Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyInstability of the electric characteristic followed by devicefailure (if the applied voltage is not stopped) is manifestedwhen deviation from linear variation is visible. In the linearregion relative uniformity of the leakage current flow over thejunction perimeter is possible. In the non-linear region ofelectrical characteristic towards breakdown, crowding of theleakage current in local spots at the interface from the junctionperimeter is possible. This current crowding can be favored bylocal non-uniformities or insufficiencies remained in theinterfacial layer from the junction passivation process.Elimination of such insufficiencies in the device manufacturingtechnology can enable linear variation of electricalcharacteristic, up to sharp vertical breakdown region given byavalanche multiplication of carriers in the junction bulk, [15].For high voltage silicon PN junctions this is not an easy task.Experimental evidence about leakage current crowding in localspots (hot spots) at the junction periphery, in the non-linearregion towards breakdown has been given in [16]. In the caseof the diode samples from Fig.2 the breakdown region ofelectrical characteristic is the result of phenomena occurring atthe silicon-dielectric interface.Further influence of the silicon-dielectric interface (theinterfacial layer) from the junction edge on device performanceis shown in Fig. 4 for an ordinary rectifier diode and acontrolled-avalanche diode from the same manufacturer andwith the same processing technology. Although the junctionarea of the ordinary diode is lower than that of the controlledavalanchediode, higher level of I R is exhibited by the ordinarydiode. Such difference in the level of I R cannot be attributed tothe junction bulk, because despites the diodes from Fig.2, thetwo PN junctions have the same physical parameters includinglow density of g-r centres, providing practically the samelifetime of charge carriers. Better quality of the dielectricsiliconinterface resulted from junction passivation provideslower level of I R for the controlled-avalanche diode sample. Asa consequence, practically vertical breakdown region isreached at room temperature for the controlled-avalanche diodebut not for the ordinary diode. At high temperature of 150 o Cdeviation from linear variation above 1200 V is exhibited forthe ordinary rectifier diode sample, whereas for the controlledavalanchediode, deviation takes place above 1800 V. For thecontrolled – avalanche diode, operation in the breakdownregion is possible without risk of failure at 150 o C, in specifiedconditions given in the data sheet. For the ordinary recoverydiode to avoid risk of failure, no increase of applied voltage,even for short time is allowed above 1200 V. In the case of thecontrolled-avalanche diodes, different level of I R has beenfound from sample to sample as shown in Fig.5 at 200 o C. Ifthe level of I R is higher, then deviation from linear variationtakes place on extended portion of electrical characteristic(sample No.1 in Fig.5). At low applied voltage up to 500 V,approximately constant level of I R exhibited by the sampleNo.2 in Fig. 5 could be attributed to the presence of the bulkdiffusion current (I RB in Fig.1) predicted in the theory. Such apossibility would be suggested as a result of significantreduction in the level of I R on behalf of its component from thejunction periphery in the interfacial layer (I RS in Fig. 1).Nonetheless change in the portion of linear variation of theReverse current (A)Reverse current (A)2.0E-031.5E-031.0E-035.0E-040.0E+00Fig. 4. Reverse current-voltage characteristics for standard recovery siliconrectifier diodes (25A, 1200 V ordinary rectifier diode, 40 A, 1200 Vcontrolled – avalanche diode)1.5E-031.0E-035.0E-040.0E+00200 o C200 oC200 o C0 500 1000 1500 2000Reverse voltage (V)Controlled-avalanche diodesSample No.1(also used in Fig.4)Sample No.2Ordinary rectifier diodeControlled -avalanche rectifier diode150 o C150 o C18 o C0 500 1000 1500 2000Reverse voltage (V)Fig. 5. Reverse current-voltage characteristics at 200 o C for two samples ofcontrolled-avalanche diode (40 A, 1200V)leakage current for the sample No.2 from Fig.5 takes placewhen the junction temperature increases from 200 to 250 o C.This change is shown in Fig.6. The portion of the electricalcharacteristic with linear variation is more reduced at 250 o C.Such behavior cannot be attributed to the bulk diffusioncurrent. At junction temperature above 200 o C change of theproperties of the interfacial layer is possible and more leakagecurrent flows through this layer. Other experimental resultsrelated to excessive leakage current at the junction edge havebeen presented in [11, 17-18]. A high level of the leakagecurrent at high temperature is a main limitation for reliableoperation of silicon devices above 150- 200 o C. Significantpower dissipation is possible due to the leakage current. For©EDA Publishing/THERMINIC 2008 145ISBN: 978-2-35500-008-9