electronics Article Investigation of Electrical Contacts to p-Grid in SiC Power Devices Based on Charge Storage Effect and Dynamic Degradation Meng Zhang 1,2, Baikui Li 1,*, Mengyuan Hua 3 and Jin Wei 4 1 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] 2 Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, China 3 Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China; [email protected] 4 Institute of Microelectronics, Peking University, Beijing 100871, China; [email protected] * Correspondence: [email protected] Received: 4 September 2020; Accepted: 11 October 2020; Published: 19 October 2020 Abstract: P-grid is a typical feature in power devices to block high off-state voltage. In power devices, the p-grid is routinely coupled to an external electrode with an Ohmic contact, but Schottky contact to the p-grid is also proposed/adopted for certain purposes. This work investigates the role of contact to p-grid in power devices based on the commonly adopted technology computer-aided design (TCAD) device simulations, with the silicon carbide (SiC) junction barrier Schottky (JBS) diode as a case study. The static characteristics of the JBS diode is independent of the nature of the contact to p-grid, including the forward voltage drop (VF) and the breakdown voltage (BV). However, during the switching process, a Schottky contact would cause storage of negative charges in the p-grid, which leads to an increased VF during switching operation. On the contrary, an Ohmic contact provides an effective discharging path for the stored negative charges in the p-grid, which eliminates the dynamic degradation issues. Therefore, the necessity of an Ohmic contact to p-grid in power devices is clarified. Keywords: electrical contacts; stored charges; dynamic performance; SiC JBS diode 1. Introduction Power devices typically feature p-grids inside the device active regions to block the high off-state voltage or to protect certain vulnerable structures [1]. With the key challenges, such as low channel resistance [2], channel mobility [3,4], and oxide reliability [5] being addressed, silicon carbide (SiC) devices are now commercially available from several vendors [6,7]. The SiC devices are widely used in power electronics—for example, SiC transistors adopted as switches in inverter circuit, SiC diodes used in rectifier circuit and as a freewheeling diode in inverter circuit. In a power metal-oxide-semiconductor field-effect transistor (MOSFET), the p-grid is grounded to the source electrode, while in a junction barrier Schottky (JBS) diode, the p-grid is connected to the anode. In blocking state, the PN junction (between p-grid and n-drift region) is reversely biased to withstand the high voltage. In power devices, the metal contact to the p-grid is mostly an Ohmic contact. However, Schottky contact is also proposed and adopted for certain purposes. For example, for an insulated gate bipolar transistor (IGBT), a Schottky contact to the p-body has been proposed to enhance the minority carrier concentration near the emitter side without any sacrifice in the off-state breakdown voltage (BV)[8]. In the emerging Electronics 2020, 9, 1723; doi:10.3390/electronics9101723 www.mdpi.com/journal/electronics Electronics 2020, 9, 1723 Electronics 2020, 9, x FOR PEER REVIEW 2 of 9 2 of 9 In the emerging gallium nitride (GaN) power transistors, a Schottky contact to the p-region is widely gallium nitride (GaN)used power to enlarge transistors, the gate a Schottky voltage contact swing to[9,10], the p-region or explored is widely as a method used to to enlarge adjust the the threshold voltage gate voltage swing [[11,12].9,10], or Furthermore, explored as a methodthe injection to adjust of minority the threshold carrier voltage through [11,12 the]. Furthermore, PN junction is likely to cause the injection of minoritybipolar carrier degradation through in the SiC PN power junction devices, is likely such to as cause body bipolar diodes degradationof SiC MOSFETs in [13,14] and SiC SiC power devices,bipolar such as junction body diodes transistor of SiC (BJT) MOSFETs [15]. If [13 the,14 metal] and SiCcontact bipolar to p-grid junction is non-Ohmic transistor —e.g., a Schottky (BJT) [15]. If the metalcontact contact is used to p-gridinstead is— non-Ohmic—e.g.,the conduction through a Schottky the PN contact junction is usedcan be instead— completely suppressed, so the conduction throughthe bipolar the PN degradation junction can beand completely poor revers suppressed,e recovery socan the be bipolar avoided degradation [16,17]. and poor reverse recovery canThis be avoidedwork aims [16 ,to17 ].discuss the role of the contact to p-grid in certain power devices, whose This work aimsmetal/p-region to discuss contacts the role are of under the contact investigation. to p-grid The SiC in certainJBS diode power is adopted devices, in this work as a case whose metal/p-regionstudy. contacts SiC power are under devices investigation. have attracted The worldwide SiC JBS diode attention is adopted because in of this the work high critical breakdown as a case study. SiCfield power in SiC devices [18]— havefor example, attracted SiC worldwide MOSFET attentionwith protruded because p-base of the [19], high split-gate critical SiC MOSFET [20], breakdown field inSiC SiC diodes [18]—for [21], example, and GaN/SiC SiC MOSFET hybrid withfield-effect protruded transistors p-base [22]. [19], In split-gate this work, SiC we present that the MOSFET [20], SiC diodescontact [21 to], the and p-grid GaN/SiC exerts hybrid little field-e influenceffect transistorson the static [22 characteristics]. In this work, of we the present SiC JBS diode, including that the contact to thethe p-gridforward exerts voltage little drop influence (VF) and on the the BV static. However, characteristics the SiC of JBS the diode SiC JBS with diode, Schottky contact to the including the forwardp-grid voltage suffers drop from (VF) severely and the BVdegraded. However, dynamic the SiC V JBSF during diode the with switching Schottky operation. contact The cause of the to the p-grid suffersdynamic from severely degradation degraded is found dynamic to beV theF during charge the stor switchingage in the operation. p-grid. An The Ohmic cause contact to the p-grid of the dynamic degradationeffectively is discharges found to be the the p-grid charge and storage eliminates in the the p-grid. dynamic An Ohmic degradation. contact to the p-grid effectively dischargesTechnology the p-grid computer-aided and eliminates design the dynamic (TCAD) degradation. Sentaurus device simulations and mixed mode Technology computer-aidedsimulations are designdeployed (TCAD) to reveal Sentaurus the internal device dynamics simulations of the devices. and mixed A Sentaurus mode Structure Editor simulations are deployedis used to for reveal structure the internal and mesh dynamics construction. of the devices. A Sentaurus A Sentaurus Device Structureis used for Editor device simulations and is used for structuremixed-mode and mesh construction.circuit simulati Aons. Sentaurus The physics Device models is used such for device as Auger simulations and Shockley–Reed–Hall and mixed-mode circuitcombination, simulations. impact The ionization physics (Lackner models such model), as Auger incomplete and Shockley–Reed–Hall ionization, high-field saturation effects, combination, impactdoping ionization dependent (Lackner transport, model), band incomplete narrowing, ionization, and anisotropic high-field mobility saturation model effects, are all considered. doping dependent transport, band narrowing, and anisotropic mobility model are all considered. 2. Device Structure and Static Characteristics 2. Device Structure and Static Characteristics Figure 1 shows the schematic structures of SiC JBS diodes with either an Ohmic contact or a Figure1 showsSchottky the schematic contact structures to the p-grid. of SiC The JBS work diodes function with eitherof the anSchottky Ohmic contact contact is or assumed a to be 4.33 eV. Schottky contact toThe the n-drift p-grid. region The work has a function thickness of of the 12 Schottkyµm, and contacta doping is concentration assumed to be of 4.33 8 × 10 eV.15 cm−3. The width of 15 3 The n-drift region hasthe ap-grid thickness in one of cell 12 µ ism, W andgrid =a 2 dopingµm. The concentration width of the junction of 8 10 gatecm field-effect− . The width transistor (JFET) region × of the p-grid in oneis cell WJFET is W= 2grid µm= unless2 µm. otherwise The width specified. of the junction The key gate device field-e parametersffect transistor of the diodes (JFET) are listed in Table 1. region is WJFET = 2 µThem unlesstemperature otherwise is set specified. as room Thetemperature key device (300 parameters K) in the following of the diodes TCAD are listedsimulations. in Table1. The temperature is set as room temperature (300 K) in the following TCAD simulations. (a) (b) Figure 1. Schematic structuresFigure 1. ofSchematic (a) the SiC structures junction of barrier (a) the Schottky SiC junction (JBS) barrier diode withSchottky Ohmic (JBS) contacts diode with Ohmic contacts to the p-grids and (b)to the the SiC p-grids JBS diode and with(b) the Schottky SiC JBS contacts diode with to the Schottky p-grids. contacts to the p-grids. Electronics 2020, 9, x FOR PEER REVIEW 3 of 9 Table 1. Key device parameters in the studied SiC JBS diodes. Parameter Value Unit n-drift thickness 12 µm Electronics 2020, 9, 1723 n-drift doping 8 × 1015 cm−3 3 of 9 cell pitch 4 µm Table 1. KeyW devicep-grid parameters in the studied2 SiC JBS diodes.µm WJFET 2 µm p-gridParameter doping Value1 × 1018 Unit cm−3 n+ substraten-drift thicknessdoping 1 12 × 1019 µm cm−3 n-drift doping 8 1015 cm 3 × − The static I-V characteristics ofcell the pitch SiC JBS diodes 4are plottedµ min Figure 2.