ON-RESISTANCE OF POWER MOSFETS Janusz ZarQbski Gdynia Maritime University, Department of Marine Electronics Keywords; MOSFET, silicon carbide parameters, Ron resistance. Abstract: The paper concerns the problem of modelling of the drain-to-source ON-Resistance (Ron) of power MOSFETs. Two kinds of the transistor structures: VDMOS and CoolMOS made of a silicon and a silicon-carbide are considered in the paper. Upornost močnostnih MOSFET tranzistorjev v prevodnem stanju Kjučne besede: MOSFET, parametri silicijevega karbida, upornost Ron Izvleček: V prispevku prikažemo probleme modeliranja upornosti izvor - ponor, Rom, močnostnih MOSFET tranzistorjev v prevodnem stanju. Opišemo dve tranzistorski strukturi: VDMOS in CoolMOS izvedeni v siliciju, oz. silicijevem karbidu. 1. Introduction Currently, silicon power MOSFETs are one of fjie most intensive developed and modified devices intended mainly for power converters. The substantial drawback of the considered class of power devices is their relatively high value of the drain-to-source ON-Resistance (Ron), what often results in unacceptable values of the energy losses at the high values of the drain current. The value of Ron depends on both the values of the device breakdown voltage and the semiconductor parameters. The decreasing of Ron, especially in the high-voltage power MOSFETs, is the very important challenge for the producers of these devices. This task can be reached by developing new structures (e.g. CoolMOS transistors), as well as by using advanced (wide band-gap) semiconductor materials, e.g. silicon-carbide (SiC). The paper presents the estimation of influence of the impurity doping of the epitaxial-layer of the selected power MOS transistors (VDMOS, CoolMOS) on their drain-to-source ON-Resistance. The considerations are performed for two semiconductors: silicon and the most popular pol-ytypes of silicon carbide. 2. The theoretical dependences Power silicon MOSFETs (Fig. 1) are commonly used in electronics and energoelectronics in the voltage range from about fifteen up to one thousand volts. The main component of the switch-ON resistance (Ron) of high-voltage VDMOSTs is the resistance represented by the epitaxial layer. The fundamental relation of Ron on the breakdown voltage (Ubr) is/1/. n+ n epitaxial-layer n+ T D Fig. 1. The considered structure of the VDMOS transistor ^ON - 4-U BR (1) Where: p - electron mobility, lo - the permittivity of free space, Ir - relative permittivity, Ec - critical electric field are a semiconductor parameters. It is seen, that Ron increases strongly with increasing of Ubr (to the second power) whereas it decreases for semiconductors characterized by higher values of (j and Ec (third power). On the other hand, at a fixed value of the epitaxial-layer thickness, the higher voltage VDMOS transistor have to be lightly doped (it concerns the epitaxial layer) according to the dependence /1/. U br - 2-q-N (2) where N - doping concentration, q - elementary (electron) charge. In turn, lower doping concentration results in higher value of Ron. The new generation of power MOSFETs - named Cool-MOS transistors (Fig. 2) described for the first time in /2, 3/, have been offered by Infineon Technologies since 1998. i; n+ n epitaxial-layer n+ Fig. 2 ÖD The considered structure of the CoolMOS transistor CoolMOS transistors (Fig. 2) belong to the class of super-junction devices presented in /4/. In CoolMOS transistors the epitaxial layer consists of parallely connected heavily doped n and p pillars of the width equal to d. At the drain-to-sourcevoltage, typically higher than 30 - 50 volts, the pillars become fully depleted by lateral extension of the depletion region from the p-pillar/n-pillar junction. On the other hand, the pillar-doping concentration depends on the pillar width. The thinner epitaxial-layer, the higher doped one. The resistance Ron and the breakdown voltage Ubr of the CoolMOS transistor are related by the following dependence /4/. U br So (3) As seen, Ron resistance is proportional to Ubr, what means that CoolMOS transistors are especially attractive for high-voltage applications. On the other hand, narrower pillars, the lower Ron, what have to result in the heavily dopped ones. 3. The semiconductors parameters As seen from Eqs. 1 and 3, two semiconductor parameters: charge mobility and critical electric field are of the fundamental importance to determine the Ron- resistance. In Table 1 the values of these parameters for silicon (Si) and three of the most popular polytypes of silicon carbide: 4HSiC, 6H-SiC and 3C-SiC are presented. Parametr Si 3C-SiC 4H-SiC 6H-SiC cm^ V-s 1400 900 700 400 Ec ' V' _cm 3.10^ 1,5.10® 2.6.10® 2,1.10® £r 11,9 9,7 9,6 10 The values in Table 1 concern lightly doped semiconductors. As results from a lot of publications, assuming the fixed values of the considered parameters is a great simplification, because their values are a function of temperature, doping concentration, pressure, etc. Fig. 3 presents the dependences of |j(N) and Ec(N) for the silicon and silicon carbide, respectively. 1e+14 1e+15 1e+16 1e+17 1e+18 1e+19 1e+20 N[cm-'] 3,0 2,5 u 2,0 1,0 0,5 -|6H-SiC )=— " "1 3H-SiC [—H] 3,0 2,8 2,6 2,4? o 2,2.^ o 2,0 lU 1,8 1,6 1e+15 1e+17 N [cm ' 1e+19 1,4 1e+21 Fig. 3. Dependencies iJ(N) and EC(N) As seen from Fig. 3 the value of the electron mobility is nearly the proper low-doped value for the doping concentration less than any critical value Ncr, which for silicon (NcrSi) is equal to 10"''* cm"^, whereas for silicon-carbide is much higher, e.g. for4H-SiC we have Ncr4HSic = 10^® cm"^. The analogical observation (Fig. 3b) concerns the critical field, for which NcrSi = 10^® cm"^ and NcreHSic H" 10^® cm"^, respectively. 4. Results On the parameter values from Table 1, the dependences of Ubr(N) forVDMOS transistors made of silicon (Si) and silicon-carbide (SIC) were calculated from Eq. 2 (Fig. 4). In turn, the dependence of Ron(Ubr) for silicon and silicon-carbide VDMOS and CoolMOS transistors at three various values of the pillars width, corresponding to the same materials parameter values are presented in Fig. 5. 1E+12 1E+13 1E+14 1E+15 1E+16 1E+17 1E+18 1E+19 1E+20 N [cm'^l Fig. 4. Dependencies of UBR on N It is seen from Ubr(N) characteristics that for silicon transistors of the breakdown voltage higher than 20V, the doping concentration not exceed 10^® cm"®, whereas the SiC transistors of the same Ubr value can be even one hundred times more heavy doped (N = 10^®). The current technology allows to manufacture the silicon CoolMOS transistors of the pillar width equal to 5 pm. For examples, forthe 1 kV CoolMOS transistor, assuming that d/2 ~ i/-Jn and d = 5 pm, the doping concentration can increase 10® times. Note, that the tenfold degreasing of the pillar with (d = 500 nm) results in the further hundredfold increasing of N doping until the value equal to 10^^ cm" ® (for Si). As it is seen from Figs 4,5 the values of Ec and |j decrease considerably at the doping concentration N > Ncr. So, this phenomenon should be taken into account, when the Ron resulting from Eqs. (1,3) is calculated. For instance, in Fig 6 the dependence of ARon/Ron as a function of Ubr of the form AR„ K -K Rn •100% = /(17, J (4) CST) 1E+1 1E+1 1E+2 U8r[V] 1E+3 1E+4 b CjHjSiC^ 1E-2 1E-3 „ 1E-4 "e o Š. 1E-5 "^"lE-e 1E-7 1E-8 1VDMOSf iSpm 10,5umt O.OSpm dCoolMOS 1E+1 1E+2 Ubr[V] IE+3 1E+4 Fig. 5. Dependencies of RON on UBR use of the semiconductor parameters of the constant values (table I) and the values obtained from Fig. 3, respectively. To get the proper value of the doping concentration N corresponding to the selected value of the device breakdown voltage, the diagram shown in Fig.4 was used. As seen, using the dependence Ec(N) and |j{N) results in the higher values of the ON-resistance, what is of the great importance, especially for silicon-carbide devices. 400 UbrM 1000 for Si and SIC VDMOS transistors is presented. In this equation the resistances Rq^'"' and T^^JJ are calculated with the Fig. 6 Dependencies of ÄRON on UBR for VDMOS transistors 5. Conclusion In the paperthe problem of modelling of the drain-to-source ON-Resistance of silicon and silicon carbide power MOSFETs is discussed. Two l