July 2007

Rev 13

1/6

AN1232

Application note

Ruggedness improvement

of RF DMOS devices

Introduction

RF amplifiers often experience impedance mismatch between output and load. Such an

impedance mismatch generates a reflected wave towards the RF power transistor, making a

much more stringent working environment for the transistor. Working conditions grow critical

when the load is disconnected from the output of the RF power transistor, since, in this case,

the reflected wave amplitude becomes comparable to the incident one.

RF transistors are able to withstand severe impedance mismatch conditions particularly

essential for applications such as plasma generators or nuclear magnetic resonators which

operate under rough conditions. DMOS devices used in such applications appeared to lack

the necessary ruggedness when operating under severe RF load mismatch conditions. This

weakness was believed to be intrinsic.

Based on the need to improve the ruggedness of RF power DMOS, an investigation was

carried out and a theoretical model simulating the failure mode mechanism was developed.

Finally, relevant corrective actions were undertaken.

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