Measuring leakage current in RF power transistors

Measuring leakage current in RF power transistors

Technology News |
By eeNews Europe

What, specifically, is “transistor leakage current”?

A transistor can be thought of as a simple “ON/OFF” semiconductor device. In an ideal sense then, the transistor only allows DC current to flow through it when it is “ON” (i.e. properly biased and with the proper DC supply voltages applied), and allows zero DC current flowing through it, when it is “OFF.” In reality, a small amount of DC current still flows through all transistors, even when they are in their “OFF” state, as long as the DC supply voltages are applied. This relatively low-level of DC “OFF” current is commonly referred to as transistor leakage current. Leakage current is present in every type of transistor, using any semiconductor technology (Bipolar, CMOS, VMOS, LDMOS, GaAs, GaN, etc.).

Note: Leakage current for bipolar junction transistors (BJT) is commonly referred to as ICEO, the collector-emitter cutoff current (base open). This is one reason why you seldom find the words “leakage current’ in older transistor data sheets and data books. With the advent of Field-Effect Transistors (FETs), and the subsequent FET technology advancements (VMOS, LDMOS, etc.), BJTs have decreased dramatically in usage as RF power transistors.

“Normal” leakage current, or the expected amount of leakage current that is within a given part’s specifications, is due mainly to imperfections and limitations in the transistor die. The actual causes of this expected leakage current are beyond the scope of this paper.

Leakage current specifications for transistors

Leakage current is specified today in virtually every transistor data sheet. For the most part, though, the leakage current specifications are rarely noticed, and are almost never a cause for concern by RF power design engineers. This is because leakage current is typically very low, usually in either the low µA range or even the nA range. Since leakage current is so low, it is only considered “design-impacting” when:

  • The transistor is used in extremely low power applications; or,
  • In rare cases, the transistor is used in designs where extremely tight bias current limitations exist; or,
  • In some cases, a given transistor unit in the field is found to be “out of specification” with regard to its published leakage current specification.

A specific example for leakage current specification

For field effect transistors (FET), leakage current is usually specified for both drain-to-source current (IDSS) and for gate-to-source current (IGSS), as in this excerpt from an actual data sheet:

Figure 1: Example of leakage current specifications for FET device. (Click on image for larger version).

Notice that the specifications for leakage current are dependent on certain conditions of the transistor device under test (DUT). In the example above, “Table 5” includes the device’s electrical characteristics, all to be tested at a case operating temperature of TC = 25˚ C (unless otherwise noted). The Zero Gate Voltage Drain Leakage Current, IDSS, is specified twice as a maximum value, with two different drain-to-source voltages (VDS = 66 VDC and VDS = 28 VDC). In each case the gate-to-source voltage is specified at zero volts (VGS = 0 VDC), i.e. the gate and the source of the DUT are shorted together to properly test IDSS. “IDSS Max,” or the maximum allowable value for drain leakage current, is clearly specified for this device as up to 10 times higher with VDS = 66 VDC than it is with VDS = 28 VDC. Gate-Source leakage current, IGSS, is specified once, with VGS = 5 VDC and VDS = 0 VDC, which means the drain and the source of the DUT are shorted to test IGSS.

Key starting point: In order to determine if the leakage current on an actual device is within its own printed specification, care must be taken to reproduce the proper testing conditions.

Figure 2: Always use proper test conditions and calibrated methods/procedures.

To determine whether or not a given transistor sample meets its leakage current specification, absolute care must be taken to properly test and evaluate the device.

Proper Conditions

  1. Always test per the manufacturer’s specifications (temperature, voltages, shorts, etc.).
  2. The device itself must be free of foreign substances, dirt, dust, and other contaminants.
  3. Isolate the DUT. The device cannot be properly isolated and tested when soldered into a board with other parts connected. 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 board (even solder flux and fingerprints) can be higher than the leakage current through the DUT itself.
  4. Always use properly calibrated laboratory-grade test equipment.
  5. Never use a battery operated “multimeter” to test leakage current.
  6. Extreme care must be taken with regard to the testing environment itself and the procedures employed in performing the tests as well. Each must:
    • Comply with all industry Electrostatic Discharge (ESD) requirements.
    • Employ proper lab grounding techniques (equipment and technician).
    • Ensure that technicians are trained properly for the task.
    • Ensure that isolation techniques are employed for the DUT…
    • Use a proper, calibrated test fixture and shielded test leads.
    • Providing shielding from light (for the DUT) and some filtering/shielding from other noise sources (AC line, RF, etc.) may be necessary to detect low nA currents.

Proper testing methods

There are at least three acceptable methods for testing leakage current in RF power transistors:

  1. Calibrated lab power supply and calibrated (µA or nA) ammeter. (Good)
  2. Calibrated programmable semiconductor tester. (Better)
  3. Calibrated curve tracer. (Best)

Figure 3: Calibrated curve tracer (the best overall choice for measuring one device).

Current conditions in “Real World” leakage current testing

Engineers and technicians know that a working transistor with too much leakage current, that is to say a working transistor with “out-of-spec” leakage current, can indeed be a problem in the field. Such a device can cause early field failures, exhibit poor performance, be an unnecessary drain 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) an otherwise good device.

Issue #4
Improper grounding/shielding/isolation methods are employed with the DUT.

Problems caused
Testing with improper shielding, grounding, and isolation will lead to erroneous and invalid measurements; and it may also damage the part.

Issue #5
Uncalibrated test equipment being used.

Problems caused
Using uncalibrated test equipment will produce erroneous and invalid measurements.

Issue #6
Devices not tested at the proper temperature.

Problems caused
If the parts are not tested to all manufacturers’ specifications, then there is no way to prove that a part does not meet its specifications.

Improper leakage current testing lead to these unfortunate consequences:

  1. Both “good” and “bad” devices are being damaged unnecessarily, and this makes further analysis impossible.
  2. Some devices that are actually within specification are being thrown away or being rejected/returned because of erroneous test results. This happens at the “incoming inspection” stage, in some cases.
  3. Root causes of other RF transistor problems are being “masked” (i.e. undetected and undiagnosed) if leakage current is improperly tested and labeled the “cause.”

Final recommendations:

  1. All RF components and subassemblies used in complex and critical designs (i.e. military, avionics, broadcast, etc.) should be parametrically evaluated in order to make sure that they meet published specifications.
  2. Component testing must be performed properly. Carefully review your test set-up and methods to insure that your leakage current (and other testing) is safe, calibrated, reliable, and repeatable.
  3. As a service offering, Richardson RFPD can help customers to perform DC testing (including leakage current) and/or RF testing on components and assemblies that we supply. In our testing facilities, fully automated test systems are employed to test volume production parts, including those parts on tape and reel. All test data is electronically stored and can be supplied per request. Critical component testing can increase field reliability, improve product performance, and save cost. Richardson RFPD uses trained technicians, fully-calibrated test equipment, and adheres to all manufacturer and industry testing specifications.

More information on this topic is available at Richardson RFPD’s RF Power Transistor Matching, Testing, and Sorting web page.

About the author

William R. (Bill) Murphy earned a Bachelor of Science degree in Electrical Engineering (BSEE) from the University of Illinois at Urbana-Champaign, an MBA degree from the University of Chicago, and an MSEE degree from the Illinois Institute of Technology (IIT). He is currently the Technical Marketing Manager for Richardson RFPD, Inc. in LaFox, Illinois. He began his career as an RF design engineer and later served in various marketing and business management roles in the telecom supplier business sector. He is also a U.S. patent holder.

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