In base stations for personal communications systems, for example used for the Global System for Mobile communications (GSM), Enhanced Data Rates for GSM Evolution (EDGE), Wideband Code Division Multiple Access (W-CDMA), radio frequency (RF) power amplifiers are among the key components. For these power amplifiers, RF power laterally diffused metal oxide semiconductor (LDMOS) transistors are a commonly used technology. They exhibit excellent high power capabilities, gain and linearity. These metal oxide semiconductor (MOS) transistors are not only used in base stations but also in radar and broadcast applications. If a RF mismatch occurs at the output of the LDMOS, power will be reflected back at the LDMOS and the LDMOS must be able to handle this. Thus ruggedness, i.e. the capacity to sink excess energy without damage, is an important issue for RF power LDMOS transistors. For low-frequency applications, ruggedness can be improved by adding external diodes with appropriate breakdown voltages. The diodes will sink electrical energy and thus prevent damage to the LDMOS. For RF LDMOS transistors with high frequency applications, external diodes would degrade the RF performance and the ruggedness improvement would be insufficient. It is also conceivable to sink electrical energy through the drain to substrate diode, if the drain-substrate diode is able to sink enough energy before the voltage reaches a level at which the parasitic bipolar transistor is turned on and the LDMOS is damaged. However, the breakdown often starts locally in a specific section of the drain, which determines a low breakdown voltage but doesn't give a high breakdown current.
It is therefore an object of the invention to improve a laterally diffused metal oxide semiconductor circuit for a radio frequency power amplifier in a way that allows the adjusting of a breakdown voltage at which breakdown will occur with a higher breakdown current.