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Building bigger GaN ICs

TriQuint builds a ring oscillator with more than a thousand transistors.
Engineersat TriQuint Semiconductor have taken the level of integration in GaN ICs to an entirely new level with a ring oscillator circuit built from 501 depletion-mode and 502 enhancement-mode HEMTs.

This effort, which employs transistors that can operate at sub-millimetre wave frequencies, will aid the development of higher performance, mixed-signal products for commercial and defence applications that now draw on silicon CMOS and SiGe technology.

Douglas Reep, Senior Director of Research in the Infrastructure and Defense Products division at TriQuint, points out that monolithic integration in GaN will allow high-speed, low-voltage switching to be combined with other attributes of GaN, such as its high power density.

“It will also allow designers to avoid the inter-chip interconnect parasitics and complex packaging solutions that limit today’s high performance, mixed-signal circuits.”

In Reep’s opinion, the most significant aspect of this piece of work by TriQuint is that it is the first report of E-mode and D-mode HEMTs, which have been grown on a single wafer, that combine a breakdown voltage in excess of 10 V with values of more than 300 GHz for maximum cut-off frequency (fT) and maximum oscillation frequency (fmax).

Transistors capable of realising a very high value of fTcan accurately produce high-speed digital waveforms. “And when combined with a sufficiently high breakdown voltage, the same devices can be used in RF power amplification circuitry addressing the W-band and beyond,” explains Reep.

The sub-30 nm gate-recessed HEMTs produced by TriQuint are not based on the most common pairing of nitride materials – GaN and AlGaN – but instead exploit the combination of GaN and InAlN. Turning to a In0.17Al0.83N barrier creates a conduction band discontinuity of 0.65 eV to GaN; spawns a large spontaneous polarization field; and leads to charge densities in excess of 2 x 1013 cm-2 in very thin layers. Thanks to these attributes, aggressive scaling is possible with manageable short-channel effects.

“At similar thickness, the carrier density for AlGaN/GaN HEMTs, with a typical aluminium mole fraction of 25 percent, would be inadequate,” argues Reep. “A thin AlN barrier could provide a similar amount of charge, but with the disadvantage of high strain, due to a large lattice mismatch between AlN and GaN.” In comparison, because it can be lattice-matched to GaN, InAlN promises higher reliability.

TriQuint’s engineers fabricated their HEMTson 6H-SiC substrates, using MOCVD-growth to deposit the layers: a 1.5 µm-thick layer of iron-doped GaN, followed by a 1 nm-thick layer of AlN and a 8 nm-thick layer of In0.17Al0.83N. A dieletric process formed the 27 nm-long Pt/Au gates, with E-mode channels defined by selective removal of the InAlN layer beneath the gate. Circuit fabrication involved an abridged version of TriQuint’s three-metal interconnect process.

Electrical measurements on the transistors revealed transconductance and drain current in excess of 1 S/mm and 2 A/mm for E-mode and D-mode devices. Off-state breakdown for the latter device is 10.7 V, and it is 11.8 V for its cousin. Values for fTand fmaxwere obtained by extrapolating data from on-wafer, S-parameter measurements made between 0.5 GHz and 110 GHz, taken with an Agilent E8361C network analyser. For the E-mode HEMT, fTand fmaxwere 359 GHz and 347 GHz, respectively, and for the D-mode variant, these values were 302 GHz and 301 GHz, respectively. Compared to a previous generation of transistors produced by TriQuint, the latest version is almost 60 percent faster, in terms of  fT. This is attributed to a trimming of parasitic capacitance and resistance, and only a small increase in RF transconductance. The IC produced by the team is an order-of-magnitude more complex than its predecessor, a 51-stage ring oscillator based on direct-coupled FET logic.

Future goals for the team include improving the manufacturability of its deeply scaled GaN, while studying its reliability and optimising it for the company’s infrastructure and defence applications.
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