Charge control devices based on wide band gap semiconductors

Abstract

Metal-Semiconductor Field Effect Transistors (MESFETs) based on Hydrogen Terminated Diamond have been fabricated according to different layouts and technologies with different gate lengths. Aluminum self-aligned and fixed drain-source distance (in the following “standard”) gates and gold ohmic contacts have been fabricated both on single crystal and polycrystalline diamonds supplied by Element Six and by Russian Academy of Sciences. Aluminum assures low ideality factor gate-source diodes, high rectification ratio and low leakage currents (basically higher for polycrystalline samples). Gold contacts, after a hard work of development and improvement, resulted in good mechanical adhesion and strength as well as contact resistivity, lower enough to allow an excellent device operation. The small LT resulting from transfer length method (TLM) analysis, allows the supposition of a completely lateral charge transfer and hydrogen terminated area dimension reduction close to gate junction does not negatively affect device performances. Hydrogen terminated layer has been deeply investigated by means of Hall bars, TLMs and gate junctions characterization: hydrogen induced two dimensional hole gas (2DHG) results in sheet resistances essentially stable and repeatable depending on substrate quality. Results achieved for self-aligned and standard FETs fabricated on nominally identical large-grain size, low quality, polycrystalline diamonds TM180 by Element Six are presented and investigated in order to obtain a better understanding of hydrogen terminated diamond transistors. DC characteristics clearly point out the presence of 2DHG as well as an operation typical of hetero-junction FETs. Anyway, achieved results seem to highlight the presence of an effective insulating layer at the aluminum-diamond interface. Self-aligned 200 nm gate length FETs show current density and transcondutance values of 100 mA/mm and 40 mS/mm, respectively. Higher values have been obtained on higher quality samples by Russian Academy of Sciences, with an IDSmax of 120 mA/mm, stable up to a VDS = -80 V (corresponding to an applied electric field EA = 2 MV/cm) and a maximum transconductance exceeding 70 mS/mm. It is worth to notice as all devices fabricated in this thesis are not affected by self-heating effect.RF small signal performances of fabricated FETs have been investigated for both self-aligned and standard devices fabricated on Element Six TM180 diamond plates. Established technology allows the fabrication of transistors with operation frequencies always in the X and C band of the radio spectrum, even if based on low quality substrates. Self-aligned 200 nm gate length FETs fabricated on TM180 achieved a maximum oscillation frequency fMAX = 15 GHz and a current gain cut-off frequency fT = 6 GHz resulting in a power output at 1 GHz of 800 mW/mm that is, up to now, the best result in literature for polycrystalline diamond based transistors. From S-parameters analysis, it has been possible to extract a FET small signal model. Best results in terms of operation frequencies have been obtained, also in this case, for single crystal and polycrystalline diamond plates supplied by Russian Academy of Sciences. Single crystal plate achieved a fMAX = 27 GHz and a fT = 14 GHz when VDS = -15 V and VGS = -0.2 V and polycrystalline diamond exceed 30 GHz for maximum oscillation frequency while 9 GHz have been achieved for fT. Completely satisfying reasons for better performances, in terms of fMAX, obtained for polycrystalline diamond have not been found, probably such a trend is due to a sort of mobility increase owing to grain boundaries and bulk interface (bulk mobility in diamond is 1600 cm2/Vs). Results of this thesis have pointed out the supremacy of the self-aligned gate structure and the development of an RF optimized layout specifically for diamond results in a DC characteristics improvement and a fMAX/fT ratio reduction that will lead to better RF performances. Quality of substrate clearly influence the hydrogen termination and so, in terms of FETs operation.