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Pseudo HBT accelerates to 845 GHz

It's IEDM time again, and that can mean only one thing: another record-breaking transistor from Milton Feng and his Urbana-Champaign research team.

Milton Feng s team of researchers from the University of Illinois at Urbana-Champaign has once again smashed the world transistor speed record, this time with a 845 GHz pseudomorphic heterostructure bipolar transistor (PHBT).

The team has used the International Electron Device Meeting (IEDM) on many previous occasions to reveal transistors with a record cut-off frequency - the semiconductor industry s equivalent of the land speed record - and this year is no different.

The latest achievement is a pseudomorphic InP/InGaAs device that actually operates at "only" 765 GHz at room temperature. At -55°C, reduced base and collector transit delays combine with smaller charging delays at the transistor s collector to deliver the faster switching speed.

Feng has long been at the forefront of the US military-funded drive towards a transistor with terahertz switching speed, and previous efforts include a 562 GHz PHEMT, and both single and double HBTs exceeding 500 GHz.

However, to reach terahertz speeds, single and double HBTs would need unrealistic current densities exceeding 300 mA/µm2.

The scalability of PHEMTs is also limited, because they have to be biased at close to their breakdown voltages to switch at these very high speeds.

Feng reckons that his PHBTs, which are so-called because their base and collector regions feature pseudomorphically-graded InGaAs structures, offer a possible solution, largely because their cut-off frequency has an accelerated dependency on current density.

The latest results demonstrate this property, and suggest that a terahertz transistor would draw a current density of only 28 mA/µm2.

Having previously made a 710 GHz PHBT, Feng and colleagues broke the speed record by reducing the size of the transistor's base finger mesa region (see SEM image). The smaller mesa drastically reduces the extrinsic base-collector junction capacitance, which enhances electron transport throughout the structure.

"By scaling the device vertically, we have reduced the distance that electrons have to travel," explained Feng's graduate student and co-author William Snodgrass. "Because the size of the collector has also been reduced laterally, the transistor can charge and discharge faster."

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