Ultrahigh-speed electronics is quickly approaching a "terahertz gap" between semiconductors that top out at hundreds of gigahertz and optical frequencies that hit hundreds of terahertz. Promising to span that breach, where wavelengths are measured in millimeters, is a new breed of metal-insulator electronics that inventor Phiar Corp. (Boulder, Colo.) has demonstrated at frequencies up to 3.8 THz.
Phiar claims its technology surmounts hurdles in many applications for which it already claims to have industry development partners, including 60-GHz antenna-edge frequency conversion, parallel flash solid-state storage drives, monolithic millimeter-wave radar, integrated terahertz detector arrays for safe "X-ray vision" systems and chip-to-chip RF interconnects.
"Our technology is the first viable alternative to semiconductors since the vacuum tube," said Adam Rentschler, Phiar's director of business development.
Phiar and Motorola Labs (Tempe, Ariz.) recently completed joint development of a 60-GHz antenna based on Phiar's metal-insulator diode in a bid to enable multigigabit wireless radios that would stream multiple channels of uncompressed high-definition video. The device conforms to the emerging wireless 60-GHz standard IEEE 802.15 TG3c. Previously, the only cost-effective devices fast enough to enable 60-GHz antennas have been expensive, discrete gallium arsenide diodes. But with Phiar's metal-insulator diode, which uses no semiconductors, Motorola has successfully prototyped 60-GHz radios and antennas.
"Motorola demonstrated multigigabit transmission and reception at very high frequencies several years ago and is currently working to improve the size, performance and overall cost of these devices," said Rudy Emrick, manager of millimeter-wave RF technology.
Using the Phiar technology, whereby 60-GHz radio signals are stepped down to a 2- to 3-GHz signal using an inexpensive analog metal-insulator circuit, high-definition video can be wirelessly trans- mitted between consumer devices. The technology might also enable inexpensive terahertz radar and imaging applications, such as airport security systems that can safely see through clothing to spot concealed weapons.
Phiar also claims to have landed a contract with an undisclosed "major U.S. flash memory manufacturer" that plans to use the metal-insulator technology to enable parallel flash memories for solid-state drives that operate at NOR speeds but achieve the memory densities of NAND.
In addition to the 60-GHz diodes, Phiar has demonstrated matched detectors and AM receivers, as well as proof-of concept prototypes of 60-GHz mixers, modulators and varactors; 500-GHz diodes; and terahertz detectors. It expects to demonstrate a terahertz-transistor prototype this year. "A metal-insulator transistor could enable a solid-state hard drive by configuring a high-density NAND memory, which is usually a serial device, for random access like a NOR memory, potentially enabling a random-access solid-state hard drive," said Vahé Mamikunian, senior analyst at Lux Research (New York).
Swapping out the semiconductor
Metal-insulator electronics substitute a second layer of insulator and metal for the semiconductor found in metal-oxide semiconductor (MOS) devices, ending up with a four-layer stack of metal-insulator-insulator-metal. What makes the technique work is that two types of metals, and their insulators, are carefully tailored to form a quantum well between the insulators that only allows high-energy tunneling. As a result, when a voltage is applied to the top metal that exceeds its threshold, a ballistic transport mechanism kicks in that accelerates tunneling electrons across the gap.
"We [facilitate] quantum tunneling across oxides whose thickness, taken together, is only about 60 angstroms," or 0.6 nanometer, said Bob Goodman, president and CEO of Phiar. "Because quantum tunneling is the transport mechanism, it happens much faster than anything in the semiconductor world--on the order of a femtosecond in our device, which would break the laws of physics for semiconductors."
As a result, metal-insulator devices have achieved recorded frequencies of up to 3.8 THz, whereas semiconductors inevitably slow electron flow to about 60 GHz maximum for CMOS and 400 GHz for SiGe.
Metal-insulator technology is also claimed to be easier to fabricate than high-speed silicon technologies, since it uses the same processing steps that are already in place at CMOS fabs and since the devices can be fabricated on almost any substrate---even on the inside of the plastic case of consumer devices.