Joel Schulman, a principal research scientist at HRL Laboratories in Malibu, California, has developed a diode structure that uses an antimony (Sb) based heterostructure to oscillate at up to 110GHz.
Representatives of the International Electron Device Meeting have invited Schulman to present the work as part of the session on quantum electronics and compound semiconductor devices.
Diodes that oscillate at high speeds have potential applications in the field of millimetre wave detection. The technology has received a lot of interest recently as a possible means to improve surveillance and security techniques.
The HRL diode is made from an InAs/AlSb/GaAlSb material system. Sb offers faster electron transfer than gallium arsenide or indium phosphide (InP), but the material is also more expensive and harder to work with than these other semiconducting compounds.
The diode structure creates an asymmetric current flow, with low forward bias current and a large backward flow. The imbalance is a result of the InAs conduction band minimum that registers below the valance band maximum of GaAlSb.
Schulman and his team have developed a technique to control the current through quantum tunnelling.
"The interesting aspect of my work is the quantum tunnelling," said Schulman. "It provides a means of detecting millimetre wave radiation. The tunnelling produces a curved voltage/current line near zero voltage, which is a desired characteristic of millimetre wave detection."
The greater the curvature of the line near zero voltage, the better the diode will be at detection. Schulman has made a diode 2X2µm in area which has an "excellent" response at 110GHz. He is confident that, by shrinking the diode further, higher frequencies will be possible.
Qinetiq, the commercial spin-off company formerly part of the Defence Evaluation and Research Agency, is using Schulman's diodes in its development of milli-metre wave technologies designed to spot solid objects through clothing and walls. Millimetre wave imaging is also very effective in difficult conditions when there is a lot of smoke, dust or precipitation.
As well as Schulman's invited paper, the session features papers from the Simon Frazer University in Canada, NEC and NTT, all of which present bipolar transistors with frequencies greater than 300GHz. NTT has developed a double heterojunction bipolar transistor (DHBT) with a peak frequency of 341GHz at a current density of 833kA/cm2, a performance record.
To achieve this, the NTT team of Minoru Ida, Kenji Kurishima, Noriyuki Watanabe and Takatomo Enoki made a 150nm thick collector constructed using a step-graded process, with an InGaAsP base which provides a high level of thermal resistivity.
The authors conclude that the step-graded design makes full use of the superior electron transport properties of the compound semiconductor materials they used.
The team from Simon Frazer University examined the frequency roll-off DHBTs have at high current densities. The team concluded that frequency roll-off happens when the base-collector field collapses as a result of travelling electron space charge.
The NEC team, from the company's Photonic and Wireless Devices Research Laboratories, worked on collector design to improve DHBT frequency performance. Using InP, the team shifted the emphasis away from reducing the thickness of the collector in order to increase frequency.
The NEC team is confident that its approach can produce DHBTs with operating frequencies above 200GHz and maximum frequencies above 250GHz.