Fabrication of 2DHG Multi-Finger Diamond Vertical MOSFETs and I-V and RF Characteristics

We fabricated 2DHG multi-finger diamond vertical MOSFETs and measured DC and RF characteristics for the first time. Diamond has a wide bandgap (5.5 eV), high thermal conductivity (22 W/cmK), and high breakdown field(10 MV/cm). Therefore, diamond is a promising semiconductor material for use in High-Power RF amplifier[1-2]. The vertical multi-finger structure is suitable for both higher breakdown voltage and integration, and it is necessary to increase RF<br/>output power density.<br/>The fabrication process is as follows. The device was fabricated on a (001)-oriented p + diamond substrate. 0.5 μm undoped layer and 1.0 μm nitrogen-doped layers were deposited on the substrate by microwave plasma vapor deposition(MPCVD) to block the vertical leakage current. Next, the<br/>depth of 3.0 μm gate trench was formed by inductively coupled plasma reactive ion etching(ICP-RIE). After gate trench etching, 200 nm regrown undoped diamond layer([N]:&lt; 1×10 16 cm -3 ,[B]:&lt;3×10 14 cm -3 ) was deposited by MPCVD. Source trench was etched to reach p+ substrate by ICP-RIE.<br/>After that, source and drain electrode (Ti/Pt/Au: 50/50/200 nm) was deposited and annealed at 600 to form TiC for ohmic contact. After Hydrogen-termination and Oxygen-termination for defining the channel region, 100 nm of Al 2 O 3 was deposited as the gate insulator by atomic layer deposition at 450 using H 2 O as an oxidant. Finally, 300nm of Al was deposited on the entire gate trench as the<br/>gate electrode.<br/>In this device, the whole p + diamond substrate operates as source. 2DHG is induced on both the surface and sidewall of H-terminated regrown layer. The hole current passes through the regrown layer from p + diamond substrate, and then flows up along the H-terminated regrown layer on gate trench sidewall toward the drain electrode on the substrate surface. The total gate width(W GT ) is<br/>defined by gate width of each finger(W GU ) and the number of fingers (N f ) as W GT = W GU ×N f ×2, since there exist two current paths from source to drain at each finger.<br/>In DC measurement, maximum drain current density(I D,max ) of 297 mA/mm at V DS = -40 V and V GS = -30V was obtained from the device with W GT = 50×5×2=500 μm, trench width (W T ) = 8 μm, and gate length(L G ) = 3.7 μm. Another device with W GT = 100×5×2=1,000 μm, W T = 8 μm, and L G = 3.7<br/>μm showed the results of I D,max = 289 mA/mm (=289 mA as actual current) at V DS = -40 V and V GS = -30V. Also, I D,max = 250 mA/mm (=500 mA as actual current) were measured from the device with W GT = 100×10×2= 2,000 μm, W T = 8 μm and L G = 3.7 μm. Therefore, the actual output current increased almost proportionally to the expansion of the gate width by increasing N f .<br/>In RF measurement without deembedding, f max of 0.88 GHz was obtained at V DS = -40 V and V GS =-30V from the device with W GT = 50×5×2= 500 μm, W T = 2 μm and L G = 3.7 μm. After gate redeposition for more suitable gate length, f max of 1.3 GHz was obtained at V DS = -40 V and V GS = -30V from the device with W GT = 25×5×2= 250 μm W T = 4 μm and L G ≒ 2 μm. The relatively low<br/>fmax is caused by parasitic capacitance C DS between the bottom source and the upper drain. It can be decreased by avoiding the overlapping of the two layers.<br/>In this work, we fabricated 2DHG multi-finger diamond vertical MOSFETs for the first time.<br/>I D,max of 297 mA/mm and f max of 1.3 GHz without emdedding were obtained from the devices. The device structure demonstrated in this work shows the possibility of efficient integration and gate width expansion as RF device, which is the advantage of diamond’s high thermal conductivity compared with GaN.<br/><br/>References<br/>[1] K. Kudara, H. Kawarada et al., Carbon, vol. 188, pp. 220-228.(2022)<br/>[2] C. Yu, Z. Feng et al., Functional Diamond, 2(1), pp. 64-70.(2022)