The Fabrication And Characterization Of A New Vertical 4H-SiC Photoconductive Switch

Based on the development and application of silicon carbide photoconductive switches,this paper introduces the advantages and disadvantages of vertical silicon carbide photoconductive switches.Aiming to improve the low efficiency of the laser utilization and the photoconductivity of the device,several optimizations has been performed on the device structure and substrate.Firstly,the comparison experiments were designed to verify the influence of the internal light intensity distribution on the photoconductive performance of the traditional vertical circular electrode silicon carbide photoconductive switch.In view of the shortcomings of vertical photoconductive switches,four vertical silicon carbide switching devices with progressively new structures were designed by intronducing an Al-doped Zn O(AZO)transparent conductive electrode and a silver mirror electrode.And several sample devices were fabricated using 4H-Si C a-plane slices with different vanadium doping concentrations or slice thicknesses as substrate materials.It is confirmed that the structure with AZO transparent electrode,the AZO sub-contact layer and the silver mirror electrode is the best device structure.The 4H-Si C a plane slice with a vanadium doping concentration of 2.4×10·16 at/cm3 and a thickness of 0.2 mm is confirmed to be the best substrate.The device sample fabricated by this has the best performance and it has an on-state resistance and conductance of 7.14 Ω and 0.14 Ω-1 at laser energy of 20 m J,respectively.By using the no-load test circuit to detect the on-state performance of the switching device,different structure switching devices with different device structures,different V-vanadium doping concentrations,and different sample thickness Si C substrates were prepared.Test result device.It is confirmed that the structure having the AZO transparent electrode,the AZO sub-contact layer and the combined silver mirror electrode is the optimum device structure;the 4H-Si C a surface having a V doping concentration of 2.4×10·16 at/cm3 and a thickness of 0.2 mm is confirmed.The wafer is the optimal substrate for the fabrication of the switch;the new vertical Si C-based switching device thus fabricated has the best conduction performance.Under the laser light intensity of 20 m J,the on-resistance and conduction conductance of the switching device are 7.14 Ω and 0.14 Ω-1,respectively,and the sample has the best conduction performance at 20 m J excitation light intensity.The on-resistance and conduction conductance of the device are 7.14 Ω and 0.14 Ω-1,respectively.Then,the performance of the new photoconductive switch sample under different bias voltages and excitation light intensity are also been characterized by the circuit without a load.The test result shows that the on-state performance(resistance,conductance,and peak current)increases as both the test bias and the excitation laser energy.The conductance of the device increases linearly with the laser energy at first,and then get saturated gradually.The device achieves its best on-state resistance and conductance of 11.34 Ω and 0.088 Ω-1 respectively at 1000 V and 8.76 m J.Considering the influence of the existence of parasitic effects on the performance of the device in the no-load circuit test,this paper quantifies the relationship between the test resistance value of the no-load circuit and the parasitic effect in the circuit by establishing a theoretical equivalent circuit.Utilizing the advantages of the new photoconductive switch,the area of laser exciting is set as a variable in the test,which is called the ‘small holes’ method in this paper.By the ‘small holes’ method,the accurate photoconductive performance of the device are characterized.The test result shows that the on-state performance(resistance,conductance,and peak current)increases as both the test bias and the excitation laser energy and the conductance of the device increases linearly with the laser energy.The device achieves its best on-state conductance,conductivity and resistance of 0.16Ω·m,6.26(Ω·m)-1 and 1.71Ω,at bias of 1 k V and optical density of 2.34 MW/cm2.Comparing the performance characterization of the device by two different characterization methods,it is found that the device does not actually reach its saturation in the test laser range.The so-called saturation phenomenon appearing under the no-load method test is not caused by the saturation of the device carriers,but because of the existence of parasitic impedance effects in the no-load circuit.When the light intensity is large enough,the resistance of the device is no longer the main impedance of the circuit.Therefore,there is a "saturation" phenomenon occurring.The result refers that the device’s conductive performance is much more than that tested in this paper.Under a larger laser energy,the device’s performance must be better.