High field transport phenomena in wide bandgap semiconductors
Author
Sevik, Cem
Advisor
Bulutay, Ceyhun
Date
2003Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
The Ensemble Monte Carlo (EMC) method is widely used in the field of computational
electronics related to the simulation of the state of the art devices.
Using this technique our specific intention is to scrutinize the high-field transport
phenomena in wide bandgap semiconductors (Such as GaN, AlGaN and AlN).
For this purpose, we have developed an EMC-based computer code. After a brief
introduction to our methodology, we present detailed analysis of three different
types of devices, operating under high-field conditions, namely, unipolar n-type
structures, avalanche photodiodes (APD) and finally the Gunn diodes. As a testbed
for understanding impact ionization and hot electron effects in sub-micron
sized GaN, AlN and their ternary alloys, an n
+−n−n
+ channel device is employed
having a 0.1 µm-thick n region. The time evolution of the electron density along
the device is seen to display oscillations in the unintentionally doped n-region, until
steady state is established. The fermionic degeneracy effects are observed to be
operational especially at high fields within the anode n
+-region. For AlxGa1−xNbased
systems, it can be noted that due to alloy scattering, carriers cannot acquire
the velocities attained by the GaN and AlN counterparts. Next, multiplication
and temporal response characteristics under a picosecond pulsed optical illumination
of p
+-n-n
+ GaN and n-type Schottky Al0.4Ga0.6N APDs are analyzed. For
the GaN APD, our simulations can reasonably reproduce the available measured
data without any fitting parameters. In the case of AlGaN, the choice of a Schottky
contact APD is seen to improve drastically the field confinement resulting in
satisfactory gain characteristics. Moreover, alloy scattering is seen to further slow
down the temporal response while displacing the gain threshold to higher fields.
Finally, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz
to 650 GHz are studied in detail by means of extensive EMC simulations. The
basic operation is checked under both impressed single-tone sinusoidal bias and
external tank circuit conditions. The width of the doping-notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency
up to a critical value, beyond which sustained Gunn oscillations are ceased. The
degeneracy effects due to the Pauli Exclusion principle and the impact ionization
are also considered but observed to have negligible effect within the realistic operational
bounds. Finally, the effects of lattice temperature, channel doping and
DC bias on the RF conversion efficiency are investigated
Keywords
High field transportUnipolar devices
Gunn diodes
Avalanche photodiodes
Ensemble Monte Carlo technique