1. Leila Moghadasi
Attachments of section 5
Evaluation of resonant tunneling Transmission Coefficient (TC) from
multilayer structures GaAlAs/GaAs
A theoretical study of resonant tunneling in multilayered GaAlAs / GaAs structures of four barriers of
GaAlAs enclosing three GaAs quantum wells as well as structure of five GaAlAs barriers enclosing
four GaAs quantum wells was presented. The spectrum of resonant energies and their dependence on
respective barrier structure were analyzed from calculated profiles of barrier transparency versus
energy computed at selected temperatures and Fermi levels. The present formalism was based on the
effective mass approximation and the results were via direct numerical evaluations.
Junctions between semiconductors of different types or semiconductors and metals (MS) form a
fundamental part of modern electronic components and circuits as well as semiconductor diode lasers
that compared to their counterparts enjoy many advantages such as being more robust, cheaper,
smaller and having an operating voltage of only 1.5-2 volts. Generally when two extrinsic
semiconductors aimed to form a diode are highly doped in so far as the Fermi level is located above
conduction band and under valence band in n-type and p-type, respectively, tunnel effect, electrons
tunneling through forbidden band, occurs. At reverse bias a large growing tunneling effect with
respect to voltage is observed, reaching to a maximum (resonant tunneling) when Fermi level in n-
type levels out valence band in p-type. With increasing forward bias, however, the tunnel effect
contributes less and less to hit a dip, producing the negative resistance portion, until finally the
forward voltage at larger amounts resembles that of an ordinary p-n junction.
Multilayer or heterostructures in definition are structures comprising of series of alternative super thin
quantum wells and barriers, alternation of 100 GaAs wells of 50°𝐴 enclosed by GaAlAs barriers of
25°𝐴, for instance. These are mostly applied in high velocity detectors in infrared regions. Initially,
the TC from multilayer GaAlAs/GaAs and the respective current was obtained and then computed
numerically for three layers in 1995, whereupon, our evaluation for four and five barriers was
expanded. The TC versus energy was drawn for bias voltages 0, 0.16 and 0.4v indicating a triple and
quadruplet peak around 0.05-0.1v for four and five barriers at bias zero, respectively. At this bias both
structures showed another following triple and quadruplet peaks around 0.30-0.45v, clearly widened
compared to first peak. At bias 0.16v, multiple peaks with more distinguishable resonance shifted to
higher energies around 0.2-0.4v.This pattern was faded away at bias 0.4v, yet showing greater
transparency in comparison with lower bias instances, however .Furthermore, drawing the diagram of
TC versus energy at various bias voltages in low energies, around 0-5× 10−3(eV), we demonstrated
an interesting characteristics of heterostructures, that is, negative resistance. A significant drop in TC
was observed when bias voltage was raised from 𝑉𝑎 = 0.4 v to 𝑉𝑎 = 0.6v. The last study on these
structures of multiple barriers concerned the dependence of resonant tunneling to temperature. The
evaluation was conducted for three temperatures of 0, 77 and 300 °K whereby respective diagrams
were drawn. As we had expected, resonant triple and quadruple peaks associated with four and five
barriers, respectively, shifted to lower energies ensuing temperature growth.
In conclusion, it was demonstrated that Increasing bias led to higher TCs and that the greater number
of barriers is applied, the more efficient detection would be attained. With respect to the effect of
temperature, although temperature rise does not contribute to better transparency, it makes tunneling
current occur in lower energies. Exclusive feature of tunneling currents, negative resistance, also was
indicated.

2. Leila Moghadasi
Attachments of section 5
Domestic publication:
Evaluation of resonant tunnelling transmision coefficient from multylayer structures
GaAlAs/GaAs, L. Moghaddasi, A. Morteza Ali and R. Sabet-Dariani, Iranian Journal of
Physics Research 4(1), 91 (2004).