Journal of Computer and Communications, 2013, 1, 50-53
Published Online December 2013 (
Open Access JCC
THz Oscillations in a GaN Based Planar Nano-Device
K. Y. Xu1, Y. N. Wang2, Z. N. Wang2, J. W. Xiong2, G. Wang2
1Laboratory of Quantum Information Technology, School of Physics and Telecommunication Engineering, South China Normal
University, Guangzhou, China; 2State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University,
Guangzhou, China.
Received September 2013
Gunn oscillations in a GaN based planar nano-device have been studied by ensemble Monte Carlo (EMC) method. Si-
mulation results show that when the channel length of the device reduces to 450 nm, THz oscillations (about 0.3 THz)
can be obtained. Also the phase of the oscillations can be controlled by the initial conditions that excite the Gunn do-
mains. Moreover, through adjusting the phase difference between the oscillations in a double-channels device, which
attained by parallel connecting two single-channel devices, the frequency of the device shifts from 0.3 THz to 0.6 THz.
This phenomenon remains in devices with shorter channel-length, unless the channel-length is too short to support
Gunn osci llations . T he possible underlying mec ha nisms are a lso discuss ed.
Keywords: THz Oscillation; Gunn; Monte Carlo; GaN; Planar Nanodevice
1. Introduction
The terahertz (1 THz = 1000 GHz) electromagnetic wave
has attracted w ide attentions as it could enable very broad
applications ranging from non-destructive imaging and
spectroscopy of biological materials, remote detection of
hidden objects and explosives, to manipulations of quan-
tum states in semiconductors [1,2]. Moreover, develop-
ment of semiconductor THz electronic devices is also cer-
tainly paramount and timely to future generation of large-
volume information processing and high-performance
computations. However, the development of THz tech-
nology is so far largely hampered by the lack of reliable,
solid-state sources operating at room-temperature [3].
One possible way of developing THz devices is to
adapt well-known mechanisms that have been already
utilized in microwave f ield for higher frequency applica-
tions. Among them, Gunn Effect in GaN is considered as
one of the most promising candidates for further THz
sources, which has attracted wide attention recently [4-8].
Heat dissipation is considered as one of the most chal-
lenging issues for practical GaN-based Gunn sour ces. To
conquer this problem, planar devices, such as SSDs [9],
may be a suitable choice, since planar architecture allows
easier design for heat dissipation [5,8]. In addition, in
planar devices the electrodes are connected side by side
to the active semiconductor layer rather than placed on
top of each other, as in conventional multilayered vertical-
structured devices, resulting in very low parasitic capa-
citances. As such, high speeds are attainable.
In this work, we focus on studying the behaviors of
Gunn oscillations in GaN based SSDs at room tempera-
ture. SSDs as shown in Figure 1 are planar nanodevices,
which not only have high operation speed, up to THz at
room temperature, but also benefit for the propagation of
Gunn domains [10]. The paper is structured as follows.
In Section 2, device structures and simulation models are
introduced. In Section 3, the time-dependent Gunn oscil-
lations are studied under different bias conditions. Then
the relationship between the initial phase of the oscilla-
tion and the exciting condition is established. And moreo -
ver the interaction of Gunn oscillations in adjacent double
channels is further studied under different phase condi-
tions. In Section 4, simulation results are further dis-
2. Device Structures and EMC Model
Figure 1(a) shows schematically the top view of a
double-channels device including two SSDs and will be
mentioned as D-SSD in follows. The device is based on a
GaN/AlGaN heterostructure, where a 2DEG is formed at
the hetero-interface with a carrier concentration of 8.0 ×
1012 cm2 [8]. For a single-channel SSD (S-SSD) framed
by green dashed line in Figure 1(a), the two L-shaped
insulating trenches are etched through the 2DEG layer,
which ensures that electrons have to pass the narrow
channel between the two trenches in order to conduct a
THz Oscillations in a GaN Based Planar Nano-Device
Open Access JCC
0.5μm0.45μm 1.0μm
terminal 2
Etched area
Etched area
Etched area
terminal 1
Figure 1. Schematic top view (a) and side view (b) of the
simulated T-SSD (not to scale). The gray areas and the
white areas in the top view represent 2DEG and insulating
trenches, respectively. The green dash-lines delineate the
typical structure of single SSDs. An interface of AlGaN/
GaN heterostructures in (b) is just 30 nm bellows the de vice
surface, at which a sheet of 2DEG forms.
current between the left and right terminals. The two
S-SSDs are both designed with a channel width of 50 nm
and a trench width of 30 nm. Other geometric parameters
are defined in Figure 1(a). In the D-SSD, the two S-
SSDs have separated left terminals, but share the same
right terminal. The design of sharing the same right ter-
minal will benefit the interactions between channels, and
that of using different left terminals will entitle us to con-
trol the Gunn oscillations through applied voltage s .
In order to obtain the operation properties of the de-
vices, a semi-classical 2D EMC method self-consistently
coupled with 3D Poisson equations is used here. This
2D-3D combined model is developed from our entirely
2D EMC model, which has been used in earlier work
[10-13]. Details of the above 2D-3D combined model
and the comparison with fully 3D EMC method [14,15],
can be found in our recent work [16]. Figure 1(b) shows
schematically the side view of the simulated SSDs. In
order to properly include the 3D electric-field coupling,
Poisson equations are solved in a domain beyond the
geometric structure of the SSDs. As one can find from
Figure 1(b), a volume with a height of one micrometer
above the device surface is also included. The dielectric
constant used in the simulations for Air, AlGaN and GaN
are 1, 8.5 and 8.9, respectively. In order to model the
influence of surface states at the semiconductor-air inter-
face, a uniform negative charge density, 0.8 × 1012 cm2,
is also added at the edge of the insulating trenches during
the simulations. All simulations are carried out at room
3. Simulation Results
In this section, EMC method is used to study the Gunn
oscillations in S-SSDs and D-SSDs. Sim ulations are firstly
carried out on S-SSDs under different bias condition to
study the voltage dependence and the initial phase of
Gunn oscillations. Then the interaction of Gunn oscilla-
tions is further studied in D-SSDs under different phase
3.1. Single-Channel Devices
For S-SSD simulations, the left terminal of the device is
always grounded and the current output from the right
terminal is recorded. Fig ure 2 shows the time-dependent
current output of an S-SSD when a serial of step voltages
are applied on the right terminal. A current peak with
sharp rise and fall is shown after each abrupt change of
the applied voltage, which is anticipated as a result of the
charging of parasitic capacitances in the device [10]. This
parasitic-capacitance-induced current-peak will be men-
tioned as PCIC peak in the following studies. The current
after each PCIC peak increases with the applied voltage
and shows obvious oscillation behaviors when the ap-
plied voltage beyond 16V. Since the length of the chan-
nel is 450 nm, the threshold electric field for Gunn os cil-
lation would be about 0.4 MV/cm, which agrees with
recent experimental results [4]. Recently, by using an
entirely 2D EMC model, SSDs with a wider channel-
width have been studied and a special mode with two
domains simultaneously forming in the channel have
been found, but the mechanism is still puzzled [8]. The
period of the oscillations shown in Figure 2 is about 3 ps,
corresponding to frequency about 0.3 THz. The period of
the oscillations is also the time for Gunn domain to travel
through the channel, so that the drift velocity of the Gunn
domain should be about 1.5 × 107 cm/s, which is close to
the saturated velocity of the electrons in GaN [17]. This
result reveals that in our case, like those in traditional
Gunn diodes, only one effective domain simultaneously
Figure 2. Current responses of a single SSD showed in Fig-
ure 1 under the application of a series voltage steps.
THz Oscillations in a GaN Based Planar Nano-Device
Open Access JCC
3.2. Phase Control
Further investigations tell us that the initial phase of the
Gunn oscillations is dependent on what time the step
voltage is applied. If the time interval of two step voltag -
es is Ti, the phase-shift between the corresponding ex-
cited-oscillations will be 2πTi/T, where T is the period of
the oscillations. Typical results are shown in Figure 3.
The applied step voltages change their values from 0 V
to 21 V at 2 ps and 3.5 ps respectively, resulting in two
PCIC peaks. Since these PCIC peaks are too strong and
with non-needing information for our topic, only parts of
them are shown in Figure 3. One can find that the phase
of the two oscillations is opposite, because the time in-
terval is just half of the oscillation period.
3.3. Double-Channel Devices
During D-SSD simulations, the right ter minal is grounded
and two different negative step voltages are applied to
the two left terminals, respectively. Results for special
time interval Ti = 0 ps and Ti = 1.5 ps are shown in Fig-
ure 4. PCIC peaks are also not shown in whole as those
in Figure 3. The time-dependent output-current induced
by step voltages with zero time-interval is similar to that
in S-SSD. However, that induced by step voltages with
1.5 ps interval shows obvious second-harmonic oscilla-
tion with period of about 1.5 ps, just half of that in S-
SSD, corresponding to a frequency of about 0.6 THz.
The distinct deviation of current output between in
phase and out phase oscillations may come from the fol-
lowing facts. The two channels are identified, so the po-
tential distributions along the two channels should be the
same for the in-phase case. But for the out-phase case,
the potential distributions must be different and might
also changes with time. As a result, in the out -phase case,
the two oscillations will mutually modulate. For simpli-
fication, the Gunn oscillations can be assumed to obey
sinusoidal function with frequency of f, so that the mu-
tual modulation could be reasonably described with the
same sinusoidal function. Consequently, the current out-
put would be proportional to the square of the sinusoidal
function, which possesses a fr e q uency of 2f .
Figure 3. Current responses of a single SSD showed in Fig-
ure 1 when the applied voltage changes from 0 to 21 V at (a)
2 ps and (b) 3.5 ps.
Figure 4. Current re sponses of a T-SSD showed in Figure 1
for (a) without and (b) with self-gating effect.
It is well known that when two oscillators are put to-
gether, they would show synchronized oscillations [18].
As such, more simulations are carried out for D-SSD in
the out-phase case to exa mine the persis tence of the above
modulations. Results show that even in a much longer
simulation time (longer than 300 ps which including at
least 600 periods of oscillations), the waveform of the
current output does not change. Moreover, further inves-
tigations show that the above phenomenon still exists in
shorter-channel D-SSDs so that oscillations with higher
frequency can be obtained. For an instance, when the
channel length is reduced to 300 nm, the corresponding
frequency is raised to about 1 THz, results not shown
4. Conclusions
In conclusions, Gunn oscillations in GaN based S-SSDs
and D-SSDs have been studied by ensemble Monte Carlo
(EMC) method in detail. We show that since the channel
length of the device is extremely short (only 450 nm) the
oscillations in S-SSDs operate with a frequency about 0.3
THz. Also the initial phase of Gunn oscillations can be
controlled by applied voltage. Moreover, for D-SSD s, by
adjusting the phases of two oscillations in each channel
to be contrary, the frequency of Gunn oscillation can be
increased from 0.3 THz to 0.6 THz. This phenomenon
may attribute to the mutual modulations between the two
oscillations and occurs even in shorter channels so long
as Gunn oscillation can occur, resulting in oscillations
with frequency up to 1 THz.
5. Acknowledgements
This work was supported by FOK YING TONG Educa-
tion Foundation (No. 122004), Natural Science Founda-
tion of Guangdong Province, China (No. S2013010012711)
and NSFC (Grands U0934002).
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