The first essential point to note is that a modulation of an output signal obtained
by modulating some input always requires a change or modulation
in some internal state of the device. | |||||||||||||||||||||||||||

And changing something always takes some time. Nothing happens instantaneously, changing
something consumes some time. We thus may start by listing the time
consuming processes that we already encountered. | |||||||||||||||||||||||||||

What kind of typical time constants in semiconductors
did we encounter so far? Think about it for a minute. Well, we had | |||||||||||||||||||||||||||

The minority carrier life time . It measures the average time that a minority carrier "lives" before it recombines with a majority carrier.
It can be rather large for very clean indirect semiconductors (t
ms), and rather small for indirect semiconductors
(ns). The numerical value of a minority carrier life time implies that you cannot change the minority carrier concentration
at a frequency much larger than 1/t.We have a first limit
to how fast you can change an internal state. | |||||||||||||||||||||||||||

The dielectric relaxation time . It measures the average time that t
_{d}majority carriers need
to respond to some disturbance of their distribution. It was rather small, typically in the ps range and given by
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Those were the two fundamental material related time constants that we encountered so far. But there are more time constants which are not so directly obvious: | |||||||||||||||||||||||||||

First, we have the "trivial" electrical time constant inherent in any electrical
system, simply given by the t_{RC} product. R · C is the ohmic resistivity, and R
the capacitance of the circuit (part) considered. C | |||||||||||||||||||||||||||

and R need not be actual resistors or capacitors Cintentionally
included in the system, but unwanted, nevertheless unavoidable, components. The resistivity of Al metallization lines
together with the parasitic capacitance of this line in a Si integrated circuit. e.g., gives a t
of roughly _{RC}10, and this value (per ^{–9} scm line length) is directly determined by the product
of the specific resistivity r of the conducting material times the relative dielectric
constant eof the dielectric separating individual wires - it is thus a rather
intrinsic _{r}material property. | |||||||||||||||||||||||||||

The physical meaning of t
is clear: It is the time needed to charge or discharge the capacitors in the system. Clearly, you cannot
change internal states very much at frequencies much larger than _{RC}1/t. And
note that space charge regions, or _{RC}MOS structures always have a capacity ,
too.C | |||||||||||||||||||||||||||

Second, if we turn to Lasers for a moment,
we have seen that we need to feed some of the light produced
by stimulated emission back into the semiconductor by using a suitable mirror assembly. | |||||||||||||||||||||||||||

Light bounces back and forth between the two mirrors in the simple system considered - and that means that even after you turned off the current through the Laser diode, some light will still bounce back and forth and thus come out until everything eventually calmed down. There is an obvious time constant | |||||||||||||||||||||||||||

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With average number of reflections, N_{r} = distance
between the mirrors, L =n refective index of the material, and _{r} =
c = vacuum velocity of light. | |||||||||||||||||||||||||||

If, for an order of magnitude guess, we take and consider L = 100 µm10
reflections; the "last" photons to come out would have to travel 10 · 100 µm = 1 mm, which takes
them a time t. _{Q} = · nN_{r} · L_{r}/c » 10^{–11} s = 10 ps | |||||||||||||||||||||||||||

In other words, for the example given, it would not be possible to modulate the light intensity
with frequencies in excess of about 100 GHz. This seems to be a respectable frequency, but keep im mind that data
can now (2001) be transmitted through fibre optics at frequncies in the THz regime. | |||||||||||||||||||||||||||

This example, while a bit far-fetched, gives us an important insight: There is
a general relation between a time constant of a system and a typical
length of a system mediated by the speed with which things move. This means that the size
of a device may be important for its frequency response. | |||||||||||||||||||||||||||

In other words, we can always ask: How much time does it take to move things
over a distance ? And whenever the output l is some distance away from the input O
, the question of how long it takes to move whatever it takes
from In to In produces a typical time constant of the system. O | |||||||||||||||||||||||||||

In straight-forward simple mechanics is linked to its time constant lt by the _{l} speed
of the moving "things" - for the photons considered above this was clearly the speed of light (in the medium,
to be correct). | |||||||||||||||||||||||||||

For our moving statistical ensembles, we have somewhat more involved relations, e.g. . | |||||||||||||||||||||||||||

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What are the moving things? Well, besides photons, we essentially are left with
electrons and holes; everything else that might be of interest is usually immobile (dopants, localized excitons), or so
slow that it should not matter for electronic signals (phonons, mechanical movements
(e.g. vibrating parts) in MEMS devices) | |||||||||||||||||||||||||||

This brings us to a first simple and important question: How long does it take electrons or
holes to move from the source to the drain in a MOS transistor. Clearly, this will give us another maximum frequency
for operating said transistor. | |||||||||||||||||||||||||||

The relevant velocity in this case is the drift velocity
v of the carriers, usually proportional to the field strength _{D} as driving force for the
movement, and better expressed via the carrier mobility E | |||||||||||||||||||||||||||

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With the source-drain distance , and the source drain voltage l_{SD}
, we have U_{SD}
and a "travel time" E = U_{SD} /l_{SD} |
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To get a feeling for orders of magnitude, we take a source-drain distance and a source-drain voltage l_{SD}
= 1 µm, obtaining a field strength of U_{SD} = 5V. Typical mobilities are
E_{SD}
= 5 · 10^{4} V/cmm for _{Si} = 1000 cm^{2}/VsSi. This gives us a drift velocity of
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Is that a large or small velocity? It might be good to look up at an old exercise at this point | |||||||||||||||||||||||||||

The "travel time" t then is_{l}
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A "1 µm " Si MOS transistor thus would not be able to switch
frequencies beyound about 10 ^{11} Hz = 100 GHzif
t would be the only limiting time constant of the system._{l} |
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Last, there are some ultimate limits that we should be aware off: | |||||||||||||||||||||||||||

Nothing moves faster than c, the the speed of light (in vacuum). The consideration
for the Laser from above already gives an example for this limit. | |||||||||||||||||||||||||||

The movement of electrons and holes has some intrinsic constant of its own: The average
time between scattering processes and the average distance
or mean free path in between. While we are not very aware of the values for these parameters, the mean free path is in the
order of 100 nm. | |||||||||||||||||||||||||||

This has an important consequence: We only can use average
quantities like drift velocities, if individual carriers could have many collisions. | |||||||||||||||||||||||||||

Turning this around implies: If we look at travel scales around and below 100 nm, everything
may change. For transistors this small, electrons (or holes) might just speed from source to drain without any collisions
in between - much faster than at larger distances. This is the case of ballistic carrier
transport which must be considered separately. | |||||||||||||||||||||||||||

© H. Föll (Semiconductors - Script)