Transistors go 3D as Intel re-invents the microchip

At an event today in San Francisco, Intel announced one of the most important pieces of semiconductor news in many years: the company's upcoming 22nm processors will feature a fundamental change to the design of the most basic building block of every computer chip, the transistor.

Intel has been exploring the new transistor for over a decade, and the company first announced a significant breakthrough with the design in 2002. A trickle of announcements followed over the years, as the new transistor progressed from being one possible direction among many to its newly crowned status as the official future of Intel's entire product line.

In this short article, I'll give my best stab at explaining what Intel has announced—the so-called tri-gate transistor. Semiconductor physics are not my strong suit, so corrections/clarifications/comments are welcome. Also, this explanation focuses solely on the "3D" part of today's announcements. Other features of the 22nm process, like high-K dielectrics and such, are ignored. (So if you see a funny term on a slide and you don't know what it means, either ignore it or hit one of the Related Links for more info.)
But before we dive into what's new about Intel's transistor design, we first have to review how traditional transistors work.
In the diagram above, you can see that a traditional "planar" transistor—the kind that was first invented at the dawn of the microchip era, and which has been the norm up until today's announcement—consists of three main parts: source, drain, and gate. (This is actually one specific kind of transistor, a MOSFET, but let's not get too deep into the weeds.)

The device may look odd, but it's really just an electrical switch. Think of the source and the drain as the two slots in a standard electrical socket; if you stuck a conducting wire (like a coat-hanger ) into both of the slots, you'd close the circuit and current would flow (and sparks would fly, flesh would burn, etc... so don't try that at home.) The transistor's substrate is sort of like a magic wire that can either conduct electricity or not, and the gate is the switch that controls whether the wire will conduct or not.

So when a voltage is applied to the metal plate that forms the transistor's gate, a tiny strip of semiconductor material between the source and the drain (our magic wire) changes from an insulator into a conductor, thereby turning the switch "on" and allowing current to flow from the source to the drain. When the voltage is removed, current stops flowing... or, at least, current is supposed to stop flowing when the switch is off. In reality, trace amounts of current will constantly flow between the source and the drain. This so-called "leakage current" wastes precious power and becomes even more of a problem as transistors get smaller and more numerous.

So to recap, the basic idea is that the transistor is a switch that works because a tiny bit of insulating material between two "electrodes" magically morphs into a conductor when a voltage is applied to it, thereby closing the circuit.