RF Power Amplifiers based on GaN HEMT Technology in the W-Band

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W-band frequency range systems such as high-speed wireless communication, remote sensors (radars) and millimetre-wave imaging require high RF power levels. The best output power available, using gallium arsenide (GaAs) and indium phosphide (InP) monolithic microwave integrated circuits (MMICs) are currently limited a few hundreds of milliwatts. Getting higher output power always desirable as the overall systems become smaller, so cheaper and lighter.Gallium nitride (GaN) is a very good candidate for high power applications, since GaN high electron mobility transistor (HEMT) has advantages of having high breakdown electric field 3.3x106 V/m ( compared to GaAs (4x105 V/m) and InP (4x105 V/m)), high electron saturation velocity 2.5x107 cm/s which is twice compared to GaAs and InP, and large energy bandgap GaN (3.4 eV) and AlN (6 eV) compared to GaAs (1.43 eV) and InP (1.34 eV) and so allows GaN devices to handle high internal electric field, high breakdown voltage provides higher output power per unit gate width. Also, GaN has a low dielectric constant (9.5) compared to GaAs (12.5) and InP (12.4) allows the device to have low capacitive loading, larger area for specified impedance and high RF currents. Good thermal conductivity of GaN (1.3 W/Kcm) allows the dissipated power to be easily extracted from the device if a silicon carbide (SiC) substrate is used. SiC has high thermal conductivity of (4 W/Kcm ) compared to GaAs (0.54 W/Kcm) and InP (0.67 W/Kcm).State of the art W-band GaN power amplifiers offer output powers of up to 3.1 W at 91 GHz using devices with gate lengths of 100 nm and which exhibit a power density 3.23 W/mm [1]. In other reported results, the device gate lengths are in the 80 - 100 nm range, with the power densities lying between 3.6 and 8.8 W/mm [2]-[3]. They all employ the AlGaN/GaN HEMT devices.Aim and objectivesThe aim of this PhD project is to design and build a GaN-based HEMT power amplifier that is operates at W-band (75-110 GHZ) with output power of over 5 W. The work entails the development of a W-band band device technology, i.e. sub-100nm gate technology, design of appropriate epitaxial layer designs either in the conventional AlGaN/GaN or less conventional AlN/GaN material systems, development of a low Ohmic contact resistance process, amplifier design, device and amplifier fabrication and characterisation.