Browsing by Subject "Linearity"

Now showing 1 - 5 of 5

Open Access
AlGaN/GaN-Based laterally gated high-electron-mobility transistors with optimized linearity
(IEEE, 2021-02-01) Odabaşı, Oğuz; Yılmaz, Doğan; Aras, Erdem; Asan, Kübra Elif; Zafar, Salahuddin; Çankaya Akoğlu, Büşra; Bütün, Bayram; Özbay, Ekmel
In this work, highly linear AlGaN/GaN laterally gated (or buried gate) high-electron-mobility transistors (HEMTs) are reported. The effect of gate dimensions on source-access resistance and the linearity of laterally gated devices are investigated experimentally in detail for the first time. Transistors with different gate dimensions and conventional planar devices are fabricated using two-step electron beam lithography (EBL). Current-voltage, source-access resistance, small-signal, and two-tone measurements are performed to evaluate the linearity of devices. Contrary to conventional planar HEMTs, the intrinsic transconductance of laterally gated devices monotonically increases with increasing gate voltage, showing a similar behavior as junction field-effect transistors (FETs). The source-access resistance shows a polynomial increase with the drain current, which can be reduced by decreasing the filling ratio of the buried gates. Through the optimization of these two competing factors, i.e., intrinsic transconductance and the source-access resistance, flat transconductance with high linearity is achieved experimentally. The laterally gated structure shows flat transconductance and small-signal power gain over a larger span of gate voltage that is 2.5 times higher than a planar device. Moreover, 6.9-dB improvement in output intercept point (OIP3)/P DC is achieved. This approach can be used to improve the linearity of AlGaN/GaN HEMTs at the device level.
Open Access
Design of a high efficiency power amplifier by using Doherty configuration
(2010) Peker, Kazım
Power amplifiers (PAs) have their highest efficiency when they are used at full power (0dB back-off). For this reason, most PAs are used at 1dB compression point (P1dB), but this point is highly nonlinear. For high linearity, PAs should be used at some back-off value (below the point of 1dB compression point). In this case the efficiency of PAs decreases drastically. Another issue is that widely used digital modulation techniques produce signals which has a large peak-to-average power ratio (PAPR). In modern systems the power is reduced automatically to use spectrum efficiently and to prevent interference and detection. These conditions force new PA designs to have both high linearity and high efficiency from P1dB point down to a few dB back-off region.Doherty Amplifier technique uses Class-AB and Class-C amplifiers in parallel, and an increase in the efficieny especially at back-off regions occurs. By the use of parallel configuration P1dB point is improved. In the thesis, the theory of Doherty Configuration is explained, a Doherty Amplifier working at 4.75GHz is designed and simulated. A balanced amplifier is also designed and the results of both amplifiers are compared. The P1dB points of balanced amplifier and Doherty Amplifier are nearly same. In the Doherty case, a significant increase in efficiency is obtained at 6-dB back-off point and a little increase in efficiency is obtained at P1dB point. A Doherty Amplifier at 2GHz is implemented and its efficiency and linearity is compared with the implemented single amplifier. Significant increases are achieved both at P1dB point and at the efficiency.
Open Access
Dynamic power supply design for varying envelope signals using envelope tracking method
(2012) Özel, Muhlis Kenan
In modern communication systems, demand for higher data-rate is consistently growing. Higher data-rate within a limited bandwidth tends to require more amplitude modulation (AM) to increase number of symbols per second. Amplitude modulated carriers should be amplified using linear power amplifiers (PAs). Although, linear PAs have good performance in terms of linearity, they are efficient only when they transmit at maximum power. However, modern communication signals have high peak-to-average-power ratio (PAPR), therefore probability of a PA transmitting at maximum power is low. As a conclusion, efficiencies are degraded to alarmingly low values and the problem translates to heat and shorter battery life issues. In this thesis, we investigate how dynamic power supplies (DPS) perform for non-constant envelope RF signals. We have designed a DPS and used it to test efficiency enhancement of two PAs compared to the same PAs operating with constant supply. Our aim was to obtain efficiency increase compared to constant supply case without introducing extra non-linearity. We have obtained satisfying results for both amplifiers.
Open Access
Improved drain lag by reduced surface current in GaN HEMT via an ultrathin HfO2 blanket layer
(IOP Publishing Ltd, 2023-04-20) Güneş, Burak; Ghobadi, Amir; Odabasi, O.; Bütün, Bayram; Özbay, Ekmel
This paper reports the influence of an ultrathin 1.5nm atomic-layer-deposited HfO2 blanket layer as a gate dielectric on GaN high-electron-mobility transistors (HEMTs) grown on a 4H-SiC substrate. Transistors with a gate length of 250nm and a source-to-drain distance of 3µmwere manufactured. The proposed technique involves HfO2 deposition at 250◦C prior to the gate metallization with no additional lithography steps. This approach reduced the drain lag by 83%compared to the conventional design with no gate dielectric. The HfO2 layer suppressed the parasitic lateral conduction from the gate, reduced surface trapping, and improved gate electrostatics. The manufactured devices exhibited nearly three orders of magnitude decreased surface leakage, better turn-on behavior, and improved cut-off frequency fT linearity by 16%. High quality metal-oxide interface formation was confirmed by the conductance method. Results demonstrate that the blanket HfO2 deposition is a promising approach to improve the current dispersion characteristics and gate electrostatics of GaN HEMTs without incurring major changes to the established fabrication techniques.
Open Access
Improved robustness, stability and linearity in GaN based high electron mobility transistors for 5G applications
(2021-06) Odabaşı, Oğuz
5G technology requires high frequency and high power transistors. GaN-based high-electron-mobility transistors (HEMTs) are promising candidates to answer these needs. Although it is studied nearly for 30 years, there are still problems with this technology such as high non-linearity, unstable behavior and uncertain-ties in lifetime estimations, to take the lead. Some of the most important problems of GaN HEMT technology are studied elaborately in this thesis. Electro-thermal simulations are used to analyze the heating behavior of HEMT power devices under operation and a new more re-alistic model is developed. Fabrication of devices is governed and highly linear transistors with 6.4 V gate voltage span are achieved by using ﬁn-like structures. 6.5 dB improvement in OIP3 is obtained compared to conventional devices. Fi-nally, the stability and robustness of these devices are studied in the view of passivation. Signiﬁcant improvement in surface morphology, DC operation, long-term stability, pulsed IV performance, and forward gate bias stress durability has been demonstrated. Findings will help the implementation of GaN HEMT devices into 5G applications.