Shifting boundaries

8 November 2012

US researchers report new RF power limiters based on III-Nitride materials that have unprecedented high operating voltages, excellent temperature stability and radiation hardness. Fully planar and simple to fabricate, these limiters could be used in nearly any type of microwave integrated circuit.

Sensitive circuits

Microwave/RF power limiters (PLs) are crucial to applications that have to protect sensitive components from high input power levels, e.g. low-noise receivers, high-sensitivity measurement equipment and transmit-receiver systems like radar. Increasing dynamic range in the power levels of newer systems makes their role even more important and difficult.

PLs are subject to conflicting demands, including low loss at low power levels (below the limitation threshold) whilst handling high power levels and ensuring fast and abrupt power limitation. They must also work in broad temperature and frequency ranges and be robust and reliable.

Simultaneously meet these requirements is a challenge for commercial PLs. These are generally fabricated using Si or GaAs Schottky or pin diodes, and so cannot withstand high powers due to low breakdown fields in Si and GaAs.  Pin diodes are relatively slow and require large active areas with high associated capacitances and significant losses. Their parameters are also strongly temperature dependent. Similar limitations apply to PLs based on silicon MOSFETs or GaAs HEMTs.

Material advantages

The PLs presented in this issue by the team from the University of South Carolina (USC), SETI Inc. and Rensselaer Polytechnic Institute uses III-Nitride materials to allow much higher operating voltages. They also use a novel two-terminal device design that has voltage-dependent capacitance, which they refer to as a capacitively coupled contact (C3) varactor. The resulting device is compact, with channel length less than half that required for a FET-based design. The devices also have advantages in fabrication with no post-growth high-temperature anneal and by eliminating gate deposition and alignment - one of the most challenging operations in FET technology.

The PLs are formed on AlGaN/GaN heterostructures. The interface of the two materials produces a very high conductivity channel referred to as a two-dimensional electron gas (2DEG) – a very thin layer with extremely high electron concentration formed at the interface between materials with different bandgaps.  

The record high-density 2DEG in the AlGaN/GaN structures provides very high conductivity giving a very small resistance between the C3 varactor electrodes. This reduces losses; where a typical PL loss may be over 1dB at frequencies above 5 GHz, the C3 design loss is as low as 0.2 dB.

The 2DEG channel was key to the success of the devices but harnessing its properties was problematic, team member Professor Grigory Simin explained, “We discovered that AlGaN/GaN, and similar heterostructures with very thin barrier layers and highly-conducting channels enable efficient RF signal injection without ohmic contacts, via strong capacitive coupling between the C3 contacts and the 2D-channel. However, transmission line effects caused by the distributive capacitance and resistance of the channel interfered with RF signal injection. Device operation was also strongly affected by the surface states and defects in the semiconductor material.”

With support from the US Office of Naval Research monitored by Dr. Paul Maki, the team overcame these difficulties using modelling and simulations to optimise the device to obtain ‘nearly ideal’ C3 electrodes.

Beyond limiters

The approach developed for C3 varactors also allowed the team to demonstrate low-loss, high-power RF switches operating in a broad frequency range (2 – 20 GHz). These switches could be used in phase shifters, filter banks and other RF systems. Fully planar and easy to fabricate, these devices can be incorporated into nearly any type of microwave IC. The team envision numerous applications in control electronics for radar, wireless networks, collision avoidance systems, medicine, and terahertz electronics.

The team is now working on C3 devices using AlInN/GaN heterostructures, offering even higher conductivity and lower loss. They are also developing patent-pending “frequency configurable electronics” technology using low-conducting layers to control electric field distribution and surface charge in high-power III-Nitride based RF microwave devices. Preliminary results demonstrate dramatic improvements in the device breakdown voltages and off-state capacitance.

The USC/SETI/Rensselaer partnership is also working on unique deep UV light sources and their applications. Professor Michael Shur said, “In the past year, we have improved their efficiency by an order of magnitude. We believe this work will have a dramatic impact on health, security, and food supply in the US and worldwide.” Simin added , “Our team is also developing III-Nitride devices for power electronics applications, including convertors, with potential to dramatically increase efficiency in the power grid and reduce energy losses and fossil fuel consumption.”     

Further reading

This article is based on the Letter: RF Power Limiter using Capacitively-Coupled Contacts lll-Nitride Varactor (new window).

A PDF version (new window) of this feature article is also available.

http://www.s-et.com/ (new window)

http://www.ee.sc.edu/people/faculty/?id=simin (new window)

Journal content

Cover of Electronics Letters, volume 50, issue 21

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