Browsing by Author "Liu, Yin"

Now showing 1 - 6 of 6
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    Design and fabrication of Emitter Controlled Thyristor
    Liu, Yin (Virginia Tech, 2001-06-15)
    The Emitter Controlled Thyristor (ECT) is a new MOS-Gated Thyristor (MGT) that combines the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. An ECT is composed of an emitter switch in series with the thyristor, an emitter-short switch in parallel with the emitter junction of the thyristor, a turn-on FET and the main thyristor structure. Numerical analysis shows that the ECT also offers superior high voltage current saturation capability even for high breakdown voltage ratings. Two different ECT structures are investigated in this research from numerical simulations to experimental fabrications. A novel ECT structure that utilizes IGBT compatible fabrication process was proposed. The emitter short FET, emitter switch FET and turn-on FET are all integrated with a high voltage thyristor. Numerical simulation results show that the ECT has a better conductivity modulation than that of the IGBT and at the same time exhibits superior high voltage current saturation capability, superior FBSOA and RBSOA. The technology trade-off between turn-off energy loss and forward voltage drop of the ECT is also better than that of the IGBT because of the stronger conductivity modulation. A novel self-aligned process is developed to fabricate the device. Experimental characteristics of the fabricated ECT devices show that the ECT achieves lower forward voltage drop and superior high voltage current saturation capability. A Hybrid ECT (HECT) structure was also developed in this research work. The HECT uses an external FET to realize the emitter switching function, hence a complicated fabrication issue was separated into two simple one. The cost of the fabrication decreases and the yield increases due to the hybrid integration. Numerical simulations demonstrate the superior on-state voltage drop and high voltage current saturation capability. A novel seven-mask process was developed to fabricate the HECT. Experimental results show that the HECT could achieve the lower forward voltage drop and superior current saturation capability. The resistive switching test was carried out to demonstrate the switching characteristics of the HECT.
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    Design and Fabrication of the Emitter Controlled Thyristor
    Liu, Yin (Virginia Tech, 2001-06-15)
    The Emitter Controlled Thyristor (ECT) is a new MOS-Gated Thyristor (MGT) that combines the ease of a MOS gate control with the superior current carrying capability of a thyristor structure for high-power applications. An ECT is composed of an emitter switch in series with the thyristor, an emitter-short switch in parallel with the emitter junction of the thyristor, a turn-on FET and the main thyristor structure. Numerical analysis shows that the ECT also offers superior high voltage current saturation capability even for high breakdown voltage ratings. Two different ECT structures are investigated in this research from numerical simulations to experimental fabrications. A novel ECT structure that utilizes IGBT compatible fabrication process was proposed. The emitter short FET, emitter switch FET and turn-on FET are all integrated with a high voltage thyristor. Numerical simulation results show that the ECT has a better conductivity modulation than that of the IGBT and at the same time exhibits superior high voltage current saturation capability, superior FBSOA and RBSOA. The technology trade-off between turn-off energy loss and forward voltage drop of the ECT is also better than that of the IGBT because of the stronger conductivity modulation. A novel self-aligned process is developed to fabricate the device. Experimental characteristics of the fabricated ECT devices show that the ECT achieves lower forward voltage drop and superior high voltage current saturation capability. A Hybrid ECT (HECT) structure was also developed in this research work. The HECT uses an external FET to realize the emitter switching function, hence a complicated fabrication issue was separated into two simple one. The cost of the fabrication decreases and the yield increases due to the hybrid integration. Numerical simulations demonstrate the superior on-state voltage drop and high voltage current saturation capability. A novel seven-mask process was developed to fabricate the HECT. Experimental results show that the HECT could achieve the lower forward voltage drop and superior current saturation capability. The resistive switching test was carried out to demonstrate the switching characteristics of the HECT.
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    Methodologies, Techniques, and Tools for Understanding and Managing Sensitive Program Information
    Liu, Yin (Virginia Tech, 2021-05-20)
    Exfiltrating or tampering with certain business logic, algorithms, and data can harm the security and privacy of both organizations and end users. Collectively referred to as sensitive program information (SPI), these building blocks are part and parcel of modern software systems in domains ranging from enterprise applications to cyberphysical setups. Hence, protecting SPI has become one of the most salient challenges of modern software development. However, several fundamental obstacles stand on the way of effective SPI protection: (1) understanding and locating the SPI for any realistically sized codebase by hand is hard; (2) manually isolating SPI to protect it is burdensome and error-prone; (3) if SPI is passed across distributed components within and across devices, it becomes vulnerable to security and privacy attacks. To address these problems, this dissertation research innovates in the realm of automated program analysis, code transformation, and novel programming abstractions to improve the state of the art in SPI protection. Specifically, this dissertation comprises three interrelated research thrusts that: (1) design and develop program analysis and programming support for inferring the usage semantics of program constructs, with the goal of helping developers understand and identify SPI; (2) provide powerful programming abstractions and tools that transform code automatically, with the goal of helping developers effectively isolate SPI from the rest of the codebase; (3) provide programming mechanism for distributed managed execution environments that hides SPI, with the goal of enabling components to exchange SPI safely and securely. The novel methodologies, techniques, and software tools, supported by programming abstractions, automated program analysis, and code transformation of this dissertation research lay the groundwork for establishing a secure, understandable, and efficient foundation for protecting SPI. This dissertation is based on 4 conference papers, presented at TrustCom'20, GPCE'20, GPCE'18, and ManLang'17, as well as 1 journal paper, published in Journal of Computer Languages (COLA).
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    Only pay for what you need: Detecting and removing unnecessary TEE-based code
    Liu, Yin; Dhar, Siddharth; Tilevich, Eli (Elsevier, 2022-06-01)
    A Trusted Execution Environment (TEE) provides an isolated hardware environment for sensitive code and data to protect a system's integrity and confidentiality. As we discovered, programmers tend to overuse TEE protection. When they place non-sensitive code in TEE, the trusted computing base (TCB) grows unnecessarily, leading to long execution latencies and large attack surfaces. To address this problem, we first study a representative sample of open-source projects to uncover how TEE is utilized in real-world software. To facilitate the process of removing non-sensitive code from TEE, we introduce TEE Insourcing, a new type of software refactoring that identifies and removes the unnecessary program parts out of TEE. We implemented TEE Insourcing as the TEE-DRUP framework, which operates in three phases: (1) a variable sensitivity analysis designates each variable as sensitive or non-sensitive; (2) a TEE-aware taint analysis identifies non-sensitive TEE-based functions; (3) a fully-declarative program transformation automatically moves these functions out of TEE. Our evaluation demonstrates that our approach is correct, effective, and usable. By deploying TEE-DRUP to discover and remove the unnecessary TEE code, programmers can both reduce the TCB's size and improve system performance.
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    Privacy-Preserving Sharing of Mobile Sensor Data
    Liu, Yin; Cruz, Breno Dantas; Tilevich, Eli (Springer, 2022-01-01)
    To personalize modern mobile services (e.g., advertisement, navigation, healthcare) for individual users, mobile apps continuously collect and analyze sensor data. By sharing their sensor data collections, app providers can improve the quality of mobile services. However, the data privacy of both app providers and users must be protected against data leakage attacks. To address this problem, we present differentially privatized on-device sharing of sensor data, a framework through which app providers can safely collaborate with each other to personalize their mobile services. As a trusted intermediary, the framework aggregates the sensor data contributed by individual apps, accepting statistical queries against the combined datasets. A novel adaptive privacy-preserving scheme: 1) balances utility and privacy by computing and adding the required amount of noise to the query results; 2) incentivizes app providers to keep contributing data; 3) secures all data processing by integrating a Trusted Execution Environment. Our evaluation demonstrates the framework’s efficiency, utility, and safety: all queries complete in <10 ms; the data sharing collaborations satisfy participants’ dissimilar privacy/utility requirements; mobile services are effectively personalized, while preserving the data privacy of both app providers and users.
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    Secure and flexible message-based communication for mobile apps within and across devices
    Liu, Yin; Cruz, Breno Dantas; Tilevich, Eli (Elsevier, 2022-11-01)
    In modern mobile platforms, message-based communication is afflicted by data leakage attacks, through which untrustworthy apps access the transferred message data. Existing defenses are overly restrictive, as they block all suspicious message exchanges, thus preventing any app from receiving messages. To better secure message-based communication, we present a model that strengthens security, while also allowing untrusted-but-not-malicious apps to execute their business logic. Our model, HTPD, introduces two novel mechanisms: hidden transmission and polymorphic delivery. Sensitive messages are transmitted hidden in an encrypted envelope. Their delivery is polymorphic: as determined by the destination's trustworthiness, it can be delivered no data, raw data, or encrypted data. To allow an untrusted destination to operate on encrypted data deliveries, HTPD integrates homomorphic and convergent encryption. We concretely realize HTPD as POLICC, a plug-in replacement of Android Inter-Component Communication (ICC) middleware. POLICC mitigates three classic Android data leakage attacks, while allowing untrusted apps to perform useful operations on delivered messages. Our evaluation shows that POLICC supports secure message-based communication within and across devices by trading off performance costs, programming effort overheads, and security1.