Journal of the Korean Physical Society, Vol. 60, No. 5, March 2012, pp. 842∼848 A RESURF LDMOSFET with a Dummy Gate on Partial SOI Behzad Ebrahimi, Behrouz Afzal and Ali Afzali-Kusha ∗ School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran Saeed Mohammadi School of Electrical and Computer Engineering, Purdue University, West Lafayette, 47907, USA (Received 19 December 2011, in final form 26 January 2012) In this paper, we propose a laterally double-diffused metal-oxide-semiconductor field-effect tran- sistor (LDMOSFET) that uses a dummy gate on top of the lightly-doped drain (LDD) region in a reduced surface field (RESURF) structure. The structure is based on the partial silicon-on-insulator (PSOI) structure to increase the operating frequency of the device. The use of the dummy gate induces another electric field peak in the structure, which increases the breakdown voltage of the transistor. It also improves the gate-drain feedback capacitance, the transconductance, and the ON-resistance of the device. For a LDD length of 3 μm, the breakdown voltage increases about 10%, the transconductance increases over 200%, and the ON-resistance decreases about 45%. When a LDD length of 10 μm is used, the increase in the breakdown voltage is 17%. In addition, we pro- pose a breakdown voltage model for the structure, which may be used to optimize the design of the structure. PACS numbers: 85.30.Tv Keywords: Breakdown voltage, Dummy gate, LDMOSFET, RESURF, PSOI DOI: 10.3938/jkps.60.842 I. INTRODUCTION A laterally double-diffused metal-oxide-semiconductor field-effect transistor (LDMOSFET) is the most promi- nent transistor for high-voltage and high-frequency ap- plications. A vertically double-diffused MOSFET (VD- MOSFET) is another structure that offers good stabil- ity, but cannot be used in high-frequency applications [1]. In LDMOSFETs, there is a tradeoff between the frequency of operation and the breakdown voltage. If high-frequency operation is not desired, the dimensions of the transistors can be large, providing a high break- down voltage. For high-frequency applications on the other hand, the transistor dimensions should be small. To achieve high breakdown voltages in small size transis- tors, one should devise new structures. The breakdown voltage in Si devices is the voltage at which the elec- tric field within the device reaches about 3 × 10 5 V/cm. The peak of the electric field usually occurs at the sur- face at the boundary of the gate and the drain [2]. As the voltage drop is the integral of electric field, one can increase the breakdown voltage by devising a structure that increases the number of electric field peaks at the surface. If these peaks have the same height, the maxi- mum breakdown voltage is achieved [2–8]. * E-mail: afzali@ut.ac.ir; Fax: +98-21-8877-8690 Two structures that increase the number of electric field peaks have been proposed. One makes use of thin- film transistors where a thickness of around 1-2 microns is used for the silicon film. Using such thin-film tran- sistors causes a reduced surface field (RESURF) effect and induces another peak at the boundary between the lightly-doped drain (LDD) and the main drain [3,4]. The second technique is based on using a dummy gate, which normally has a fixed bias voltage and is placed on the LDD structure somewhere between the gate and the drain. This structure also increases the breakdown volt- age by adding one more electric field peak [5]. The gate- drain feedback capacitance of this structure is typically smaller than that of the device without the dummy gate. As a consequence, it can have higher operating frequen- cies [9]. The use of a silicon-on-insulator (SOI) substrate also increases the operating frequency due to smaller capacitances of the transistor relative to the substrate. These structures, however, suffer from an increased self- heating effect. To minimize the effect, instead of com- pletely separating the thin-film from the substrate by us- ing a buried oxide, a partial oxide isolation is used, which facilitates the heat transfer. This structure is known as a partial SOI (PSOI) structure [10]. Some works (e.g., Refs. 6 – 8) have also proposed stepped or triangular buried oxide to modulate the electric field and to increase the breakdown voltage. In this paper, we propose a LDMOSFET device that -842-