Gallium Nitride

Gallium Nitride devices have potential to realise high power, high temperature and high frequency applications for the following reasons: Band diagram for AIGaN/GaN hetero-structure

- Wide bandgap material - leading to high breakdown voltages
- Ability to form heterojunctions with wider bandgap materials such as AIN and AIGaN
- High 2-Dimensional-Electron-Gas (2DEG) concentration at heterojunction interfaces
- Mobility of 1500-2000cm2/VS in 2DEG channel

Power device figure of merit between GaN, GaAs and SiThese devices are expected to contribute significantly towards efficiency improvement and downsizing of power supplies since the devices have the potential for realising higher breakdown voltages and lower on-state resistances in comparison to silicon-based devices conventionally used (a projected 100x performance advantage), and to provide unprecedented microwave power amplification (>10x performance advantage over GaAs-based counterparts).

Enhancement-Mode AIGaN/GaN devices

Operation principle for E-mode devicesMost of the development to date in GaN-based HEMTs has been focused on depletion-mode (D-Mode) AIGaN/GaN devices which operate at negative gate threshold voltages.

Applications for E-mode devices include:
- Electrical inverters
- Switched mode power supplies
- Motor drive circuits
- Logic circuits
We are currently working on a patent for a method developed at Glasgow in realising E-mode devices.

Thermal Management

Gallium Nitride is grown on foreign substrates, most commonly Silicon Carbide and Sapphire.

Silicon Carbide has higher thermal conductivity than Sapphire (4W/cmºC compared to 0.45W/cmºC) which means it removes heat almost 10 times more efficiently. Devices fabricated using SiC as the substrate are therefore a lot more efficient.

The downside to using SiC as the substrate is that it is about 10 times more expensive than using Sapphire. Therefore, it would be desirable to find a way in which devices fabricated using Sapphire could be made to be as efficient as ones using SiC.

At Glasgow, we are currently developing a method which will hopefully make this possible.


Cross section of AIN/GaN MOS-HEMTPotential to create higher power amplifiers than AIGaN/GaN HEMTs due to:
- Higher sheet carrier concentration
- Higher electric field breakdown voltage
This technology has been of limited use due to high contact resistance (>1Ω.mm) and high leakage currents and currently there are no reported power amplifiers using this material. At Glasgow, AIN/GaN MOS-HEMTs have been developed which display very low contact resistance (lowest to date) and low leakage current. RF devices have been fabricated with ft= 80GHz and fmax= 65GHz.

SEM micrograph of fabricated AIN/GaN HEMT     RF characteristics of fabricated device

Power Amplifier Design

Work at Glasgow is focussed on developing high efficiency power amplifiers operating in the X-band (i.e. 8-12 GHz) which can deliver 10W of power, using AIN/GaN MOS-HEMTs and conventional AIGaN/GaN epi-layers.

High efficiency is very desirable as it saves on battery lifetime in wireless devices, and reduces overall operational costs. X-band amplifiers reported on GaN have efficiencies of <50% due to their design configuration.

Work includes:
- DC and RF characterisation of HEMTs
- HEMT modelling
- Circuit design
- Fabrication of monolithic microwave integrated circuits (MMICs)