Pouya Valizadeh

Investigation of the impact of Al mole-fraction on the consequences of RF stress on Al/sub x/Ga/sub 1-x/N/GaN MODFETs

Variations of the low-frequency noise (LFN), power, and dc characteristics of a variety of SiN/sub x/ passivated AlGaN/GaN MODFETs with different values of Al mole-fraction, gate length, and gate drain spacing upon RF stress are investigated. It is experimentally evidenced that the variation of Al mole-fraction (x) of the barrier Al/sub x/Ga/sub 1-x/N layer from 0.2 to 0.4, has no considerable impact on the drain and gate low-frequency noise current characteristics. The most noticeable variation on the device characteristics upon long-term RF stressing has been on the pinch-off voltage. Although no material degradation by increasing the Al mole-fraction has been evidenced through the low-frequency noise data, it is observed that the variation of pinch-off voltage upon RF stressing becomes more important as the Al mole-fraction increases.

Effects of RF and DC stress on AlGaN/GaN MODFETs: a low-frequency noise-based investigation

The impact of radio frequency (RF) and DC stress on passivated and unpassivated AlGaN/GaN modulation-doped field effect transistors (MODFETs) is investigated by means of DC and low-frequency noise (LFN) measurements. Unpassivated devices endure significant changes in the output resistance, gate, and drain noise current level after RF and DC stress. RF and DC stress of unpassivated devices leads to different degradation time constant and gate noise current. Besides, a positive shift in the pinch-off voltage is found to take place only after RF stress. In contrast to unpassivated devices, passivated devices do not show any considerable variation in the output resistance and gate, and drain noise current characteristics upon RF or DC stress. However, a positive shift in the pinch-off voltage upon RF stress is observed for both types of devices.

Impact of RF stress on dispersion and power characteristics of AlGaN/GaN HEMTs

The impact of RF stress on dispersion and power characteristics of AlGaN/GaN HEMTs are reported. Reduced drain current ( /spl sim/ 67 mA/mm in the saturation region) and similar output power and power-added efficiency were found after RF stress. Transconductance dispersion is small before and after RF stress while output resistance dispersion reduces after RF stress. Tests performed under UV light suggest that the observed results may be attributed to trapping in the AlGaN/GaN HEMT layers.

High-Temperature Very Low Frequency Noise-Based Investigation of Slow Transients in AlGaN/GaN MODFETs

The variations of the very low frequency noise (i.e., 100 mHz to 100 kHz) and the dc characteristics of unpassivated AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with temperature from 300 to 500 K are investigated. The rise in temperature to 500 K is shown to reveal generation-recombination (G-R) noise characteristics within the 100 mHz to 1 Hz frequency range. It is experimentally evidenced that these manifestations can predict the existence of very slow transients in the drain-current characteristics of AlGaN/GaN MODFETs. Due to the very long time constant of the transients at room temperature (i.e., on the order of several hours or even days), the observed features can be classified as semipermanent. The comparison between the energy levels predicted by the noise data and the levels previously reported in literature supports these observations.

Fin- and Island-Isolated AlGaN/GaN HFETs

The effects of the variation of the size of isolation mesa of AlGaN/GaN heterojunction field-effect transistors (HFETs) on the device characteristics are presented for the first time. Studies on the direct current and pulsed drain and gate current-voltage characteristics demonstrate a correlation between the pinchoff voltage and the size of the isolation mesa. In this paper, devices fabricated on narrow mesas (i.e., 16 × 40 μm2 fins) and also a device fabricated on an array of very small size mesas (i.e., 16 × 7 μm2 islands) are compared with AlGaN/GaN HFETs of traditionally sized mesas (i.e., 70 × 100 μm2). All these devices show maximum extrinsic gate transconductance greater than 220 mS/mm, whereas the pinchoff voltage is observed to become less negative by reducing the size of the individual mesas. The island-isolated HFETs also enjoy a relatively higher gate transconductance.

Influence of transferred-electron effect on drain-current characteristics of AlGaN/GaN heterostructure field-effect transistors

An analytical model, with incorporation of transferred-electron effect, for drain-current characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) is presented. The transferred-electron effect is often neglected in modeling the drain-current of III-V HFETs. The broad steady-state electron drift-velocity overshoot of GaN in comparison to other direct semiconductors such as GaAs and InP, in addition to the larger difference between the peak and saturation drift-velocity, and the wider band gap of this semiconductor suggest the importance of the incorporation of transferred-electron effect (i.e., steady-state drift-velocity overshoot) in modeling the drain-current of these devices. Simulation results are compared with the results of the adoption of a saturating drift transport model, which has been recently used in modeling the drain-current of these devices. Comparisons between the two models demonstrate the importance of the consideration of transferred-electron effect, especially as t...

Characterization and analysis of gate and drain low-frequency noise in AlGaN/GaN HEMTs

The gate and drain low-frequency noise (LFN) characteristics of 0.15/spl times/200 /spl mu/m/sup 2/ AlGaN/GaN HEMTs are reported. The measured gate noise current spectral density is low and insensitive to the applied high reverse bias voltage between the gate and the drain. Typical gate noise level values vary from /spl sim/1.9/spl times/10/sup -19/ to /spl sim/3.4/spl times/10/sup -19/ (A/sup 2//Hz) as the drain voltage increases from 1 V to 12 V (V/sub G/=-5 V) at 10 Hz. The calculated Hooge parameter is /spl sim/5.9/spl times/10/sup -4/, which is comparable to traditional III-V FETs. Lorentz noise components were observed when V/sub DS/ is higher than 8 V. The peak of Lorentz component moves toward higher frequency when V/sub DS/ increases and V/sub GS/ decreases. The exponent /spl gamma/ of the 1/f/sup l/ was found to reduce from 1. 17 to 1.0 1 when V/sub DS/ increases from 8 V to 16 V. The observed trends are discussed in terms of electric field, carrier velocity and trapping-detrapping considerations.

Analytical Modeling of Current Collapse in AlGaN/GaN HFETs According to the Virtual Gate Concept

GaN-based heterostructure field effect transistors (HFETs) have gained considerable attention in high-power microwave applications. So far, unsurpassed current levels and high output power at microwave frequencies have been achieved. However, the dominant factors limiting the reliability of these devices under high-power operation are still unsettled. Drain current collapse is one of the major encumbrances in the development of reliable high-power devices in this technology. In this paper, an accurate and versatile analytical model based on the concept of virtual gate formation due to the existence of acceptor-type surface states is developed to model the current-collapse phenomenon. The presented model is considerably simple, and at the same time, it is more precise than the other analytical models previously proposed in literature. The implementation of this analytical model demonstrates superb agreement with the experimental observations of permanent/semipermanent current collapse in AlGaN/GaN HFETs. To demonstrate the versatility, results of this model are also compared with an existing recently developed analytical model of comparable degree of complexity.

Low-Frequency Noise-Based Degradation Prediction of

Degradation prediction of AlGaN/GaN MODFET is explored based on characterization of gate and drain low- frequency noise. Heterostructures grown by molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) are used for this purpose. Devices from the former category were unpassivated while those of the latter were passivated. Despite the highly variable gate noise current characteristics among unpassivated MBE devices and between MBE and MOCVD-based devices, it is demonstrated that the drain noise current characteristics of the two groups of devices have considerable resemblance. Moreover, it is shown that the drain noise current level can be used as a means for gate degradation prediction

hboxAl_xhboxGa_1 - xhboxN/GaN

Investigation of the reliable operation of AlGaN/GaN HFETs at elevated temperatures is attempted. In this paper, a Monte Carlo-based temperature-dependent mobility model, with incorporation of steady-state velocity overshoot, is employed in modeling the drain current-voltage characteristics of AlGaN/GaN HFETs at 300, 400, and 500 K. One of the major merits of this model is that it employs a very small set of fitting parameters. The modeled drain current-voltage characteristics have been successfully matched to the experimental characteristics at the aforementioned temperatures. While confirming that a brief measurement at these temperatures is of no reliability concern on the quality of the metal-semiconductor contacts, this matching proves that the temperature dependency of the electron drift velocity is the cause of the degradation of drain current within the aforementioned range of temperature. In producing the aforementioned match for the long-gate AlGaN/GaN HFETs, it is also shown that the accurate modeling of the temperature dependency of the low-field drift transport is more consequential than the accurate representation of the transport in the medium-to-high electric fields.