Leakage current

leakage current

1. Leakage (electronics)
From Wikipedia, the free encyclopedia
In electronics, leakage may refer to a gradual loss of energy from a charged capacitor. It is primarily
caused by electronic devices attached to the capacitors, such as transistors or diodes, which
conduct a small amount of current even when they are turned off. Even though this off current is an
order of magnitude less than the current through the device when it is on, the current still slowly
discharges the capacitor. Another contributor to leakage from a capacitor is from the undesired
imperfection of some dielectric materials used in capacitors, also known as dielectric leakage. It is a
result of the dielectricmaterial not being a perfect insulator and having some non-zero conductivity,
allowing a leakage current to flow, slowly discharging the capacitor.
[1]

Contents
 1 In capacitors
 2 In semiconductors
 3 See also
 4 References
In capacitors[edit]
Another type of leakage occurs when current leaks out of the intended circuit, instead flowing
through some alternate path. This sort of leakage is undesirable because the current flowing through
the alternate path can cause damage, fires, RF noise, or electrocution.
[2]
Leakage of this type can be
measured by observing that the current flow at some point in the circuit doesn't match the flow at
another. Leakage in a high-voltage system can be fatal to a human in contact with the leak, as when
a person accidentally grounds a high-voltage power line.
[3]

Leakage may also mean an unwanted transfer of energy from one circuit to another. For example,
magnetic lines of flux will not be entirely confined within the core of a power transformer; another
circuit may couple to the transformer and receive some leaked energy at the frequency of the
electric mains, which will cause audible hum in an audio application.
[4]

Leakage current is also any current that flows when the ideal current is zero. Such is the case in
electronic assemblies when they are in standby, disabled, or "sleep" mode. These devices can draw
one or two microamperes while in their quiescent state compared to hundreds or thousands of
milliamperes while in full operation. These leakage currents are becoming a significant factor to
portable device manufacturers because of their undesirable effect on battery run time for the
consumer.
[5]

In semiconductors[edit]
In semiconductor devices, leakage is a quantum phenomenon where mobile charge carriers
(electrons or holes) tunnel through an insulating region. Leakage increases exponentially as the
thickness of the insulating region decreases. Tunneling leakage can also occur
across semiconductor junctions between heavily doped P-type and N-type semiconductors. Other
than tunneling via the gate insulator or junctions, carriers can also leak between source and drain
terminals of a Metal Oxide Semiconductor (MOS) transistor. This is called subthreshold conduction.
The primary source of leakage occurs inside transistors, but electrons can also leak between
interconnects. Leakage increases power consumption and if sufficiently large can cause complete
circuit failure.

2. Leakage is currently one of the main factors limiting increased computer processor performance.
Efforts to minimize leakage include the use of strained silicon, high-k dielectrics, and/or
stronger dopant levels in the semiconductor. Leakage reduction to continue Moore's law will not only
require new material solutions but also proper system design.
Certain types of semiconductor manufacturing defects exhibit themselves as increased leakage.
Thus measuring leakage, or Iddq testing, is a quick, inexpensive method finding defective chips.
Increased leakage is a common failure mode resulting from non-catastrophic overstress of a
semiconductor device, when the junction or the gate oxide suffers permanent damage not sufficient
to cause a catastrophic failure. Overstressing the gate oxide can lead to stress-induced leakage
current.
In bipolar junction transistors, the emitter current is the sum of the collector and base currents. Ie =
Ic + Ib. The collector current has two components, minority carriers and majority carriers. The minority
current is called the leakage current
[c larific ation needed]
.
Leakage current is generally measured in microamperes. For a reverse-biased diode it is
temperature sensitive. Leakage current must be carefully examined for applications that work in wide
temperature ranges.

What is Leakage Current?
In low voltage and electronics Leakage Current is any current that flows when the ideal current is zero.

In medium and high voltage applications it is the current that flows either through the body or over the
surface of an insulator. Leakage current often arises when outdoor insulation is submitted to conductive
fog (sea mist/spray, industrial smogs) or when a pollution layer containing soluble salts is formed on the
insulator surface and then wetted, for example by dew or mist. These currents can reach several
hundred mA and may lead to flashover of the insulation.

What is leakage current of a pn-junction and how to measure it?

A leakage current is the current which is flowing thought a pn-junction when is
inversely polarized. As the definition says in order to measure the leakage current
of a pn-junction you simply have to apply a voltage in the reverse direction of the
junction and measure the resulting current. A schematic picture of this process is
presented in Figure 1.
Due to the fact that on an integrated chip, e.g. SDRAM chip, there are a huge
number of pn-junctions, leakage currents are of a paramount importance and
should be kept as small as possible in order to reduce the current consumption of
the whole chip.

3. Figure 1: Leakage current of a pn-junction
The most important reason for leakage currents are defects in the substrate, which can
appear during the implantation process of the ph-junctions. Such “implantation defects” can
be eliminated by a suitable anneal after the implantation, however if the anneal procedure is
not well optimized not all implantation defects will be eliminated, which in turn will increase
the leakage current of the junction.
Another reason for high pn-leakage currents are the so called TiSi-grains. In order to make
a contact to a pn-junction usually a thin layer of Titanium (Ti) is sputtered on/in the
contact. After that the contact is annealed. During the annealing process Ti is reacting with
Si to form TiSi. The interface between TiSi and Si-substrate is not smooth, but has a
“grain”-like structure (see Figure 1), which can degrade the leakage current significantly if
are not well controlled by the annealing process.