The Theoretical And Experimental Study For 4H-SiC Buried-channel MOSFET

4H-and 6H-SiC are attractive semiconductor materials for high-power and hightemperature electron devices because they have excellent physical properties such as awide bandgap, high breakdown voltage, high thermal conductivity, and high saturationelectron drift velocity. In addition, SiC has the advantage of a thermally grown nativeoxide, which provides the opportunity for its application in metal-oxide-semiconductor(MOS) devices. The bulk mobility of 4H-SiC is approximately twofold higher than thatof 6H-SiC. However, the reported surface channel mobility of 4H-SiC MOSFET isextremely low. It is believed that the high density traps at 2.9eV above the valence bandedge at the SiO₂/4H-SiC interface lower the surface channel mobility. One possiblesolution to raise the channel mobility is to keep the channel electrons away from theSiO2/4H-SiC interface. So, buried-channel(BC) MOSFET become promising devicestructure. The main studies and contributions of this dissertation are as follows.(1) The operational modes of BC-MOSFET are studied comprehensively. The surfaceof semiconductor can be in surface depletion, surface accumulation and flatband statesat different gate bias. Different surface states also make the BC MOSFETs operated inseveral modes: surface channel mode, buried-channel mode and surface-buried channelmixed mode. The one-dimension Poisson equation is solved to analyze electricalcharacteristics in surface depletion region for buried-channel MOS structure. The effectof incomplete ionization is also included. The analytical results show that incompleteionization almost has no any effect in depletion duo to field-dependent ionization, butthis effect in channel is obviously.(2) The distortion of C-V characteristics of SiC buried-channel MOS structure isinvestigated in chapter 3. It becomes more complex and difficult to characterize gatecapacitance because there is a pn junction in buried-channel MOSFET. The pn junctionhas no function when buried-channel does not pinch-off, but the total surfacecapacitance is the series of surface depletion region capacitance and pn junctioncapacitance when the channel is pinched off by he surface depletion region and n-sidespace-charge region of pn junction This makes gate capacitance decrease quickly, andthe distortion of C-V characteristics happens. The analytical expression of gatecapacitance in pinch-off mode is proposed by solving Poisson equation. A fundamentalphysical understanding of C-V characteristics in pinch-off mode is analyzed and alsothis model is verified by experimental results. The parameter extraction method usingC-V characteristic is discussed.(3) The I-V characteristics for SiC buried-channel MOSFETs based on an averagemobility model are presented. The drain current model proposed in this dissertation issimplified by an average capacitance and a simple expression of channel averagemobility with gate voltage. Three different operational modes are included in thecalculation of drain current, and the expressions of drain current in various modes arederived. A good agreement is obtained between calculated and measured result. Thesubthreshold characteristics for SiC Buried-Channel MOSFETs are studied based on anequivalent channel thickness model. The expression of equivalent channel thickness isdeveloped with the solution of Poisson's equation firstly, and the results show that theequivalent channel thickness is independent of gate voltage if N D+ NA? ≤43. The peakpotential in the subthreshold region varies with gate voltage linearly, and a simpleexpression of drain current in subthreshold region is obtained, in which the effect ofinterface state is included. The expression can be used to extract some importantparameters.(4) Buried-channel MOSFETs with a thermally grown gate oxide on 4H-SiC isfirstly fabricated successfully in our country. The experiment process and deviceparameters are designed based on 4H-SiC material. A layout for 4H-SiC BC-MOSFETwith implanted channel is designed. The depth and doping concentration of implantedchannel are designed with depletion approximation. Channel depth has amaximum x i,maxand a minimum x i,min for playing a good role and assuring normal work.Pearson IV equations are used to design the implanted dose and energy. Thedependences of oxide velocity in dry, wet, NO and N2O on temperature are discussed.The research status of ohmic contact for SiC is reviewed, and a measurement methodfor specific ohmic contact resistance is presented.(5) The devices fabricated by the designed process are measured. C-Vcharacteristics are used to extract device parameters: doping concentration in channel,channel depth, oxide thickness and flatband voltage. The threshold voltage is obtainedby transfer characteristics, and its temperature dependence is measured and analyzed.I-V characteristics at room temperature and 120℃are measured. Most devices havegood linear and saturation characteristics;Drain current and transconductance at 120℃is greater than the value at room temperature;The maximum effective mobilities forsample 1 and sample 2 are about 40 and 90cm2/Vs;the effects of series resistance andinterface states on mobility are discussed, and the effective mobility without the effectof series resistance and interface states for sample 1 can reach 77cm2/VsThe obtained effective mobility indicates the superiority of SiC BC-MOSFET,which shows the feasibility of improving mobility with SiC BC-MOSFET structure. Butgreat difference between experimental and expected mobility is existent. So the furthermodificative scheme is brought up based the measurement and analysis.