Operation Manual

WHAT IS AN LDMOS?
• LDMOS (laterally diffused metal oxide semiconductor) transistors are used in
microwave/RF power amplifiers. These transistors are often fabricated on p/p+ silicon
epitaxial layers. The fabrication of LDMOS devices mostly involves various ion-
implantation and subsequent annealing cycles. As an example, the drift region of this
power MOSFET is fabricated using up to three ion implantation sequences in order to
achieve the appropriate doping profile needed to withstand high electric fields.
• Silicon-based LDMOS FE Ts are widely used in RF power amplifiers for base-stations
as the requirement is for high output power with a corresponding drain to source
breakdown voltage usually above 60 volts. Compared to other devices such as GaAs
FE Ts they show a lower maximum power gain frequency.
• We use the BLF189XRA from Ampleon USA Inc:
• We have performed a test of ruggedness with a controlled load mismatch through all
phases, enabled by the test fixture. The mismatch unit is connected to a network
analyzer. Through a network control, we can set it to work through all phases of the Smith
chart. This mismatch unit will create voltage standing waves with ratios from 55: 1 up to a
maximum of 125: 1. With the normal situation (standard, 50 volts 1200 watts into a 50-
watt load) the amp is operating at full power. The mismatch unit now replaces the dummy
load connected to the output of the amp and the test set up is powered up again. We then
vary the mismatch unit to expose the LDMOS transistor to a range of extreme mismatch
conditions and positions on the Smith chart. The extreme conditions range from a VSWR
of 55: 1 to 125: 1. The transistor is still alive by going back to the original load and
showing the 1200-watt output power. It still works! Would you ever treat a transistor worse
than this? Check out the next test.
• Even though the transistor is normally used at 50 volts, we then test it at 55 volts to
challenge the extremely rugged LDMOS. The transistor was powered at 55 volts 1200
watts into 50 ohms when we suddenly disconnect the output load, creating a very
unfriendly open circuit situation. Then, we powered up the transistor to 1200 watts. A
short was introduced to the output of the amp while the transistor is powered up. When
placed back into the 50-ohm load and powered up to 1200 watts, we see that it again
survives and works perfectly. This is what we mean by extremely rugged. This transistor
seems to be unbreakable.
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