An Analytical Study of High Power Schottky Barrier Diode on 4H Silicon Carbide (SiC) Wafers

Abstract

Silicon carbide (SiC) has been under intensive investigation as an enabling material for a variety of new semiconductor devices in areas where silicon devices cannot effectively compete. These include high-power high-voltage switching applications, high temperature electronics, and high power microwave applications in the 1 - 10 GHz frequency range. In recent years, activity in silicon carbide (SiC) device development has increased considerably due to the need for electronic devices capable of operation at high power levels and high temperature. The main strength of silicon carbide is that it can resist high field strengths; it offers better heat-conducting capacity than copper at room temperature and it has a large energy band gap, which means that electrical components continue functioning even when the temperature rises. With very high thermal conductivity (~5.0 W/cm), high saturated electron drift velocity (~2.7 x 107 cm/s) and high breakdown electric field strength (~3 MV / cm), SiC is a suitable material of choice for high temperature, high voltage, high frequency and high power applications. Schottky barrier diodes (SBD's) are used as high-voltage rectifiers in many power switching applications. Whenever current is switched to an inductive load such as an electric motor, high-voltage transients are induced on the lines. To suppress these transients, diodes are placed across each switching transistor to clamp the voltage excursions. PN junction diodes could be used for this application, but they store minority carriers when forward biased and extraction of these carriers allows a large transient reverse current during switching. Schottky barrier diodes are rectifying metal-semiconductor junctions, and their forward current consists of majority carriers injected from the semiconductor into the metal. Consequently, SBD's do not store minority carriers when forward biased, and the reverse current transient is negligible. This means the SBD can be turned off faster than a PN diode and dissipates negligible power during switching. SiC Schottky barrier diodes are especially attractive because the breakdown field of SiC is about 8 higher than in silicon. In addition, because of the wide bandgap, SiC SBD's should be capable of much high temperature operation than silicon devices. SiC SBD can be fabricated with the 4H/SiC material and with Ti, Ni and Au etc Schottky metals. But the performance of Schottky diode will depend on the barrier height of the metal. Lower barrier height gives less voltage drop in forward bias but gives a higher leakage current. Special edge termination is required to minimize field crowding at the edge of the metal contact. A 4H-SiC Schottky Barrier diode has much higher breakdown voltages because of a ten times greater electric field strength of SiC compared with silicon. 4H-SiC unipolar devices have higher switching speeds due to the higher bulk mobility of 4H-SiC compared to other polytypes. The most important parameters that quantify the efficient design of 4H-SiC Schottky barrier diode are blocking voltage ( ), Specific-on-resistance ( ) and forward voltage drop ( ). For rectifiers the static on-state losses can be expressed in the forward voltage drop over the diode ( ) and the specific on-resistance ( ) in the drift region, which accommodates the specified blocking voltage. In SiC SBD the epilayer plays an important role in device design. For Schottky diode the switching power losses are very low and design strategy is to minimize the static power losses for a rated blocking voltage. The most important design parameters are consequently the drift resistance, epitaxial doping and Schottky contact properties (barrier height and current ideality factor).At the time of fabrication of SiC SBD the drift region is lightly doped so that it can support the high blocking voltage and could be used for high power application. But when it is lightly doped the on-resistance of device will increase and power dissipation across the device will also increase .So there is a tradeoff between the doping level and the on-resistance of the device. So it is necessary to optimize the device performance at a particular level according to the application requirements. The research work carried out here on 4H-Silicon Carbide Schottky barrier diode has been an attempt to understand the performance of the device with respect to power dissipation and breakdown voltage for a linearly graded doping profile in the drift region of the device. The doping profiles used are uniformly doped and linearly graded doping profiles. Although a lot of work has been has been described in the literature over the last two decades, no specific work has been reported in which the graded profiles have been used in the drift region of 4H-SiC Schottky Barrier Diode (SBD) for this type of analysis. The ultimate aim for making this study is to provide a linearly graded profile in the drift region of SBD with lower doping at the top of the device to a higher doping near the drain. This type of profile will help in increasing the breakdown voltage while at the same time will reduce the resistance at the lower end of the device and thereby reduce the overall specific on-resistance. In this work, it has been established that the power dissipation is minimum in the linearly graded profile evaluated at a current density of 1000 Amps/cm2. Hence it is possible to design and develop 4H-SiC SBD’s which can yield higher breakdown voltages at a lower device thickness by using linearly graded drift region. Thinner devices with higher breakdown voltages and lower power dissipation can be developed by using linearly graded profiles in the epitaxially grown drift regions of 4H-SiC Schottky barrier diodes.