PORTLAND, Ore.—Post-silicon era transistors fabricated from sheets of pure carbon—graphene—are pioneering a new paradigm for digital circuitry, but what about analog circuits? Now Rice University researchers have demonstrated analog graphene transistors that can not only amplify like p-type and n-type silicon transistors, but can also exploit the ambipolar ability of graphene in a novel frequency-multiplication mode.
The Rice researchers demonstrated how such triple-mode graphene transistors can be used to build simpler phase-shift keying and frequency-shift keying circuitry.
Silicon field-effect transistors (FETs) are called unipolar, because they only use one charge carrier—either electrons of holes—depending on whether they are n- or p-type. On the other hand, carbon ambipolar transistors, like those constructed from graphene, can conduct both electrons and holes, depending upon whether a positive or negative voltage is applied. As a result, novel analog circuits can be constructed that amplify using both types of carriers, depending on whether the input signal is above or below the gate's bias voltage, according to the researchers.
Three-mode transistors, according to Rice University, permit simpler circuits to be constructed, potentially shrinking the size and complexity of common analog functions when using graphene transistors. To prove the concept, the Rice University researchers constructed circuits in common-source, common-drain, and frequency-multiplication modes, including modulation schemes for phase-shift and frequency-shift keying.
The demonstrations were carried out by professor Kartik Mohanram in collaboration with professor Alexander Balandin at the University of California-Riverside, and doctoral candidates Xuebei Yang (Rice) and Guanxiong Liu (UC Riverside).
Funding for the project was provided by the National Science Foundation and the DARPA-Semiconductor Research Corporation's Focus Center Research Program.
It is called ambipolar because it changes its behavior. It acts like a p-channel FET when the gate is at one polarity and an n-channel FET when the gate is at the other. So, it wanders ("ambi", think "amble" or like "ambidextrous") between polarities. What I am curious about is whether there is a dead band between polarities and is that controllable. I would also like to see a couple of simplified schematics using the device.
Life could become really interesting; does ambipolar therefore not need a substrate bias to prevent latchup? Could we discard our old 'nominal 0V' substrate and run everything in true differential? Every signal could have a mirror in the opposite polarity, giving potential for low crosstalk and low supply voltages/signal swings... could also make it easier to recycle charge locally instead of burning it in resistive losses.
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