February 02, 2004
High Priests & Gurus
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Peggy Aycinena - Contributing Editor

by Peggy Aycinena - Contributing Editor
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There's magic in semiconductor physics and the high priests of that magic are a pretty interesting bunch of guys. Their magic is based on the fact that semiconductors live in limbo - neither are they fully conductors, nor are they fully insulators. The high priests have known for a long time that they can alter the characteristics of a semiconductor by introducing impurities into its molecular structure. The type and quantity of impurity that the high priests introduce into a semiconductor - a process called “doping” - is one of the founding principles of their magic, one that involves all kinds of closely guarded rituals, secret sauces, and soothsaying. But we'll talk more about the magic in a minute.

Meanwhile, go to your bookshelf and pull down your textbook on electronic devices. Remember that semiconductors basically come in two flavors, p-type and n-type. (That's “p” for “positive” and “n” for “negative.”) In p-type semiconductors, the majority carriers are non-negative charges referred to as “holes.” In n-type semiconductors, the majority carriers are negative charges.

Then, remember that a transistor is a device created by sandwiching various semiconductor materials together. Transistors can be pnp or npn depending on how the semiconductor materials are layered. Remember that transistor device behavior is dependent on both the type of semiconductor material and the type/amount of doping inside of that material. Remember also that transistor behavior can be tweaked and twiddled by applying various voltages across the different terminals on the device and/or by subjecting the device to a range of environmental conditions.

Now remember that the BJT (bipolar junction transistor, invented around 1947 at Bell Labs) is a current-controlled device that consists of a thin base region (only lightly doped) sandwiched between the larger emitter and collector regions (both more heavily doped). In the case of the pnp BJT (p-type emitter region, n-type base region, p-type collector region), a voltage is applied across the emitter-base junction, causing majority carriers to flow from the emitter region into the base region - “holes” flow from p-type emitter region into the n-type “hole-depleted” base region.

Meanwhile, a “reverse-bias” voltage applied across the base-collector junction causes “minority” carriers to flow from the base region into the collector region - positive charges flow from the n-type base region to the p-type collector region. Don't forget, though, that you've also got some small amount of current that can also be bled off of the base. (Of course, if you've got an npn-type device on your hands, the vice versa of all of this applies.)

The end result is that the emitter current is the sum of the current flowing out of the collector (lots) plus the current flowing out of the base (little), and the BJT is a current-controlled device. The large amount of current coming out of the collector (in the range of mAmps) is “controlled” by the small amounts of current coming out of the base (in the range of microAmps). Okay, now turn to a different chapter in your textbook. Remember that there's another class of transistors, the FETs or Field Effect Transistors, and that those bad boys come in two flavors - the metal-oxide semiconductor FET (MOSFET) or the junction field-effect transistor (JFET). They were invented by Shockley in 1955, although if your textbook is worth what you originally paid for it, you'll see that the history of the FET is actually quite complicated. (Even today it's somewhat controversial and you should go read the December 1997 issue of the IEEE Journal of Solid-State Circuits, vol. 32, No.12. You should also check out www.bellsystemmemorial.com/belllabs.transistor1.html.)

Meanwhile, your textbook should be reminding you that you used to know, way back when, that FETs differ from BJTs in that they're voltage-controlled devices and have only one kind of charge carrier moving around in there. Hence, they're “unipolar” devices - none of this majority carrier/minority carrier nonsense that's happening over in the BJT chapter.

Even better, FETs have sources, drains, and gates, which is far more intuitive than that BJT emitter-collector-base deal. And, the other really cool thing about JFETs in particular - they're channeling Jenny Craig in such a way that one minute they've got the anorexic wasp-waste thing going one and the next minute it's, “Oh my, better move the old belt buckle out a notch or two.” Which is just really cool.

Okay, okay, fine. Enough with the textbook. It's time to get back to the high priests of semiconductor magic. Two of the best and brightest among them are currently working side-by-side at the Agilent Technologies' Worldwide Process and Technology Center in Santa Rosa, CA.

Dr. David Root is the more senior of the two; Dr. Masaya Iwamoto is the newer member of the team. They spend their days working spells and incantations over small cauldrons full of a variety of toxic ingredients, trying to coax ever more magical and mysterious BJT behaviors out of the lumps that they pull up out of their brew. Oh, and not just timeworn, age-old silicon-based brews for these guys.

Root and Iwamoto are busy with the newer, trendier broths - stuff like gallium arsenide (GaAs) and indium phosphide (InP), stuff that makes their fellow high priests go all a-tingle with the dark, bubbly mystery of what can be done (particularly in those truly spooky wireless devices) when clever mixes and matches are made here.

(Well, okay, okay. In truth, Root and Iwamoto aren't really manufacturing semiconductors, or BJTs for that matter. What they're really doing all day is making models of what the lumps and glumps and BJTs would be like if somebody actually were to manufacture them along the lines of the spells and incantations that Root and Iwamoto design. Or in their words: “We feed back our understanding based on modeling, analysis, and characterizations, to those actually designing and fabricating the transistors.” An important distinction, no doubt, but you get the idea.)

Anyway, it's important to note at this point that Root has had his GaAs high priest credentials in place for quite some time, and not just in BJTs, mind you, but in FETs as well. The Root Model (as in “David Root,” not as in “root” of the problem) was first announced in a 1991 paper, “Technology Independent Non Quasi-Static FET Models by Direct Construction from Automatically Characterized Device Data” (with Fan and Meyer) in the 21st European Microwave Conference Proceedings. The article defines a measurement-based FET model for large-signal (nonlinear) simulations of GaAs FETs in both the time and frequency domain. Even today, other high priests are still referencing those seminal incantations.

(For those looking for a more “accessible” version of those particular incantations, Root's article, “A Measurement-Based FET Model Improves CAE Accuracy,” was published in Microwave Journal at the same time.)

Iwamoto, meanwhile, has more recently proven his merit to his fellow high priests in several publications. His papers, “Linearity Characteristics of InGaP/GaAs HBTs and the Influence of Collector Design” (with Asbeck, Low, Hutchinson, Scott, Cognata, Qin, Camnitz, and D'Avanzo in Transactions on Microwave theory and Techniques) and “Large-signal HBT Model with Improved Collector Transit Time Formulations for GaAs and InP Technologies” (with Root, Scott, Cognata, Asbeck, Hughes, and D'Avanzo in 2003 IEEE MTT-S International Microwave Symposium Digest), were both published right here in the new millenium, and were of a caliber that, in conjunction with his Ph.D. thesis (researched and certified by the ECE Department at U.C. San Diego) and numerous summer internships with Agilent, earned Iwamoto the right to become an official high priest working alongside the established and esteemed Root in the Santa Rosa labs. (A run-on sentence, if ever there was one.)

Recently, I was lucky enough to be able to talk to the high priests Root and Iwamoto. And I knew as soon as the conversation started, that these guys are smart. Boy, are they smart! How do I know? Well, you can tell a lot by listening to the nuance in voices when you talk on the phone. The smart ones? They're really authentic in the way they describe things - especially high priests - and they're really, really modest. They'll talk about their own work, but they'll also tell about the great stuff that others have done, the models and incantations that other high priests have developed for coaxing cool stuff out of the cauldrons and bubbling goo.

Root told me lots about the great stuff that Iwamoto has done. Iwamoto and Root, together, told me about the great stuff that Peter Asbeck and Lovell “Stretch” Camnitz have done (fellow GaAs high priests, one of whom is 6' 8”). It remained for Joe Civello (ADS Platform Manager for Agilent EEsof EDA) and Lisa Hebert (PR Manager for Agilent) to tell me about Root's important accomplishments.

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-- Peggy Aycinena, EDACafe.com Contributing Editor.

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