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This paper presents an annular-structure lateral-diffused metal-oxide-semiconductor (LDMOS) RF transistor using a 0.5-μm LDMOS process. This paper also examines the dc, small-signal, and large-signal characteristics of RF LDMOS transistors with different closed structures. In particular, the problem of evaluating the LDMOS aspect ratio for annular structure is addressed. The capacitance characteristics improvement in the LDMOS device design using the annular structure was also investigated. The power gain and efficiency of annular structure give nearly 5% enhancement compared to the traditional structure with 80-μm gatewidth at 1.9 GHz. Results show that the annular structure appears to be a better layout design for RF LDMOS transistors.