PORTLAND, Ore. Graphene transistors could enable complex new circuits using relatively simple architectures, according to Massachusetts Institute of Technology researchers who recently demonstrated a single transistor frequency multiplier.
Graphene transistors can transport both electrons like a silicon transistor, and holes like a GaAs transistor, enabling graphene field-effect transistors that outperform both silicon and gallium arsenide.
"Our new device is based on a property of graphene called 'ambipolar transport'," said MIT professor Tomas Palacios. "In conventional semiconductors, you normally have to choose between one of two different carriers: electrons [negative particles] and holes [positive particles].
"In graphene, because of the ambipolar transport, you don't need to choose, and you can have both of them in the same material by just changing the voltage applied to the graphene layer," Palacios added. "Our device uses this property to double the frequency of the input signal."
The new circuit consists of one graphene transistor and a resistor. "It is therefore a lot simpler than conventional circuitry where you need a filter to select the desire output signal," said Palacios. "It is also expected to work at much higher frequencies than conventional devices."
|A single transistor frequency multiplier demonstrated by MIT combines the functions of both silicon and GaAs transistors.|
The basic diagram for the MIT circuit is a single transistor in common source configuration with a pull-up resistor. The main difference is how the graphene transistor is biased. Unlike silicon, which only transports electrons, and GaAs, which only transports holes, the gate of a graphene transistor induces the transport of electrons or holes depending on whether the input signal is positive or negative.
"Independently of the nature of the carriers, the current always flows through the resistor in the same direction, which allows for the frequency multiplication effect," said Palacios.
For frequency multiplication, whenever the alternating current (AC) input signal passes through zero and becomes positive, it induces an'electron current through the drain. When the input goes negative, it induces a'hole current. In both cases, the output voltage'starts at zero, then ramps up with peaks and valleys in the AC input voltage, thus doubling its frequency.
The proof-of-concept demonstration has yet to be optimized for high-frequency applications, but the researchers claim that the high electron mobility of graphene transistors--100-times greater than silicon--should allow the frequency multiplier to be cascaded to repeatedly double output, perhaps into the terahertz range.
Graphene is composed of a single-crystalline lattice of carbon atoms whose electron mobility is higher than both silicon and GaAs, but which are more difficult to fabricate. The MIT researchers are growing their own graphene sheets, which so far have only been demonstrated in small flakes. Their goal is to fabricate entire wafers of the graphene material.
"For commercialization, it is important to fabricate these devices in large area graphene sheets or wafers," said Palacios. "We have recently achieved progress in this direction by growing large graphene flakes on silicon substrates using chemical vapor deposition."
Palacios estimated that it will take as long as two years to demonstrate wafer-scale fabrication of the graphene circuitry.