Introduction to GaN technology

A Boston-based research firm Strategy Analytics forecast the market for Gallium Nitride (GaN) microelectronic devices would grow to approximately $375M by 2014. The 2011 International Microwave Symposium (ISM) this past June showcased several devices based on GaN technology, including the world’s smallest 5 W power amplifier MMIC solution based on a 28 V, 0.5 µm gate length GaN HEMT. In addition, backroom workshops included sessions entitled “High-Power, High-Efficiency GaN Power Amplifiers”, “Innovative GaN Power Amplifiers” and “GaN (and LDMOS) Linear Power Amplifiers,” confirming that GaN-based technology is, indeed, a hot topic.

What is Gallium Nitride (GaN)?

GaN is a binary III-V direct bandgap semiconductor commonly used in LEDs since the 1990s. Its wide-band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. Because GaN offers very high breakdown voltages, high electron mobility, and saturation velocity it is also an ideal candidate for high-power and high-temperature microwave applications like RF power amplifiers at microwave frequencies and high-voltage switching devices for power grids. Solutions that use GaN-based RF transistors are also replacing the magnetrons used in microwave ovens.

GaN transistor models have evolved from GaAs (gallium arsenide) transistor models; however there are many advantages GaN offers:

• Higher operating voltage (over 100-V breakdown)
• Higher operating temperature (over 150°C channel temperature)
• Higher power density (5 to 30 W/mm)
• Durable and crack-resistant material

GaN devices are often grown on SiC (silicon carbide) substrates, but to achieve lower-cost GaN devices, they can be grown on sapphire and silicon wafers. GaN’s wide bandgap allows for higher breakdown voltages and operation at high temperatures. The high thermal conductivity of SiC makes it a better substrate than silicon for power amplifier applications that require good heat sinking.

Commercial Applications

GaN-based devices began to surface in commercial applications about a year ago. They are used for low-frequency L, C and S-band RF applications like cable TV and power management. Higher frequency applications are in development.

Initially, it seemed GaN-based devices would be affordable only for military applications, such as the development of electronic warfare, radar, and high security communications systems. However, material maturity, improvement in yield, expansion to 4” wafers and lower-cost substrates have reduced GaN-based device costs and therefore an economical option for commercial applications as well.

Some companies using GaN technology today are Cree, Avago, Efficient Power Conversion (EPC), Toshiba, and RF Micro Devices.

GaN Products

One example of a GaN-based device is Cree’s 2-W, 20 to 6,000-MHz CMPA0060002F GaN High Electron Mobility Transistor (HEMT) based MMIC. This MMIC employs a distributed (traveling-wave) amplifier design approach, which allows extremely wide bandwidths to be achieved in a small footprint, screw-down package (total product size 0.5 x 0.5 in) featuring a copper-tungsten heat sink. This device is suitable for applications like ultra broadband amplifiers, fiber drivers, and test instrumentation.

The part typically provides 17 dB of small signal gain and 2-W saturated output power with an associated power added efficiency of more than 20 percent. The wideband amplifier’s input and output are internally matched to 50 Ω. Additional specs include a 3-W typical P_SAT, and an operation up to 28 V.

Avago Technologies’ ATF-551M4 is a high dynamic range, super low noise, single supply E pHEMT GaAs FET housed in a thin miniature leadless package. The combination of small device size (1.4 x 1.2 x 0.7 mm), super low noise (under 1 dB Fmin from 2 to 6 GHz), high linearity and low power makes the ATF-551M4 ideal for LNA or hybrid module designs in wireless receivers in the 450 MHz to 10 GHz frequency band. Applications include Cellular/PCS/ WCDMA handsets and data modem cards, fixed wireless infrastructure in the 2.4, 3.5 GHz and UNII frequency bands, as well as 2.4 GHz 802.11b, 5 GHz 802.11a and HIPERLAN/2 Wireless LAN PC-cards. Additional specs include 24.1 dBm output 3rd order intercept, 14.6 dBm output power at 1-dB gain compression, and 17.5-dB associated gain.

EPC is the first to introduce enhancement-mode gallium-nitride-on-silicon transistors (eGaN) for applications in servers, netbooks, notebooks, cell phones, base stations, flat-panel displays, and Class D audio amplifiers.

GaN Foundry Services

The increase in GaN devices also brings additional competition in GaN foundry services. At IMS2011, RF Micro Devices announced the qualification of their GaN power semiconductor process for 65 V and its availability to foundry customers through RFMD’s Foundry Services business unit. Previously, the company’s GaN1 power semiconductor process technology had been qualified for 48-V operation. The increase in operating voltage to 65 V enables miniature, 0.5-kW power devices with high operating efficiency for L- and S-Band military and civilian radar applications.