WHEN the transistor exploded on the scene half a century ago, it seemed to sound the death knell for the thermionic valve or vacuum tube. Transistors, like valves, can amplify an electrical signal, and they can be made far smaller and consume much less power.
But now the valve is back. A team at the University of California in San Diego (UCSD) is using the latest chip fabrication techniques to build a valve on a microchip. And their purpose is up-to-the-minute too: to amplify the microwave signals used in cellphones and wireless connection technologies such as Wi-Fi and Bluetooth.
The devices are the brainchild of Sungho Jin, a veteran of Bell Labs, where the transistor itself was invented. Jin is now a materials scientist at UCSD. His valves work in the high end of the microwave frequency band, above 2.4 gigahertz, which is now being exploited by 3G phones and Wi-Fi.
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Transistors that amplify these high-frequency microwave signals burn up a lot of power 鈥� which in battery-powered devices like PDAs and cellphones is in short supply. That鈥檚 because power is soaked up by the electrical resistance of the semiconductor material used to make the transistors. The high resistance also limits the frequencies they can amplify. According to a patent application filed by the UCSD team (WO 2004/32275), the microscale valves-on-a-chip will avoid these difficulties.
In a valve, electrons flow through a near-vacuum 鈥� where they experience hardly any resistance 鈥� between the negatively charged cathode, which emits electrons, and the positively charged anode, which attracts them. This flow is controlled by a negative voltage applied to a so-called grid electrode, which lies between the anode and the cathode. A negative voltage on the grid counters the pull of the anode, and so limits the electron current. Because small changes to the voltage on the grid cause large changes to the current flow from cathode to anode, the valve acts as an amplifier.
Traditional valves are bulky and hard to make because the electrodes must be separately machined and then accurately aligned in a glass envelope. They also need a separate heater coil to raise the temperature of the cathode and make electrons 鈥渂oil off鈥� it, into the vacuum, and this consumes a lot of power.
Jin鈥檚 idea avoids the need for a power-hungry heater. To make his micro-valve, he etches three hinged flaps a few micrometres long in the surface of a silicon wafer (see Graphic). To do this he has adapted the technique that is routinely used to make the micro-mirrors that steer light to produce images in some video projectors.
The flap that will act as a cathode is then coated with ultrafine carbon fibres, to form a hairy-looking surface. When a negative voltage is applied to the cathode, a very high charge density builds up on the tips of the fibres, so they shed electrons without any need for a heater.
The other two flaps are coated with silicon and work as a grid and an anode. All the flaps contain a trace of ferromagnetic metal, so that when a magnetic field is applied, the flaps flip upright, like the masts in a 鈥渟hip in a bottle鈥�, and stay there. The device is then sealed in a vacuum.
Jin has so far refused to comment on how his devices perform, but his patent filing predicts they should use only one-tenth as much power as solid-state transistors when used as amplifiers.
