Yuanjie Lv

Polarization Coulomb field scattering in AlGaN/AlN/GaN heterostructure field-effect transistors

Using the measured capacitance-voltage curves of Ni Schottky contacts with different contact areas and the current-voltage characteristics for the circular and rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) at low drain-source voltage, we found that the polarization Coulomb field scattering has an important influence on the two-dimensional electron gas (2DEG) electron mobility in both the circular and rectangular AlGaN/AlN/GaN HFETs. Moreover, the polarization Coulomb field scattering in AlGaN/AlN/GaN heterostructures is relatively weaker compared to that in AlGaN/GaN heterostructures, which is attributed to the AlN interlayer in AlGaN/AlN/GaN heterostructures to enlarge the average distance between the 2DEG electrons and the polarization charges.

Extraction of AlGaN/GaN heterostructure Schottky diode barrier heights from forward current-voltage characteristics

Ni Schottky contacts on AlGaN/GaN heterostructures have been fabricated, and one of the prepared samples has been annealed at 700 °C for half an hour. The barrier heights for the prepared samples were measured by internal photoemission. Based on the measured forward current-voltage (I-V) characteristics and using the two-diode model, the Ni Schottky barrier height at zero bias has been analyzed and calculated by self-consistently solving Schrodinger’s and Poisson’s equations, and the correlation expression between the barrier height at zero electric field and that at zero bias has been derived for Schottky contacts on AlGaN/GaN heterostructures. The calculated Schottky barrier heights corresponding to zero electric field for the prepared Ni Schottky contacts on AlGaN/GaN heterostructures agree well with the photocurrent measured results. Thus, the method for extraction of AlGaN/GaN heterostructure Schottky barrier heights from forward I-V characteristics is developed and determined.

Influence of the side-Ohmic contact processing on the polarization Coulomb field scattering in AlGaN/AlN/GaN heterostructure field-effect transistors

Using the measured capacitance-voltage curves of Ni Schottky contacts with different areas and the current-voltage characteristics for the circular and rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with the side-Ohmic contacts, it is found that the polarization Coulomb field scattering caused by the polarization charge density variation at the AlGaN/AlN/GaN interfaces is closely related to the Ohmic-contact processing, and the side-Ohmic contact processing greatly weakens the polarization Coulomb field scattering in the AlGaN/AlN/GaN HFETs.

Theoretical model of the polarization Coulomb field scattering in strained AlGaN/AlN/GaN heterostructure field-effect transistors

The theoretical model of the polarization Coulomb field scattering (PCF) caused by the polarization charge density variation at the AlGaN/AlN interface in strained AlGaN/AlN/GaN heterostructure field-effect transistors has been developed. And the theoretical values for the electron drift mobility, which were calculated using the Matthiessens rule that includes PCF, piezoelectric scattering, polar optical-phonon scattering, and interface roughness scattering, are in good agreement with our experimental values. Therefore, the theoretical model for PCF has been confirmed.

Evaluating AlGaN/AlN/GaN heterostructure Schottky barrier heights with flat-band voltage from forward current-voltage characteristics

Both circular and rectangular Ni Schottky contacts on AlGaN/AlN/GaN heterostructures have been fabricated. Both of the Schottky barrier heights were measured by internal photoemission. The flat-band voltage (V0) for the AlGaN/AlN/GaN heterostructure Schottky contacts was analyzed and obtained from the forward current-voltage (I-V) characteristics. Based on the forward I-V characteristics and with the obtained flat-band voltage, the Schottky barrier heights for the circular and rectangular diodes have been analyzed and calculated by self-consistently solving Schrodinger’s and Poisson’s equations. The evaluated Schottky barrier heights for the prepared circular and rectangular Ni Schottky diodes agree well with the photocurrent measured results.

Effect of Polarization Coulomb Field Scattering on Parasitic Source Access Resistance and Extrinsic Transconductance in AlGaN/GaN Heterostructure FETs

With the measured parasitic source access resistance under a different forward gate-source current for the AlGaN/GaN heterostructure FETs (HFETs) with different device sizes, the variation of the measured parasitic source access resistance with the polarization charge distribution at the AlGaN/GaN interface was compared and analyzed. It is found that the variation of the parasitic source access resistance originates from the polarization Coulomb field (PCF) scattering, and the effect of PCF scattering on the parasitic source access resistance is more significant for the device with a longer gate length or a shorter gate-source distance. The behaviors of the measured extrinsic transconductance for the fabricated AlGaN/GaN HFETs confirm the effect of PCF scattering. The possibility is demonstrated to take advantage of this effect to improve the performance of AlGaN/GaN HFETs.

Influence of the ratio of gate length to drain-to-source distance on the electron mobility in AlGaN/AlN/GaN heterostructure field-effect transistors

Using measured capacitance-voltage curves with different gate lengths and current–voltage characteristics at low drain-to-source voltage for the AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) of different drain-to-source distances, we found that the dominant scattering mechanism in AlGaN/AlN/GaN HFETs is determined by the ratio of gate length to drain-to-source distance. For devices with small ratio (here, less than 1/2), polarization Coulomb field scattering dominates electron mobility. However, for devices with large ratio (here, more than 1/2), longitudinal optical (LO) phonon scattering and interface roughness scattering are dominant. The reason is closely related to polarization Coulomb field scattering.

Effects of rapid thermal annealing on the electrical properties of the AlGaN/AlN/GaN heterostructure field-effect transistors with Ti/Al/Ni/Au gate electrodes

In this study, we investigated the electrical properties of the AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with Ti/Al/Ni/Au gate electrodes using the measured capacitance-voltage, current-voltage characteristics, and micro-Raman spectroscopy. We found that the uneven distribution of the strain caused by the Schottky metals was a major factor that generates the polarization Coulomb field scattering in AlGaN/AlN/GaN HFETs, and after appropriate rapid thermal annealing (RTA) processes, the polarization Coulomb field scattering was greatly weakened and the two-dimensional electron gas electron mobility was improved. We also found that the Schottky barrier height and the DC characteristics of the devices became better after appropriate RTA. Of course, the electrical performances mentioned above became deteriorated after excessive annealing.

Influence of Different Gate Biases and Gate Lengths on Parasitic Source Access Resistance in AlGaN/GaN Heterostructure FETs

The AlGaN/GaN heterostructure FETs with different gate lengths were fabricated. Under different gate biases or for the devices with different gate lengths, the measured parasitic source access resistance values were different. By the analysis of different scattering mechanisms and polarization charge distribution, it is found that the gate bias and gate length can change the polarization Coulomb field scattering, and then affect the parasitic source access resistance. At last, the systematic scattering theories were adopted and the influence of different gate biases and gate lengths on parasitic source access resistance was further confirmed in theory.

AlGaN/GaN Polarization-Doped Field-Effect Transistors With Graded Heterostructure

The design, fabrication, and characterization of AlGaN/GaN polarization-doped field-effect transistors (PolFETs) with graded heterostructure are presented in this paper. The 3-D profiles of carriers are obtained across the graded AlGaN/GaN heterostructure grown on a sapphire substrate, which brings about some novel features. The dc transfer and frequency characteristics exhibit bias-insensitive throughout the low- and high-voltage operating regions, demonstrating the potential for high linearity applications. In addition, dynamic I-V measurement was carried out to analyze the trapping behaviors. Negligible current collapses were observed in the unpassivated PolFETs with graded heterostructure, which can be explained in the aspect of unique energy band profiles of AlGaN/GaN graded heterostructures.