Measuring leakage current in RF power transistors: Page 4 of 6

October 26, 2011 //By William R. Murphy, Richardson RFPD
Around the globe, engineers and technicians using RF power devices have had many concerns regarding the specifications for leakage current, what the specifications mean in terms of the part's performance in the field, and most importantly, how to properly test/verify that a given part is meeting its printed leakage current specification...
on the battery (only in very low power applications), and even induce noise into the channel. When problems occur in a circuit, it is good troubleshooting procedure to properly test the key devices and make sure they are performing within the manufacturer’s specifications.

All too often today, one or more of the following field-testing issues are encountered:

Issue #1
Technicians literally grabbing an RF power transistor and testing it “free form” with a battery powered multimeter (using either the ohmmeter or diode setting).

Problems caused
It is impossible to properly test leakage current with a battery powered multimeter. The output voltage and the meter’s impedance are completely unknown. This will certainly lead to invalid and/or unreliable readings. Remember, the DUT must be tested under the correct voltages, and the correct conditions, to guarantee reliable and repeatable results. Using a multimeter, no details regarding the DUT’s actual leakage current level can actually be measured or recorded in any meaningful way. A battery-powered multimeter can only reliably be used to test whether a clearly already defective DUT is either “open” or “shorted.”

Issue #2
Technicians attempting to test an RF power transistor for leakage current while the DUT is still “in-circuit” (i.e. still plugged-into or still soldered into the circuit).

Problems caused
Trying to test leakage current while the device is “in-circuit,” even if the circuit is powered-down, will most certainly provide erroneous and invalid measurements. The device cannot be properly isolated and tested when connected in a circuit with other parts connected to it. These other parts will alter the test results because they are indeed part of the test. Also, the leakage current attributed solely to contaminants on the printed circuit board (even solder flux and fingerprints) can be higher than the leakage current through the DUT itself.

Issue #3
Static discharge is applied to the DUT (improper device handling).

Problems caused
Static discharge can and likely will ruin (forever)

Design category: