Reliability Characterizations of Power Transistors: from Silicon to Oxide Semiconductors

Yen-Pu Chen, Purdue University

Abstract

Semiconductor power electronics find widespread use in miscellaneous applications, including power management in smart grids, electrical motors in self-driving cars, satellite power systems, and so on. In these high voltage systems, it is inevitable that the reliability issues at the transistor level, e.g., hot carrier degradation (HCD), bias temperature instability (BTI), and self-heating effect (SHE), would be prominent and must be carefully investigated. The geometrical and doping complexities of power transistors make their reliability issues very distinct from classical low-voltage logic transistors. Other than the classical material such as Si, commercialized wide bandgap (WBG) materials such as GaN and SiC, and emerging oxide semiconductors such as β-Ga2O3 and In2O3 have attracted researchers’ attention. For example, β-Ga2O3’s ultra-wide bandgap (4.6 to 4.9 eV) makes it a promising candidate to compete with GaN and SiC. Amorphous In2O3, grown by atomic layer deposition (ALD), is demonstrated to possess high electron mobility (>100 cm2V−1 s −1). The reliability issues of the two emerging materials are worth exploring. As a result, this thesis can be divided into two major parts: silicon and oxide semiconductor. Our main contributions are new characterization techniques and reliability models, which are essential for integrated power systems. In the first part of the thesis, the localized HCD in Si-based lateral diffused MOSFETs (LDMOSs) with different geometries and structures are explored by two new characterization methods. The first one is called ”three-point I-V spectroscopy” and the other is called ”Super Single Pulse Charge Pumping (S2PCP)”. The former technique extracts the mobility degradation percentage (∆µ) in the channel and drift regions individually. The latter extracts the localized interface trap generation (∆Nit). S2PCP is developed for the source-body-tied (SBT) LDMOS, in which the classical charge pumping techniques cannot function properly. The results from the two techniques compare well with each other, providing cross-validation of the techniques. For different types of LDMOS transistors under study, the channel region degradation is enhanced under higher VG bias. This channel degradation was then observed to be HCD-assisted anode hole injection (AHI) because of the stronger recovery, positive temperature activation, and negligible temperature dependence in gate leakage. In the second part of the thesis, two emerging oxide semiconductors, β-Ga2O3 and In2O3 are studied. For β-Ga2O3, SHE is identified as a critical issue due to its low thermal conductivity. Therefore, the self-heating was included in circuit simulations, and indeed reveals a degradation in the efficiency of DC-DC boost conversion. The thermal resistance (θth) of the bilayered structure is modeled, and its maximum current (Imax) and power (Pmax) metrics are derived and estimated through analytical calculations. The choice of high-thermalconductivity substrate, wafer thinning, etc, can help in mitigating SHE. However, extra effort is still vital to further improve β-Ga2O3 performance to outperform GaN and SiC. On the other hand, 1.2-nm-thick amorphous In2O3thin-film transistors (TFTs) demonstrate promising electrical performances. Grown in a relatively low temperature (∼225 °C), it is recognized as a back-end-of-line (BEOL) compatible transistor. The reliability studies such as positive bias temperature stress (PBTS) and HCD are explored and modeled. Unlike traditional logic transistors, HCD is strongly correlated to PBTS, which is caused by the much stronger vertical field compared to the lateral field in the ultra-thin devices. Overall, the high-performance BEOL-TFTs are remarkably reliable, with a relatively small threshold voltage shift under PBTS/HCD stress conditions at room temperature.

Degree

Ph.D.

Advisors

Alam, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Optics

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