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Transistor Teams Meet The Need For Speed

A handful of research teams are closing in on terahertz transistor speeds, with a variety of InP-based devices.

Researchers in the US and Switzerland are inching ever closer to the production of RF and digital components capable of operating at speeds faster than 1 THz.

At the International Electron Devices Meeting (IEDM) held in Washington, DC, last week, groups working on both HEMT and HBT designs revealed their latest progress, with Northrop Grumman s Richard Lai reporting the first maximum oscillation frequency (termed fmax) in the terahertz regime.

Although that figure is calculated by extrapolation of genuine measurements, Lai told IEDM delegates that reaching the 1.2 THz milestone with an InP-based PHEMT was "groundbreaking".

Lai and his team are aiming to develop the first RF amplifiers operating at terahertz speeds.

And, although they are yet to quite achieve that, their cutting-edge device designs are behind a record-breaking three-stage amplifier that delivered a gain of 16 dB at 340 GHz.

Key features of the latest devices, which Lai says are being fabricated in volume with a wafer yield of more than 90 per cent, include improved electron-beam lithography and a refined epitaxial structure.

Previous designs with a 70 nm gate delivered an fmax value of 700 GHz, but Lai and colleagues have scaled the gate down to just 35 nm (the actual gate size varies between 30 nm and 50 nm) to make the faster transistors.

The pseudomorphic epitaxial structure, which has been similarly scaled-down, features an InAs/InGaAs composite channel grown by MBE, and a highly-doped cap layer that reduces Ohmic contact resistance.

Calculating both fmax and the cut-off frequency (ft) is a complicated affair, and a number of different methods can be employed. But, according to Lai, whatever approach is taken, Northrop Grumman s InP HEMTs feature an fmax of more than 1 THz and an ft of around 450 GHz.

HEMTs to HBTs
Another team from Northrop Grumman s Redondo Beach, California, operation is working on InP-based HBT designs, where the emphasis is more on the ft figure of merit.

That team has now produced double-HBTs with an InGaAs base that show an ft of 400 GHz. Using 1,100 of these HBTs, fabricated in a high-yielding process, Cedric Monier and colleagues have also made a 35 Gb/s integrated circuit, and demonstrated a static divider with a record speed of 153 GHz.

Milton Feng s group at the University of Illinois at Urbana-Champaign (UIUC) has long targeted the terahertz regime, and at IEDM William Snodgrass described that team s latest achievement "“ an InP HBT with an ft measured at 745 GHz.

This is similar to the figures of merit that Feng s laboratory has previously produced (see related article), but what the team has now been able to do is to scale up the breakdown voltages of HBTs - something that is inherently traded-off against raw speed (see related feature articles).

Working on so-called "type II" HBTs grown by MBE, a 30 nm-thick GaAsSb base layer gave an ft of 480 GHz and a breakdown voltage of 4.3 V.

Further scaling that base layer to 20 nm yielded a faster ft of 630 GHz, with a breakdown voltage of 3.2 V.

Outside of the US, Colombo Bolognesi s group at ETH Zurich has also been pursuing super-fast InP HBT structures with a graded GaAsSb base layer, although this time using MOCVD.

For those devices, they have now extrapolated an ft of 612 GHz when measured at room temperature. Cryogenic cooling to 5 K yielded a speed of 705 GHz, which Bolognesi cites as record performance for any kind of double HBT.

What s more, Bolognesi believes that it will be possible to reach an ft of 1 THz while maintaining a healthy breakdown voltage of 2.5 V.

So while it may be a little premature to say that the terahertz "gap" has been closed, transistor developers are certainly homing in on their ultimate goal.

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