar design, gates were lain across a flat silicon substrate – not 3-D enough for marketeers, however.
This non-planar construction, by the way, is why you'll hear Intel marketeers exalting the Tri-Gate design as "3-D". Well, yeah, that's not entirely untrue, but the gate-wrapped fin is hardly 3-D enough to warrant such buzz-lines as this one from the Wednesday presentation: "Transistors have now entered the third dimension! – and, yes, that's Intel's italics and Intel's exclamation point.
In any case, you might reasonably ask: "So the fin sticks up in the gate? So %$#@!ing what?" Bohr's answer to that perfectly reasonable question is equally reasonable: "The key advantage of this structure – and it comes from the fact that the gate wraps around this fin – [is that] it provides fully depleted operation of the transistor."
Bohr's answer, of course, is only reasonable if you know why a fully depleted transistor is a good thing – which takes a bit of background to explain.
When a traditional microprocessor transistor is in the "on" state, current flows from source to drain through what's called the transistor's "inversion layer", which interacts in Intel's designs with the transistor's high-k metal oxide interface with the gate. That's all well and good.
However, when the transistor is in the "off" state, some charge can still trickle through the silicon substrate. When it does, the transistor is regarded as not being fully depleted. That's bad.
When voltage hangs around like this, it degrades what chip boffins call the "sub-threshold slope" of the transistor – which Bohr defined as "essentially the transistor turn-off characteristics," or which we can think of as how "off" the off state actually is.
When the transistor's off state is fully depleted, it helps minimize power usage. Also, a fully depleted transistor can have a lower threshold voltage – meaning that the voltage needed to switch it from off to on can be lower, again saving power.
"When you operate at a lower threshold voltage," Bohr explains, "you have improved performance. Or you can operate at lower voltage, and if you operate at lower voltage, there's a significant active-power saving – and that's really probably the most important advantage of Tri-Gate."
So, a fully depleted transistor both leaks less power and requires less power. All good.
Planar processors are hell to make fully depleted. In their traditional state, there's a lot of room in the silicon substrate to house errant voltages. You can, however, add an oxide insulator below the source and the drain to create what's called a partially depleted silicon-on-insulator (PDSOI) design.
You can go all the way to fully depleted (FDSOI) without going FinFET by depositing an extremely thin SOI layer on top of the oxide – but reaching full depletion this way is quite expensive. According to Bohr, baking a chip this way adds at least 10 per cent to the total wafer cost.
Tri-Gate, by comparison, is cheap. "Another nice thing about the Tri-Gate devices," Bohr says, "is that they're not that expensive to add. Compared to a planar version of 22 nanometers, Tri-Gate transistors add only about 2 to 3 per cent cost to the finished wafer."
The Tri-Gate way of reaching full depletion is to stick that silicon fin up into the gate, have the inversion layer on both sides and the top of the fin, with the high-k metal oxide of the gate snug against the inversion layer. Presto – no room for nasty voltages to accumulate, plus the larger wrap-around inversion layer and metal-oxide interface allow for more current, and thus better performance. Bonus!
Another nifty Tri-Gate trick is that you can have multiple fins in the same transistor: more fins, more current, better performance. Or, as Bohr puts it succinctly: "You can gang together these fins to create larger transistors with higher drive currents."
This "How many fins do you want?" capability will help Tri-Gate to populate throughout Intel's product line, from low-power single-fin transistors to multi-fin structures that will find their way into Xeons – and even into Itaniums, when that benighted chip reaches its Kittson incarnation, two generations from today's 9300 series.
Next page: All well and good, but so what?
Re: Interesting thought...
No it wouldn't. Merging with someone like AMD would kill ARM.
AMD licensing ARM designs would be more interesting, and any of ARM's current licensees moving to the same 22nm Tri-Gate process would effectively kill Intel's chance of ever gaining any momentum in the high-performance, low-power market.
It's about time ARM made significant inroads into the desktop market. Intel have been making our (programmer's) lives hell for long enough.
"Or, for that matter, Intel could license the ARM architecture and start buiding its own ARM variants in its own fabs, using its 22nm Tri-Gate process"
I'd have thought, more like a dead cert, than unlikely.
If Intel has the best process technology for low-power devices, ARM without question has a better CPU architecture for low-power devices like Smartphones. Put them together and what do you get? The best possible Smartphone CPU, that can either double battery runtime, or allow for a large cut in the weight of the phone without any loss of runtime.
If Intel suffers from the "not invented here" syndrome, Smartphone manufacturers will have to choose between i86 architecture running on the best Silicon, or ARM running on less good silicon. It won't be so long before TSMC or some other chip foundry catches up with Intel enough to put ARM back at the front of the pack. Best for Intel if it's Intel that makes the best mobile device chips.
Whatever Intel can do with FinFET then ARM can also do
@"The move to a 22nm Tri-Gate process architecture is an important step for Intel's entire microprocessor line"
It is important because once the ARM A15 design is here, Intel will start to loose the future server market on processing power per watt. (The ARM A15 will allow the design of servers with more processing power for less electrical power than an Intel CPU based design. That's win win for ARM and Checkmate for Intel's bloated x86 design).
Intel's market lead and dominance up until now has largely depended on Intel's ability to define what each new generation of x86 design should be, so they were always first to market with each new x86 generation. That meant each time the x86 design changed, AMD had to spend time playing catch up to add each new addition to the ever more bloated and increasingly complex x86 design. This constantly changing x86 design gave Intel the marketing lead over AMD, because Intel were always going to be first to market with each new generation.
Unfortunately for Intel, ARM are not playing that same game. ARM processor designs are more power efficient than x86 designs. So whatever chip making process Intel uses, they still can't win, as an ARM design based on that same chip making process will win over the x86 design. Ironically for Intel, their bloated x86 design that was so useful for holding back AMD, is now holding them back from competing with ARM.
Which leaves software (not hardware) as Intel's remaining x86 strength and even that is just in its legacy support. Its a pain to recompile programs and some old programs won't be recompiled (the companies who created the code may not even be around any more). But for all new code, its really not an issue. Plus ARM has a lot of software support. For example Linux has been on ARM for years. Android and iPhone already support ARM. Even Microsoft are looking to support ARM. Also ARM software development has had 28 years to evolve to a very high level of industry support with good free tools. So ARM is strong in software support as well as beating the x86 design in processing power per watt.
So Intel are in trouble. AMD isn't Intel's biggest competitor, its the over 200 companies that all license the ARM designs that are Intel's biggest threat, because together these over 200 companies could seriously harm Intel's market dominance. So Intel really are in trouble.
The article made even someone as thick as me think that they understand the concepts; thanks.
"The answer- to blow sugar up your ass."
I'm not sure that's all it's cracked up to be.