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Comparative Study of Power MOSFET Device Structures Rakesh Vaid & Naresh Padha

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Comparative Study of Power MOSFET Device Structures Rakesh Vaid & Naresh Padha Indian Journal of Pure & Applied Physics Vol. 43, December 2005, pp. 980-988 c Comparative study of power MOSFET device structures Rakesh Vaid & Naresh Padha Department of Physics & Electronics, University of Jammu, Jammu, 180006 Received 19 April 2005; revised 13 October 2005; accepted 26 October 2005 In this paper, a comprehensive comparative study of various power MOSFET device structures designed and developed s: durin~ the past decade has been presented. Various design issues related with power MOSFET have been studied to look C into their on-resistance (RON) versus breakdown voltage (Bv) trade off. Some of the existing power MOSFET device M topologies have been compared with respect to their ROIVBV. The study reveals that the low-doped n epi region which gives an square law relationship between RON and Bv in the conventional power MOSFET is being constantly engineered for co optimizing RON-BV trade-off subsequently led to many structural modifications in its basic design giving rise to many new power MOSFET device structures such as SSCFET (Silicon Semiconductor Corp. FET), JBSFET (Junction barrier sa controlled Schottky FET), superjunction (SJ)ICOOLMOS™ transistor, semi-superjunction devices and FLlMOSFET pe (power MOSFETs with vertical floating islands) so as to overcome the conventional silicon limit. 2' Keywords: Power MOSFET, v-groove-MOS, Vertical double-diffused MOS, Trench power MOSFET, COOLMOS™, Vertical Floating Islands MOS (FLIMOSFET) 2.1 IPC Code: HOIL29176 trai typ 1 Introduction biased body-to-drain junction spreads into short sur As the power handling capability and frequency channel, resulting in breakdown at relati vely low san response of silicon devices has improved, new voltage. Thus, the resulting device would not be gro applications for these devices have been created. capable of handling the high voltages typical of + n 2 During the past decade, there has been an increasing power-transistor applications. For this reason, new the acceptance of the usage of power MOSFETs. Their structures had to be found for fabricating short- Sidf high input impedance and excellent safe operating channel (1 to 2 urn) MOSFETs with high breakdown to ( area make them important candidates for many voltages. The basic design principle of power met; applications. They are being used in audio/radio MOSFETs is the same as that of classical MOS difft frequency circuits, high-frequency inverters used in transistors. The SMPS, lamp ballasts and motor control circuits, etc'". Power MOS transistors can be classified into five simp The low power MOSFET structure is not suitable for families: dime high-power applications. To appreciate this fact; (1) Structures having co-planar drain, gate and each recal that the drain current of an n-channel MOSFET source electrodes with aluminium gate, horizontal and curre operating in the saturation region is given by: constant doped channels or refractory gate or with prese field plate over gate; (2) Structures having co-planar capac electrodes and horizontal channel fabricated by the ... (1) speed process of double diffusion (DMOS); (3) Structures cham having non-planar electrodes and horizontal channel It follows that to increase the current capability of with uniform doping in the channel region; i.e., source the MOSFET, its width W should be made large and or drain on the bottom with a meshed pattern for the 'There its channel length L should be made as small as gate;(4) Structures having non-coplanar electrodes; everyor possible. Unfortunately, however, reducing the source and gate on top and drain on bottom, and Internal TMOS, channel length of the standard MOSFET structure, horizontal channel fabricated by the process of double diffusion with multi-cell source configuration; the SIP results in drastic reduction in its breakdown voltage. Technol Specifically, the depletion region of the reverse- (5) Structures fabricated by chemical etching of the process silicon; that is isotropic etching or anisotropic etching structure known as VMOS. Most promising among these • Fax No. +91-191-2453079, E-mail: [email protected] families are: VAID & PADHA: COMPARATIVE STUDY OF POWER MOSFET DEVICE STRUCTURES 981 (a) The VMOS or UMOS transistors fabricated by anisotropic etching of the silicon around the grooves; (b) The VDMOS (TMOS, DMOS, HEXFET, SIPMOS, TRIMOS. ... according to the manufacturer* concerned) fabricated by the process of double diffusion. There has been a trade-off in the on-resistance (RON) and breakdown voltage (Bv) while designing power MOSFETs and recently many power MOSFET configurations have been used for optimizing the RON ,- Bv relationship such as Trench power'" MOSFET, SSCFET'O• , JBSFET10,12,13,superjunction (SJ)14,20/ " ed N drift ik COOLMOS™ devices, semi-superjunction" ce MOSFETs, Novel high voltage sustaining structure", es and FLIMOSFET23'25, etc. In this paper, we present a Dr comparative study of various power MOSFET device w structures and discuss their device operation and er + . .T performance with regard to RotrBv trade-off. N substrate vi 2 Various Power MOSFET Device Structures drain 2.1 VMOS transistor (V groove) Fig. 1-Schematic design of a V-groove MOSFET (VMOSFET). Fig. 1 shows a section of the VMOS or (V groove) Structure shown represents one cell of the device transistor. It is produced from silicon epilayer of the type n' «: with a p-Iayer diffused throughout the mobility is lower than VDMOS. Since the V groove is short surface. Diffusion zones n' are also carried out in the formed by chemical etching process, which leaves .y low same window. The silicon is chemically attacked and many sodium ions on the etched surface whose lot be grooves (V's) are opened in the middle of the diffused presence creates a lot of reliability problems, so that cal of n: zones. This V formation is obtained by etching of the VMOS devices are not used presently'". I, new the silicon through windows in an oxide by hydrazine. 2.2 VDMOS transistor short- Sides of V are thermally oxidized and then metallized At present, the most popular structure for a power kdown to constitute the gate. The source contact is also MOSFET is the vertical double-diffused (VDMOS) power metallized, which short-circuit in the n and p" transistor as shown in Fig. 2. It starts with a heavily MOS diffusions. The drain is on the lower side 'bf device. doped n-type substrate in order to minimize bulk The advantage of this type of transistor is its portion of the channel resistance. An n' epi layer is to five simplicity and the precise control of its geometric grown on it and two successive diffusions are made, a dimensions, in particular, the channel length. Finally, p-zone in which proper bias will generate the channel :e and each groove produces two channels, doubling the and an n' into it defining the source. Next the thin, tal and current capability and reducing the surface area. The high quality gate oxide is grown followed by the r with presence of the n drift zone gives a high voltage phosphorous-doped polysilicon thus forming the gate. planar capacity. These transistors have good switching Contact windows are opened on top defining the by the speeds and can operate in the VHF range. In VMOS, source and the gate terminals while the whole bottom ictures channel is formed on {Ill} plane, so, its surface of the wafer makes the drain contact. With no gate hannel bias, the n+ source and n+ drain are separated by source p-zone and no current flows (transistor is turned-off). 'or the 'There are many power MOSFET manufacturers and almost With a positive gate bias, the minority carriers in the rodes; everyone has his own process optimization and trade name. p-zone (electrons) are attracted to the surface 1, and International Rectifier pioneered the HEXFET, Motorola builds underneath the gate plate. As the bias increases more TMOS, Lxys fabricates HiPerFETs and MegaMOS; Siemens has louble electrons are being confined to this small space, the ration; the SIPMOS family of power transistors and Advanced Power Technology, the Power MOS IV, to name a few. Whether the local minority concentration becomes larger than the of the process is called VMOS, TMOS or DMOS it has a horizontal gate hole (P) concentration and inversion occurs. Now an n tching structure and vertical current flow past the gate. channel is formed in the p material right under the these 982 INDIAN J PURE & APPL PHYS, VOL 43, DECEMBER 2005 SOlUTe SOlUTe " + Pbase region P 1 1 t N-(hift N buffer layer N substrate N+substrate drain Fig. 2 - Schematic design of a VDMOSFET (DMOS) transistor. Fig. 3-Schematic design of a Trench gate MOSFET (UMOSFET) transistor. Structure shown represents half cell of the device Structure shown represents half cell of the device t s gate structure connecting the source to the drain and expensive when compared to the planar DMOS t current can now flow. The gate bias controls the flow process. The industry has also to deal with reliability r- of current between source and the drain 1.3. problems associated with high electric fields at the c The power MOSFET is nothing but a structure trench corners, which must be solved by rounding the containing a multitude of cells like the one described trench corners and buffering the electric field 'using in Fig. 2 connected in parallel. And, like any the p' regions. Further, the extension of the gate into paralleling. f.. identical resistors, the equivalent the drift region increases the coupling between the resistance is .L/n-th. of the single cell's RDS(ON)' The drain and the gate leading to higher Miller larger the die, the lower is its on-resistance but at the capacitance and gate charge, which can adversely same time larger parasitic capacitances and therefore, affect switching performance.
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