Polycrystalline boron-doped diamond with an oxygen-terminated surface channel as an electrolyte-solution-gate field-effect transistor for pH sensing

Polycrystalline Boron-doped Diamond Electrolyte-solution-gate Field-effect Transistor Applied to the Measurement of Water Percentage in Ethanol

“…The specification of the boron-doped diamond film of BDD SGFET used in this study was same as our previous study. 9 By an AC Hall measurement, the sheet carrier density, mobility, and sheet resistance of the BDD film before oxygen treatment were 3.8 × 10 13 /cm 2 , 8.5 cm 2 /Vs, and 22 kΩ/square, respectively. When the solvent in which the BDD-SGFET, the Si-ISFET, and glass electrode were immersed was varied from a pH 7 buffer solution to a 10% ethanol solution, the BDD-SGFET achieved good stability within less than 9 s, whereas the Si-ISFET and glass electrode required over 40 s and over 10 min, respectively.…”

“…9,10 A CVD (chemical vapor deposition)-synthesized polycrystalline diamond substrate with dimensions of 10 × 10 mm and 0.5 mm thickness was purchased from a Japanese company. The typical specification of diamond is described in our previous work.…”

“…Two Ti-Au contact pads (20 nm/100 nm) were deposited as the drain and source electrodes on the hydrogenterminated BDD surface, and were patterned by lift-off photolithography using the same process as in our previous work. 9,10 The contact pads were positioned to obtain a channel width and length of 0.4 and 9 mm, respectively, and electric wires were connected to the source/drain pads with a conductive adhesive for applying the external bias. Then, the source/drain electrodes were encapsulated with nonconductive epoxy resin in order to protect them from the solution so that only the area of the boron-doped diamond channel surface between the pads would be exposed to the solution.…”

“…The typical conditions were the same as described in our previous report. 9 For evaluating the BDD film, a Model Resi-Test 8300 AC Hall analyzer (Toyo Technica Co.) was used with the van der Pauw method. Two Ti-Au contact pads (20 nm/100 nm) were deposited as the drain and source electrodes on the hydrogenterminated BDD surface, and were patterned by lift-off photolithography using the same process as in our previous work.…”

“…Although Si-ISFETs with tantalum pentoxide (Ta2O5) and silicon nitride (Si3N4) show good response compared to the glass-type pH sensor, they still require over 30 s for obtaining a stable output. Recently, we have reported a no-gate-insulator electrolyte-solution-gate field-effect transistor (SGFET) with a single-crystal diamond surface-channel, [5][6][7][8] with a polycrystalline diamond surface-channel, and with a borondoped diamond surface-channel, [9][10][11] and have fabricated oxygenterminated and nitrogen-terminated diamond SGFETs with good pH sensitivity.…”