Leakage current Issues

Published:2011/8/30 2:35:00 Author:Phyllis From:SeekIC

The specification for leakage current is a big concern for engineers and technicians using RF power devices. Problems like what the specifications mean in terms of the part’s performance in the field, and how to properly test/verify that a given part is meeting its printed leakage current specification are getting more and more popular.

A transistor can be regarded as a simple “ON/OFF” semiconductor device. The ideal effect is, the transistor only allows DC current to flow through it when it is “ON” and only zero DC current when it is “OFF.” As a matter as fact, even when all the transistors are OFF, a small amount of DC current still flows through them 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.

The expected amount of leakage current is within a given part’s specifications and is mainly due to imperfections and limitations in the transistor die.

Leakage current is specified in every transistor data sheet today. Though the leakage current specifications are rarely noticed 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 when the transistor is used in designs where extremely tight bias current limitations exist; 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.

There are at least three acceptable methods for testing leakage current in RF power transistors: Calibrated lab power supply and calibrated (µA or nA) ammeter; Calibrated programmable semiconductor tester; and Calibrated curve tracer.

A working transistor with too much 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, 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.

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