Semiconductor Fundamentals
Covalent Bonding of Silicon
HOLE FLOW
Hole flow is the same thing as electric current, which is in the opposite direction of the actual flow of electrons. The term hole flow is usually only applied to semiconductor
Around 1752,
Benjamin Franklin
developed his
theory on the flow of
electricity. Franklin
believed that electricity flows like a
fluid, and this fluid
flows from areas of
positive
charge to areas of
negative charge. It
would be over 100 years before it was understood that current flow was
actually the movement of
charged particles.
By the time science understood that electric current was
the movement of negatively charged electrons, it was too late to change the
standards, the textbooks, the schematic diagrams, and the generally accepted
theory. The direction of current flow was set as opposite to the actual flow
of the charge carriers, which we now know flow from areas of negative charge
to areas of positive charge.
In most cases, this is little more
than a historical curiosity. In basic
circuit analysis, it
doesn't really matter that everything is backwards, because the standards
and models still work. The behavior of the circuit is predictable. It
doesn't really matter which direction the electrons are moving, because the
concepts of
voltage drop,
electric fields,
magnetism, and all the
physical world that they interact with behave as we expect them to.
Electron flow direction, however,
played an important part in the invention of the
vacuum tube. It was
important to understand that the negative charge carriers were the ones that
were moving in order to understand the
Edison effect (the
boiling off of electrons) which formed the basis for the device, as well as
to develop the
control grid which
makes a vacuum tube act as an
amplifier.
The term "hole flow" came into use after the transistor was invented. A semiconductor depends on "holes" in the crystal lattice of the transistor caused by doping the semiconductor material. These holes are empty areas that the electrons can flow through in order to allow the material to act as a conductor rather than an insulator. In a p-n junction, which in its most simple case forms a diode, electrons can only freely travel from the p-type semiconductor layer (the side with the holes) to the n-type semiconductor layer. In the other direction (that is, n-type to p-type), the electrons do not see holes to enter the semiconductor material and the diode acts as an insulator.
From this, we can see that
the difference between current flow and electron flow makes a big difference
in truly understanding the physics behind semiconductor operation. To
maintain the positive to negative flow standard when talking about
semiconductors, we need to talk in terms of hole flow. That is, rather than
discuss the direction the electrons are traveling (which would be the
opposite direction of the electric current), we discuss the direction the
holes are "traveling". This is very
counter-intuitive, as empty space obviously
does not actually move, but necessary to maintain the standards set by
Benjamin Franklin back in 1752.