Abstract
LDMOS transistors have become the device choice for microwave applications. An overview is given of the LDMOS technology improvements at 3.6 GHz over the last decade, and RF performance of LDMOS microwave products for S-band radar is presented.
I. INTRODUCTION
More than ten years ago LDMOS transistors were introduced as a replacement of bipolar transistors for RF power applications [1,2]. Nowadays LDMOS technology is the leading RF power technology for base station applications, in particular for GSM-EDGE applications at 1 and 2 GHz, WCDMA applications at 2.2 GHz and more recently for WiMax applications around 2.7 GHz and 3.8 GHz.
One of the last niche application areas in which bipolar devices were used was the 3-4 GHz microwave area, such as S-band radar. Main reason for this was that earlier generations of LDMOS showed a similar performance at 3 GHz compared to bipolar, which did not justify redesign of complex radar systems.
The main driver for LDMOS is a high volume application, which enables continuous improvement of the LDMOS technology [3,4], and this has resulted in the latest generation LDMOS, which outperforms bipolar at S-band frequencies with some additional advantages such as ruggedness and better thermal behaviour. In this article an overview is given of the LDMOS improvements at 3-4 GHz and the LDMOS performance for microwave products is presented.
II. LDMOS ADVANTAGES
LDMOS transistors are voltage-controlled devices, so no gate current is flowing as in bipolar devices. This voltage control allows a much simpler and cheaper bias circuitry compared to bipolar devices. The drawback of a weak gate-oxide does no longer hold, since nowadays ESD devices are standard.
Another advantage is the source connection to the bulk-backside of LDMOS, while bipolar devices have a collector back side. Therefore bipolar uses isolating BeO packages in combination with bond wires. LDMOS allows for a replacement of the toxic BeO packages by environment friendly ceramic or plastic packages. This is a major advantage for LDMOS. A picture of such a ceramic SOT502 packages is shown in Fig. 1. Internal input and output (inshin) matching is provided within the package to transform the impedance levels and reduce RF losses. The bulk source is eutectically soldered to the package without the need for source bond wires. Without the source wires LDMOS does not have the additional source inductance resulting in a high gain of the LDMOS power transistor.
LDMOS also has a better temperature stability than bipolar. Bipolar devices have a positive temperature coefficient leading to thermal runaway. Bipolar therefore needs elaborate temperature compensation like ballast resistors to protect the device against failures. At high current, LDMOS has a negative temperature coefficient automatically turning off the device when fully powered. This leads to a natural advantage with respect to thermal properties and ruggedness.
LDMOS devices have high flexibility with respect to pulse duration, as is important for microwave applications. The common source configuration of LDMOS stabilizes the device and prevents oscillations at lower pulse durations.
The RF performance of LDMOS at 3-4 GHz frequencies has also dramatically improved in the last decade to become significantly better than bipolar performance. In section IV the LDMOS RF performance is shown for 3.6 GHz, but first state of the art LDMOS technology is described in section III.

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