Fundamentals on Bionanotechnologies - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 14

a b

I

negative differential
resistance region

E F1

E res

E F2

E c

eV

V

Fig. 1.13 ( a ) Energy band representation of a resonant tunneling diode, and ( b ) a typical current-
voltage characteristic

a single quantum process through the whole structure, this expression can be used
also if sequential tunneling occurs.
The most notorious device based on the resonant tunneling phenomenon is
the resonant tunneling diode (RTD), displayed in Fig. 1.13 . The RTD is a biased
double-barrier structure, which has a single resonant energy level E res in the
quantum well region that filters the electrons from the left contact (emitter). In
consequence, only the electrons with energy E res from all electrons in the emitter
with energies between the Fermi energy E F1 and the bottom of the conduction band
E c can tunnel toward the contact on the right (collector) with Fermi energy E F2 .
As a result, the I V characteristic of the RTD displays a negative differential
resistance (NDR) region since E res drops below E c at a sufficiently high bias and
electron tunneling into the collector is no longer possible. Consequently, the current
decreases drastically. The NDR of the RTD occurs even at room temperature.
RTDs have numerous applications as bistable elements, oscillators, or logic circuit
components ( Dragoman and Dragoman 2009a ).
When a constrained structure such as a quantum well, quantum wire, or quantum
dot is positioned in the immediate neighborhood of another constrained structure of
the same type, the interaction strength between electrons in the two structures
depends on the height and width of the barrier separating the confined structures.
The quantum wells (quantum wires or quantum dots) are said to be coupled when the
electrons interact and the electron wavefunction expands over the whole structure,
whereas when the electrons in neighboring structures do not interact, we have a
series of noninteracting structures, called multiple quantum wells (quantum wires
or quantum dots). In the last case, the electrons localized in one quantum well
(quantum wire or quantum dot) are transferred to the other confined structure
only through sequential tunneling. Nanoscale structures separated by finite barrier
regions are coupled by the exponentially decaying electron wavefunction in the
barriers, which allows overlapping of the envelope electron wavefunctions in
neighboring wells if the potential barriers are thin enough.