Hamid Amini Moghadam

Active defects in MOS devices on 4H-SiC: A critical review ☆

Abstract The state-of-the-art 4H-SiC MOSFETs still suffer from performance (low channel-carrier mobility and high threshold voltage) and reliability (threshold voltage instability) issues. These issues have been attributed to a large density of electrically active defects that exist in the SiO2–SiC interfacial region. This paper reviews the earlier and the latest results about the responsible defects for the performance and reliability issues of SiC MOS devices, in the context of the evolution of physical understanding of these defects. The aim of this critical review is to clarify possible confusions due to inconsistencies between the earlier and the latest results. Specific clarifications relate to the physical position of the active defects (whether they are located at or near the SiO2–SiC interface) and the energy position of their energy levels (above or below the bottom of conduction band).

Transient-Current Method for Measurement of Active Near-Interface Oxide Traps in 4H-SiC MOS Capacitors and MOSFETs

Measurements of the near-interface oxide traps (NIOTs) aligned to the conduction band of silicon-carbide (SiC) are of particular importance as these active defects are responsible for degradation of the channel-carrier mobility in 4H-SiC MOSFETs. In this brief, a new method for measurement of the active NIOTs with energy levels aligned to the conduction band is proposed. The method utilizes transient-current measurements on 4H-SiC MOS capacitors biased in accumulation. Nitrided oxide and dry oxide are used to illustrate the applicability of the proposed measurement method.

Gate-Voltage Independence of Electron Mobility in Power AlGaN/GaN HEMTs

The mobility of current carriers in the channel of FETs usually depends on the applied gate voltage. This paper presents experimental evidence that the electron mobility in the 2-D electron gas under the gate of AlGaN/GaN high-electron-mobility transistors (HEMTs) is actually independent of the gate voltage. This demonstration of the gate-voltage independence of the electron mobility relates to power HEMTs, and it was achieved by introducing a new method for the mobility measurement. The gate-voltage independence of the electron mobility was observed for a wide range of temperature, from 25 °C to 300 °C. Furthermore, it is confirmed that the HEMT mobility decreases with increased temperature according to the power law (T-k) and with a quite high value of the power-law coefficient (k = 2.45).

Power-switching applications beyond silicon: The status and future prospects of SiC and GaN devices

Following a review of the key power-switch requirements and the fundamental limitations of silicon as a material, this paper describes the technical issues and the reasons that motivated the development of commercially available Schottky diodes and MOSFETs in SiC. In the second part, the paper analyzes the potential of GaN to enable further technical progress beyond the theoretical limit of Si and even significant price reduction of power-electronic switches.

Quantified Density of Active near Interface Oxide Traps in 4H-SiC MOS Capacitors

This paper presents a new method to quantify near interface oxide traps (NIOTs) that are responsible for threshold voltage instability of 4H-SiC MOSFETs. The method utilizes the shift observed in capacitance–voltage (C–V) curves of an N-type MOS capacitor. The results show that both shallow NIOTs with energy levels below the bottom of conduction band and NIOTs with energy levels above the bottom of the conduction band of SiC are responsible for the C–V shifts, and consequently, for the threshold voltage instabilities in MOSFETs. A higher density of NIOTs is measured at higher temperatures.

Dipole Type Behavior of NO Grown Oxides on 4H–SiC

In this paper, we present surprising MOS capacitor C–V bias instability observed in NO-grown oxides, with distinctly different behavior compared to that of conventional NO-annealed oxides on 4H-SiC. Using sequential back-and-forth and bias-temperature stress C–V measurements, it was demonstrated that the C–V shift direction of NO-grown oxides was opposite to that of NO-annealed oxides. A model based on bias-temperature stress orientated near-interfacial dipoles is proposed to explain this unique behavior of NO-grown oxides.

The effect of charge redistribution on flat-band voltage turnaround in 4H-SiC MOS capacitors

The existence of a turnaround in flat-band voltage shift of stressed MOS capacitors, fabricated on N-type 4H–SiC substrates, is reported in this paper. The turnaround is observed by room-temperature C–V measurements, after two minutes gate-bias stressing of the MOS capacitors at different temperatures. The existence of this turnaround effect demonstrates that a mechanism, in addition to the well-stablished tunneling to the near-interface oxide traps, is involved in the threshold voltage instability of 4H–SiC MOSFETs. This newly identified mechanism occurs due to charge redistribution of the compound polar species that exist in the SiO2–SiC transitional layer.

Impact of Interface Traps on Current-Voltage Characteristics of 4H-SiC Schottky-Barrier Diodes

This paper presents a physical model based on interface traps to explain both the larger barrier heights of practical Schottky diodes in comparison to the theoretically expected values and the appearance of a knee in the log I–V characteristics. According to this model, acceptor-type interface traps near the valance band increase the Schottky barrier height, which shifts the log I–V characteristic to higher forward-bias voltages. In addition to the acceptor traps, donor-type interface traps can appear near the conduction band, and when they do, they cause the knee in the log I–V characteristics as their energy level falls below the Fermi level and the charge associated with these traps changes from positive to neutral.

Selection of SPICE Parameters and Equations for Effective Simulation of Circuits with 4H-SiC Power MOSFETs

Power MOSFETs based on 4H-SiC have recently been commercialized and so circuit designers require SPICE models for simulation purposes in a range of applications including switch-mode power supplies. We present a selection of SPICE LEVEL 3 parameters and equations that can be used for effective circuit simulation of these MOSFETs, taking into account their unique characteristics for both static and dynamic operation.