Page 46 - A Practical Guide from Design Planning to Manufacturing

22 Chapter One

reduces the long-term reliability of the transistors. Any material will con-
duct electricity if a sufficient electrical field is applied. In the case of insu-
lators this is called dielectric breakdown and physically melts the
material. At extremely high electric fields the electrons, which bind
the molecules of the material together, are torn free and suddenly large
amounts of current begin to flow. The gate oxides of working MOSFETs
accumulate defects over time that gradually lower the field at which the
transistor will fail. These defects can also reduce the switching speed
5
of the transistors. These phenomena are particularly worrisome to
semiconductor manufacturers because they can cause a new product to
begin failing after it has already been shipping for months or years.
The accumulation of defects in the gate oxide is in part due to “hot”
electron effects. Normally the electrons in the channel do not have enough
energy to enter the gate oxide. Its band gap is far too large for any sig-
nificant number of electrons to have enough energy to surmount at
normal operating temperatures. Electrons in the channel drift from
source to drain due to the lateral electric field in the channel. Their aver-
age drift velocity is determined by how strong the electric field is and how
often the electrons collide with the atoms of the semiconductor crystal.
Typically the drift velocity is only a tiny fraction of the random thermal
velocity of the electrons, but at very high lateral fields some electrons may
get accelerated to velocities much higher than they would usually have
at the operating temperature. It is as if these electrons are at a much
higher temperature than the rest, and they may have enough energy to
enter the gate oxide. They may travel through and create a current at
the gate, or they may become trapped in the oxide creating a defect. If a
series of defects happens to line up on a path from the gate to the chan-
nel, gate oxide breakdown occurs. Thus the reliability of the transistors
is a limit to how much their dimensions can be scaled. In addition, as gate
oxides are scaled below 5 nm, gate tunneling current becomes significant.
One implication of quantum mechanics is that the position of an elec-
tron is not precisely defined. This means that with a sufficiently thin
oxide layer, electrons will occasionally appear on the opposite side of the
insulator. If there is an electric field, the electron will then be pulled
away and unable to get back. The current this phenomenon creates
through the insulator is called a tunneling current. It does not damage the
layer as occurs with hot electrons because the electron does not travel
through the oxide in the classical sense, but this does cause unwanted
leakage current through the gate of any ON device. The typical solution
for both dielectric breakdown and gate tunneling current is to reduce
the supply voltage.

5
Chen, “Dynamic NBTI of p-MOS Transistors.”