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INDUSTRY ENGINEERED SUBSTRATES


Figure 2. GaN-on-diamond structures are formed by: bonding the GaN face to a temporary carrier; etching away the substrate and transition layers; depositing a 35 nm-thick dielectric and then a diamond layer on the backside of GaN; and removing the temporary carrier.


that the GaN-on-diamond originally comes from a silicon substrate; and the second compares device performance to the GaN-on-SiC HEMT, which is the industry’s prevailing GaN technology.


Recently, engineers at the US Air Force Research Laboratory (AFRL) have independently investigated whether the team’s GaN epi-flip and diamond deposition process is detrimental to GaN epitaxy, and whether it can lead to any deterioration in device performance. Their examination involved the analysis of thousands of GaN-on-diamond and GaN-on-silicon HEMTs. To make the comparison as fair as possible, the GaN- on-silicon was grown at the same time as the GaN that was to be transferred to diamond. Dimensions of the HEMTs (identical on both silicon and diamond substrates) included a gate width of 300 mm (2 x 150 mm), a gate length of 0.15 mm and source-drain distance of 4.5 mm.


Epiwafers, plus devices formed from them, were scrutinised with a wide range of measurements. Entire wafers were mapped for sheet resistivity, carrier mobility, carrier density, contact resistance, sheet resistance and buffer isolation current, while measurements on passivated devices assessed values for transconductance, maximum DC drain current, saturated DC drain- source current, threshold voltage, gate leakage and knee voltage. On top of this, engineers obtained passivated RF data for the devices, including values for breakdown voltage, fmax


(MAG), ft ,


current-voltage and transfer curves. Aside from one exception − surface gate leakage − no statistically significant differences were uncovered between DC


Table 2. A summary of average (and standard deviation) DC and RF measurements made by AFRL on identically designed GaN-on-diamond and GaN-on-silicon HEMTs.


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and RF measurements for GaN-on-silicon and GaN-on-diamond (see table 2 for a summary of the results).


RF performance of both types of device was then assessed with continuous-wave Maury load pull measurements at X-band (10 GHz) frequencies and various drain voltages. Select devices were matched for best power-added efficiency and biased at a quiescent drain current of 30 mA, corresponding to 100 mA/mm, in a class AB configuration. Measurements on wafers held at 25 °C on a vacuum chuck using drain voltages between 15 V and 25 V revealed that switching the foundation from silicon to diamond boosted output power by typically 1-1.5dBm and increased power-added efficiency by 7 percentage points (see Figure 3).


Engineers at AFRL also compared the current droop in both types of HEMTs.


This study revealed that GaN-on-silicon HEMTs are more sensitive to pulse lengths than GaN-on-diamond HEMTs (see Figure 4), due to increased self-heating.


A combination of infrared thermography and micro-Raman techniques unveiled the thermal performance of the GaN- on-diamond and GaN-on-silicon HEMTs. Due to a ‘spot-size’ larger than that of the entire transistor, infrared measurements were only qualitative. These measurements made on-wafer, un-attached to a stage, involved devices operating at a drain-voltage of 25 V and drain current of 130 mA. Values for thermal resistance − the difference between the observed region’s hottest temperature and that at the base of the substrate, divided by the product of the drain voltage and current – were just 7.44 K W-1


mm-1 for GaN-on-diamond


HEMTs, compared with 16.6 K W-1 and 11.5 K W-1


mm-1 mm-1 for those with silicon


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