Systematic studies have been performed on the sensitivity of GaN HEMT (high electron mobility transistor) sensors using various gate electrode designs and operational parameters. The results here show that a higher sensitivity can be achieved with a larger W/L ratio (W = gate width, L = gate length) at a given D (D = source-drain distance), and multi-finger gate electrodes offer a higher sensitivity than a one-finger gate electrode. In terms of operating conditions, sensor sensitivity is strongly dependent on transconductance of the sensor. The highest sensitivity can be achieved at the gate voltage where the slope of the transconductance curve is the largest.
While GaN-based microchemical sensors have shown very promising performance characteristics, there has not been much understanding on how sensor sensitivity can be engineered or improved. This work provides critical information about how the gate electrode of a GaN HEMT, which has been identified as the most sensitive among GaN microsensors, needs to be designed, and what operation parameters should be used for high sensitivity detection.
The figure shows Ids (source-drain current) response to SF6 exposures measured using the GaN HEMT sensors fabricated with W = 5, 10, 25, and 50 μm at L = 2 μm. The sensors clearly demonstrate a higher sensitivity with an increasing gate width. Ids response measured using GaN HEMT sensors fabricated with L = 2, 4, and 8 μm at W = 50 μm and with L = 2 μm at W = 25 μm; (in these sensors the sourcedrain distance is DS = L + 4 μm) show decreasing sensitivity with an increasing gate length.
Comparison between L4W50, L8W50 and L2W25 sensors, which correspond to DS8W50, DS12W50, and DS6W50 respectively, indicates that the sensor sensitivity is not simply proportional to Ids or W/L (or W/DS). The higher sensitivity achieved with the L2W25 sensor compared to the L4W50 sensor indicates that the shorter gate length plays a significant role. The results shown here suggest that sensor sensitivity is not simply proportional to the size of the gate electrode or the amount of Ids of the sensor, and that a short gate length and a source-drain distance are important factors in determining the sensitivity of the sensor.
The robust, high-sensitivity GaN HEMT chemical sensors can be applied to NASA missions including in-situ detection of signatures of extraterrestrial life and in-situ planetary atmosphere analysis during planetary exploration. The sensors can be also used for health and habitat environmental monitoring for astronauts during manned missions. Due to the high thermal and chemical stability and excellent radiation hardness, GaN HEMT sensors can operate in all planetary conditions, which has not been possible for the conventional Si-based sensors. This innovation can offer a key capability for future NASA missions in extreme environments including Venus surface missions and Europa and Titan flagship missions. These micro GaN chemical sensors will be beneficial for Mars astrobiology field labs, comet nucleus sample returns, Titan in-situ, Europa surface and sub-surface, and giant planet deep probe missions.