Very low subthreshold swing normally-off diamond FET and its logic inverters

A normally-off field effect transistor with 15 nm LaB₆ gate material directly deposited on hydrogen-terminated diamond surface has been fabricated and characterized. The reason for the normally-off operation could be attributed to the low work function of LaB₆ layer, which can deplete holes in the channel. Due to the formation of C-B bond of interface and the thin thickness of LaB₆, the subthreshold swing is as low as 70.6 mV/dec and interface state density is as low as 2 × 10¹¹ cm⁻²eV⁻¹. The maximum drain current density (I_DSmax), extrinsic transconductance (G_m), threshold voltage (V_TH), and on/off ratio for the device with 8-μm gate length are −60 mA/mm, 5 mS/mm, −1.1 V, and 10⁹, respectively. The low-field mobility (μ₀) without vertical-field degradation is 187 cm²/V s. The trapped charge density and fixed charge density are 8.75 × 10¹¹ and 1.26 × 10¹² cm⁻², respectively. The LaB₆/H-diamond logic inverters coupling with different load resistors show distinct inversion characteristics with a peak gain of 4.7 V/V.