Elsevier

Microelectronic Engineering

Volume 238, 1 February 2021, 111508
Microelectronic Engineering

Research paper
2DEG transport properties over temperature for AlGaN/GaN HEMT and AlGaN/InGaN/GaN pHEMT

https://doi.org/10.1016/j.mee.2021.111508Get rights and content

Highlights

  • Analysis of 2DEG transport properties of AlGaN/GaN HEMT and AlGaN/InGaN/GaN pHEMT.

  • Analysis of temperature dependency of 2DEG for both the devices.

  • A degradation nature of 2DEG density is found for both the devices with GaN HEMT showing greater degradation than GaN pHEMT.

  • Transistor performance enhancement is noticed for AlGaN/InGaN/GaN pHEMT with recorded transconductance (gm) of 309.26 mS/mm and 2DEG sheet carrier density (ns) of 1.241 × 1013 cm−2.

  • AlGaN/InGaN/GaN pHEMT performs better 2DEG stability than AlGaN/GaN HEMT.

Abstract

For the commercial implementation of GaN-based high electron mobility transistor (HEMT) and GaN-based pseudomorphic HEMT (pHEMT), the temperature dependency of the two-dimensional electron gas (2DEG) is crucial. Herein, the temperature dependency of 2DEG for AlGaN/GaN HEMT and AlGaN/InGaN/GaN pHEMT has been analyzed theoretically and experimentally over the temperature range of −40 °C to 150 °C. From the experimental and numerical data, a decreasing nature of 2DEG density is noticed for both the devices. The reduction rate in magnitude is higher for AlGaN/GaN HEMT compared to AlGaN/InGaN/GaN pHEMT. It is identified that the reduction of conduction band offset at higher temperatures is responsible for this decreasing nature. Incorporation of an extra 5 nm thick pseudomorphic layer of InGaN in AlGaN/InGaN/GaN pHEMT improves the 2DEG transport properties and enhances the overall transistor performance. Comparatively, the AlGaN/InGaN/GaN pHEMT is found to be exhibiting better 2DEG stability with temperature than AlGaN/GaN HEMT.

Introduction

GaN-based high electron mobility transistor (HEMT) is a heterostructure device with multiple layers of lower and higher bandgap semiconductor materials. Until now, it is one of the fastest transistor technologies available due to the formation of quantum well when a higher bandgap material (AlGaN) is placed alongside a lower bandgap material (GaN). At the interface, the electric field is perpendicular to the hetero-interface. This allows the flow of electrons parallel to the interface and gives rise to a two-dimensional electron gas (2DEG) [1].

For high frequency and high-power operation, GaN-based HEMTs have superiority over Si and GaAs counterparts due to the outstanding material properties of GaN [2]. With a higher breakdown field, larger band-gap, higher saturation velocity, GaN-based devices possess a very high current density [3] and high channel-temperature operation [4]. Due to piezoelectric and spontaneous polarization, GaN-based devices possess sheet carrier density in access of 1013 cm−2 at the hetero-interface [5] which provides superiority in mobility and consequently in transistor speed over other materials and technologies. Due to the high resistivity and thermal conductivity of silicon carbide (SiC), the fabrication of GaN devices on the SiC substrate helps to avoid unrestrained heating of the devices [[6], [7], [8]].

However, the huge potential of GaN technology to be used vastly for commercial purposes is limited due to several unwanted scattering mechanisms. Scattering mechanisms such as charged impurity scattering, phonon scattering, dislocation scattering, and interface roughness scattering cause electronic traps [[9], [10], [11], [12], [13], [14]] and high leakage current [15]. These restrict the maximum achievable mobility by affecting the 2DEG and corresponding sheet carrier density of AlGaN/GaN HEMTs. For high-frequency application, when the gate length is made to shrink, a short channel effect is observed and it can be minimized by using a thinner barrier layer of AlGaN. However, it may lead to a reduction in sheet carrier density and can cause current collapse as well as a large leakage current as the surface traps will be closer to the GaN channel. The introduction of a thin InGaN channel layer in between the AlGaN and GaN layers can suppress the short channel effect as well as the alloy-disorder scattering as a part of the 2DEG penetrates the GaN layer in a very thin pseudomorphic InGaN channel layer [16]. The lattice mismatch of thin InGaN channel is entirely accommodated as strain and the new heterostructure of AlGaN/InGaN/GaN is called pseudomorphic high electron mobility transistor (pHEMT). The introduction of an extra InGaN layer in AlGaN/InGaN/GaN pHEMT, provides better carrier confinement, larger conduction band discontinuity, higher carrier density, and enhanced electron transport with an increase in threshold voltage compared to traditional AlGaN/GaN HEMTs.

For successfully implementing the AlGaN/GaN HEMT and AlGaN/InGaN/GaN pHEMT devices for commercial purposes, it is crucial to study the performance of 2DEG transport properties over temperature. The temperature stability is crucial for HEMTs especially at higher temperatures where the ohmic and Schottky contacts degrade and the device performance depends heavily on the transport properties of 2DEG. No or very little study of 2DEG transport properties over temperature is provided in the literature [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. In [17], simulation-based characteristics study on 2DEG transport properties for different mole fractions of AlGaAs/GaAs and AlGaN/GaN HEMTs have been provided. In [18], the high-temperature dependence of 2DEG in Al0.18Ga0.82N/GaN heterostructure has been studied. In this paper, we study the 2DEG transport properties for both AlGaN/GaN HEMT and AlGaN/InGaN/GaN pHEMT. We also investigate the 2DEG performance for both AlGaN/GaN HEMT and AlGaN/InGaN/GaN pHEMT over temperature using experimental data of the fabricated devices. Besides, we investigate how transistor performance is enhanced with the introduction of an InGaN pseudomorphic layer in AlGaN/InGaN/GaN pHEMT in comparison to AlGaN/GaN HEMT.

Section snippets

Device fabrication and measurement

The epitaxial layers and top views of the devices are shown in Fig.1. The MOCVD or metal-organic chemical vapor deposition technique is used for epitaxial growth. An undoped AlGaN spacer layer (25.3% Al) of 5 nm is grown on an undoped GaN buffer layer of 1.5 μm. The substrate consists of 400 μm SiC layer and a graded AlN nucleation (relaxation) layer of 1.5 nm is grown in between the SiC substrate and GaN buffer for minimization of threading dislocation. A 5 nm GaN cap layer to prevent Al

2DEG Transport Properties

With the growth of an epitaxial layer consisting of a higher bandgap material upon a comparatively lower bandgap material, e.g., AlGaN over GaN, there develops a quantum well. Carriers are constricted within the well and can travel along with the interface. The conduction band is close to the Fermi level and there is a sudden discontinuity of the conduction band at the interface. This phenomenon is known as 2DEG or two-dimensional electron gas. This 2DEG provides high mobility and high-speed

Conclusion

In this paper, we studied the temperature dependency of 2DEG for both the AlGaN/GaN HEMT and AlGaN/InGaN/ GaN pHEMT over the temperature range of −40 °C to 150 °C. We found a degradation nature of 2DEG density in both the devices. Experimental data is aided by simulation data and shows higher degradation for AlGaN/GaN HEMT. Transistor performance enhancement is noticed for AlGaN/InGaN/GaN pHEMT with recorded carrier concentration (NC-V) of 14.23 × 1021 cm−3, transconductance (gm) of 309.26

CRediT authorship contribution statement

Md. Abdul Kaium Khan: Conceptualization, Methodology, Formal analysis, Data curation, Software, Writing - original draft. Mohammad Abdul Alim: Supervision, Writing - review & editing, Project administration. Christophe Gaquiere: Resources, Writing - review & editing, Validation

Declaration of Competing Interest

We wish to confirm that there are no known conflicts of interest associated with this publication.

References (25)

  • A. Pérez-Tomás et al.

    Temperature impact and analytical modeling of the AlGaN/GaN-on-Si saturation drain current and transconductance

    Semicond. Sci. Technol.

    (2012)
  • M.A. Alim et al.

    Thermal characterisation of AlGaN/GaN HEMT on silicon carbide substrate for high frequency application

  • Cited by (18)

    • RF/Linearity figures of merit estimation for GaAs and GaN/SiC-based Nano-HEMTs

      2022, Micro and Nanostructures
      Citation Excerpt :

      Besides GaN devices grown on SiC substrates, it is commonly recognized that they have improved thermal conductivity. In addition, GaAs offer more sophisticated device technology, including better contacts and higher electron mobility [6–11]. Thus, the conclusions here are qualitatively predictable.

    • Ferroelectric domain induced giant enhancement of two-dimensional electron gas density in ultrathin-barrier AlGaN/GaN heterostructures

      2022, Applied Surface Science
      Citation Excerpt :

      AlGaN/GaN heterostructures with high electron mobility 2DEG are widely used in power and radio frequency devices [1–9]. However, due to limited spontaneous and piezoelectric polarization in AlGaN/GaN, the 2DEG density is limited around 1013 cm−2 [10–13]. Therefore, further increasing the concentration of 2DEG is a yet solved challenge attracting more and more attention.

    • Estimation of channel temperature and thermal sensitivity for a 0.15 μm GaN HEMT

      2021, Microelectronic Engineering
      Citation Excerpt :

      GaN-based devices can be fabricated on different substrates like SiC, Si, and Sapphire. Among these substrates, SiC is more suitable for GaN transistors due to its good lattice matching and high thermal conductivity which keep them safe from unreasonable heating [2,7,8]. Therefore, temperature-dependent behavior analysis is of the essence for understanding the device performance specially GaN on SiC based HEMTs.

    • An Extended Temperature Model Based on the Trapping Effect for Drain-Source Current in GaN HEMTs

      2024, Physica Status Solidi (A) Applications and Materials Science
    View all citing articles on Scopus
    View full text