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The fastest transistors in the world.

The fastest transistors in the world

In the race to make computers computer faster and communication systems convey more information, the speed at which electronic components operate is of paramount importance. Last week, scientists at the University of Illinois at Urbana-Champaign, along with researchers at General Electric Laboratory in Syracuse, N.Y., pulled ahead of the pack by announcing that they had developed the world's fastest transistor. Depending on whom one speaks to, however, the Illinois-G.E. team may be running neck and neck with a research group working on a different kind of transistor at Lincoln Laboratory in Lexington, Mass.

Illinois team leader Hadis Morkoc estimates that his group's transistor, a type of semiconductor device called a modulation-doped field-effect transistor (MODFET), has a maximum operating frequency of 230 billion hertz (GHz) -- about one and a half times higher than the previous MODFET record. Phillip Smith at G.E. says he expects that the device, with refinement, could achieve 400 GHz or more. The higher the maximum operating frequency, the more information a device can receive or transmit when it's used in analog circuits. And a high maximum operating frequency means that the device, when used digitally in computers, has a fast switching speed.

Gerald L. Witt of the Air Force Office of Scientific Research in Washington, D.C., calls the 230-GHz number "astounding." Smith thinks the new device will have a "significant impact" in communications and he anticipates that "most people now working on conventional MODFETs will drop them in favor of this new device." He says the new MODFET also has a lower noise level than any other transistor in the making.

Morkoc's group has improved upon conventional MODFETs by sandwiching a 100-angstrom-thick layer of indium gallium arsenide (InGaAs) between the layers of aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs) found in conventional devices. The velocity of electrons is much greater in InGaAs than in GaAs or AlGaAs. The disadvantage of trying to grow inGaAs atop GaAs is the spacing between atoms in the InGaAs lattice differs from that in GaAs; this mismatch strains the crystal, causing lattice defects that impede electron motion. But Morkoc's group made the layer thin enough to minimize the number of defects.

Researchers at Lincoln Laboratory, however, say the new MODFET's maximum operating frequency is comparable to what they have achieved with a device called the permeable base transistor (PBT), which consists of metallic, comb-like teeth embedded in gallium arsenide layer. But Morkoc says he thinks the calculations of his MODFET's maximum operation frequency are much much accurate than the estimates of the PBT's. Moreover, he and others note that PBTs, unlike conventional MODFETs, have yet to be produced outside of a laboratory, so chances are that Morkoc's new MODFET will reach the commercial winner's circle much sooner.
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Title Annotation:modulation-doped field-effect transistor and permeable base transistor
Author:Weisburd, Stefi
Publication:Science News
Date:Oct 18, 1986
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