Karami, Kaivan (2023) GaN HEMT technology for W-band frequency applications. PhD thesis, University of Glasgow.
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Abstract
Owing to the technological advancement, and rapid industrial growth which stimulate a rapidly growing demand for more efficient transistor devices for high-power high-frequency applications, gallium nitride (GaN) material has been one of the most intensively researched semiconductor materials in the past two decades. Apart from its exceptional material properties, GaN exhibits a unique attribute of an ability to generate a sheet of high density highly mobile two dimensional electron gas (2DEG) without the need of any intentional doping. Due to the high mobility (~2000 cm2/V.s) and concentration (1X1013 cm-2) of the 2DEG, and the large energy band gap (3.4 eV), gallium nitride devices can efficiently deliver high-power at high operating frequencies. Its low intrinsic carrier density (~10-12/cm3) enables operating GaN devices at much high operating temperatures than other conventional semiconductor materials.
Despite this propound potential, due to some critical performance and reliability challenges, GaN transistors are yet to meet an acceptable industrial performance which leads to still limited deployment in the semiconductor market for electronic applications. The focus of this project is to improve in the device processing technology which has been one of the major causes of poor performance of GaN devices in high-power high-frequency applications. GaN devices suffer from high ohmic contact resistance, gate-to-source capacitance CGS, and inefficient heat dissipation property which severely results in high power losses, low efficiency, and low cutoff frequency. These affects the output power and high-frequency parameters (such as the unit power gain fmax and unit current gain ft cut-off frequencies). The conventional method of realising low ohmic contact using heavily doped GaN contact layer requires complex and time-consuming regrowth processes. In this work, we present a new approach of realising low ohmic contacts using the heavily doped GaN cap layer technique, but without regrowth. Instead of using the usual undoped 2 nm GaN cap layer, the approach involves growing a heavily doped 5 nm GaN cap layer with a Si-doping density of 1x1019 cm3 on the AlN/GaN HEMT using molecular beam epitaxy (MBE). This technique is cost effective and minimises complexity and processing time. We obtain a very low ohmic contact resistance of 0.132 Ω.mm with 428 Ω/sq sheet resistance, for AlN (aluminium nitride) barrier GaN high electron mobility transistors (HEMTs).
Reduction of gate length is required to realise a high-frequency device. However, such reduction results in high gate resistance which affects the maximum cut-off frequency of the device. A T-shape structure of gate is normally used to reduce the gate resistance. Because of the need of very small gate lengths in high-frequency devices, any further reduction of the gate length to sub-100 nm, could lead to a severe instability due to weakening mechanical strength of the gate structure. This has become a serious reliability concern, and consequently the T-shape gate is conventionally supported using thick passivation layer of dielectric materials such as Si3N4. This layer in turn results in an unwanted parasitic capacitance which affect the frequency performance of the device. In this work, we present a new fabrication technique which yields a robust and stable T-shape gate structure without the use of any supporting insulator such as Si3N4. While this approach has not been tested on a full wafer (>4 inches) yet, it shows promising potential for using it in commercial manufacturing.
In another strand of the research, we have demonstrated the benefit of AlGaN/GaN HEMTs on diamond as efficient heat extraction mechanism for GaN devices by using three identical AlGaN/GaN on diamond wafers with varying thicknesses of GaN buffer and the diamond substrates. Due to the efficient heat extraction property, a transistor with high power density, effective unity power gain and current-gain cut-off frequencies of 32.04 W/mm at VGS = 0 V and VDS = 60 V, 90 GHz and 128 GHz are realised, respectively. We analysed the impact of the buffer and substrate thicknesses and found that self-heating of the device is less in devices with thinner diamond substrate and even lesser when the buffer is thinner.
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