Page 273 - Semiconductor For Micro- and Nanotechnology An Introduction For Engineers

Interacting Subsystems
two processes involved in sequence and thus we must go beyond first
order perturbation theory. The consequence is that the two transition
probabilities for the processes multiply to give the total transition proba-
bility and thus we may immediately identify indirect interband transi-
tions as less probable.

Intra-band transitions are indicated as (A) in Figure 7.9, e.g., between the
heavy hole and light hole valence band. In this case the electrons change
from one subset to another and remain in the same band. We therefore
count these processes for the same category as the interband transitions.
A very important process is the so called free carrier absorption. The
electrons remain in their subset of either the valence or conduction band.
These processes have to be distinguished from transport effects happen-
ing inside a subset of a band.

Emission of photons from electron in the conduction band has been
observed even from within industrially applied devices. Due to sub-
micron gate lengths in modern Field Effect Transistors the electric field
5
under certain operating conditions can exceed 10 Vcm⁄ . Some of the
carriers accelerated by such field strengths may gain up to 1 eV in energy.
A small fraction of these carriers has been shown to be able to give up
this kinetic energy by emission of a photon. A pretty illustration of this
light emission is found in [7.18]. A ring oscillator consisting of 47 invert-
ers, as it is usually found in microprocessor clocks, has been observed by
means of a photomultiplier. A time integrated image of the ring oscillator
shows infrared and far infrared activity spatially resolved in the single
drain regions of the inverter devices. This observation technique is
assumed to become even a standard diagnostic tool for monitoring the
functionality of modern integrated circuit devices [7.18].

7.4.2 Semiconductor Lasers
We start our discussion on the laser effect in semiconductors with a short
explanation of the principle effect by means of a two level atom model.

270 Semiconductors for Micro and Nanosystem Technology