hole speed in semiconductor

Discussion in 'Electronic Basics' started by steve, Feb 21, 2004.

1. steveGuest

Could anyone explain why hole speed in semiconductor material is less
than electron speed?

TIA
Steve

2. Joe McElvenneyGuest

Hi,

The simple answer, but one that begs the question, is that the
effective mass of a hole is greater than that of an electron and so it
requires a greater force to accelerate it to a given velocity. Of
course you might rightly think that a hole doesn't have any real mass.
However, for it to move in the valence band, it has to become involved
in the transport of an electron and it is this process that slows it
down making it appear heavy. Up in the conduction band though,
electrons really fly at around twice the speed.

Cheers - Joe

3. steveGuest

However, for it to move in the valence band, it has to become involved
Could you suggest which book which discuss this? In all of books I've
read they are not discussing it, perhaps it is for higher level
learning?

I would really like to know how it is calculated though.

Do you teach this subject? perhaps I can attend a class which teach

Thank you very much
Steve

4. Joe McElvenneyGuest

Steve,

No, I am not an instructor and was simply quoting that bit on effective
hole mass from memory. I've retained a reader from a university course of
years ago, and sometimes dip into it, but don't know of a book that covers
the subject you want in any depth. In fact, generally in engineering texts,
it wouldn't serve much of a purpose to go too deeply into the physics of
carrier movement beyond that needed to understand device properties. You

http://en.wikipedia.org/wiki/Effective_mass

As for measuring carrier effective mass, one method was to use a form of
cyclotron where a crystal of the material under investigation was placed in
a magnetic field inside a very low temperature waveguide system. Swept
frequency RF energy was fed into the waveguide at one end and, at certain
frequencies (around 10^10Hz for the electron, I understand), the carriers
would move in circles consuming energy from the RF field. At the other end
the absorption peaks were measured against frequency and their effective
mass deduced from the relationship -

m* = B x e / 2 x pi x f(res)

m* is the effective mass
B is the magnetic flux density
e is the charge on an electron
f(res) is the absorption peak frequency

That's it - that's all I know! In fact I had to look up the last bit

Cheers - Joe

5. Roy McCammonGuest

Steve,

a theoretical construct has the properties that it has
is that it makes the theory work.

The best theory we have for what is going on is quantum
electrodynamics (QED) which is intractable (not impossible)
for most engineers (myself included). But the results of
that theory applied to semiconductors can be boiled down
to a simpler theory in which there are "holes" that are
visualized to behave as positively charged heavy electrons.

The theory works pretty good, but that doesn't mean
that the holes have objective existence. They don't
need a reason to be slow, other than that's what the
experiments suggest.

Now the following is just hand waving speculation,
but if it helps, you can think of it this way:
when an electron moves, it just moves. When a
hole moves, the lattice of electrons holding the
crystal together are rearranged.

6. Kevin AylwardGuest

Well, I'm going to quibble a bit here on the technicalities. Solid state
physics (or condensed matter physics) doesn't really require/use QED in
any direct sense. Quantum Mechanics on its own usually sufficient.

Holes are simply a method of noting that certain electrons go in the
other direction.
Arguable, electrons suffer the same fate.

"Physical concepts are free creations of the human mind, and are
not, however it may seem, uniquely determined by the external world" -
Einstein
Again, this suggests some sort of lack of theoretical backup. Basic
Quantum Mechanics fully and completely explains why holes have lower
mobility. Experimental support is pretty much a trivial consequence.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"That which is mostly observed, is that which replicates the most"
http://www.anasoft.co.uk/replicators/index.html

"quotes with no meaning, are meaningless" - Kevin Aylward.

7. Roy McCammonGuest

quibble noted. I won't argue that point. QM is also intractable
for most engineers. Instead, we have a theory involving holes and
currents and densities and numerous empirical constants. Its wonderful
if you have the math skills and believe in QM and can calculate
those constants, but all you've done is explain one theory by
deriving it from another. Qm also has empirical constants (though
not as many) and the only answer as to why those constants have
the value that they have is that it makes the theory work.
yes, I agree. But I didn't want to have to defend it.
I think we be going meta-physical here. Experimental support
is the reason for the existence of QM, QED, etc.

But, I'm eager to see your explanation, using basic qm and
tractable math as to why the holes are slow. I'll bet you
an ayl that less than 1 in 10 of the readers here will be

8. Kevin AylwardGuest

QM was certainly arrived at by a trial and error processe, both
experimental and theoretical, but QED is a bit more of a pure
theoretically based extrapolation/extension to QM.
I agree. That's why in some of my posts I keep pushing the black box
approach. One needs to know what something does, not why it does it.
Ultimately, if you have no control over it as an electronics design
engineer, it doesn't help much knowing why. The why in any case, is an
arbitrary why. We just reduce things to other e equally unexplainable
causes that we seem happy to accept as a self evident truth of the
universe, even its not.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

"That which is mostly observed, is that which replicates the most"
http://www.anasoft.co.uk/replicators/index.html

"quotes with no meaning, are meaningless" - Kevin Aylward.