Dietmar Schroeder

Model for the voltage and temperature dependence of the soft breakdown current in ultrathin gate oxides

The voltage and temperature dependence of the soft breakdown conduction mechanism in metal-oxide-semiconductor capacitors and transistors with ultrathin dielectric layers is investigated. A physical derivation of the quantum point contact model and its parameters is presented, which incorporates the smearing of the Fermi function at the electrodes as well as the effect of thermal vibrations of the constriction’s bottleneck. The model also takes into account the boundary conditions at the two ends of the breakdown path by considering the semiconductor band bending occurring in the nondamaged surrounding device area. Good agreement between model and experimental curves is found. Because of its analytical nature, the proposed model can be implemented in circuit simulators.

Memristive operation mode of floating gate transistors: A two-terminal MemFlash-cell

A memristive operation mode of a single floating gate transistor is presented. The device resistance varied accordingly to the charge flow through the device. Hysteretic current-voltages including a resistance storage capability were observed. These experimental findings are theoretically supported by a capacitive based model. The presented two-terminal MemFlash-cell can be considered as a potential substitute for any memristive device (especially for reconfigurable logic, cross-bar arrays, and neuromorphic circuits) and is basically compatible with current Si-fabrication technology. The obvious trade-off between a memristive device based on a state-of-the-art silicon process technology and power consumption concerns will be discussed.

Modeling random telegraph signals in the gate current of metal–oxide–semiconductor field effect transistors after oxide breakdown

Measurements of random telegraph signals (RTS) in the gate current of n-channel metal–oxide–semiconductor field effect transistors (MOSFETs) after oxide breakdown are presented. Two types of behavior of the time constants and the relative amplitudes of the signals as a function of gate voltage are observed. A theory relating time constants and relative amplitudes of the fluctuations to the energetic and geometric trap location in the oxide is developed. This theory is also applicable to the commonly observed RTS in the drain current of undamaged MOSFETs.

MemFlash device: floating gate transistors as memristive devices for neuromorphic computing

Memristive devices are promising candidates for future non-volatile memory applications and mixed-signal circuits. In the field of neuromorphic engineering these devices are especially interesting to emulate neuronal functionality. Therefore, new materials and material combinations are currently investigated, which are often not compatible with Si-technology processes. The underlying mechanisms of the device often remain unclear and are paired with low device endurance and yield. These facts define the current most challenging development tasks towards a reliable memristive device technology. In this respect, the MemFlash concept is of particular interest. A MemFlash device results from a diode configuration wiring scheme of a floating gate transistor, which enables the persistent device resistance to be varied according to the history of the charge flow through the device. In this study, we investigate the scaling conditions of the floating gate oxide thickness with respect to possible applications in the field of neuromorphic engineering. We show that MemFlash cells exhibit essential features with respect to neuromorphic applications. In particular, cells with thin floating gate oxides show a limited synaptic weight growth together with low energy dissipation. MemFlash cells present an attractive alternative for state-of-art memresitive devices. The emulation of associative learning is discussed by implementing a single MemFlash cell in an analogue circuit.

Fully Implantable Multi-Channel Measurement System for Acquisition of Muscle Activity

This paper presents intramuscular electromyogram (EMG) signals obtained with a fully implantable measurement system that were recorded during goal directed arm movements. In a first implantation thin film electrodes were epimysially implanted on the deltoideus of a rhesus macaque and the encapsulation process was monitored by impedance measurements. Increase of impedance reached a constant level after four weeks indicating a complete encapsulation of electrodes. EMG recorded with these electrodes yielded a signal-to-noise ratio of about 80 dB at 200 Hz. The EMG recorded during goal-directed arm movements showed a high similarity to movements in the same direction and at the same time presented clear differences between different movement directions in time domain. Six classifiers and seven time and frequency domain features were investigated with the aim of discriminating the direction of arm movement from EMG signals. Reliable recognition of arm movements was achieved for a subset of the movements under investigation only. A second implantation of the whole measurement system for nine weeks demonstrated simple handling during surgery and good biotolerance in the animals.

An Ultra Low-Noise CMOS Operational Amplifier with Programmable Noise-Power Trade-Off

A programmable operational amplifier (OpAmp) concerning noise and power consumption is described. Key design issues for achieving programmability of a preselected OpAmp architecture are discussed. Experimental results for a 0.35 mum CMOS OpAmp show either ultra low noise of 2 nV/radicHz or low power consumption of 140 muW. The OpAmp remains stable over the whole range of programmability

Systematic design of programmable operational amplifiers with noise-power trade-off

A methodology for the systematic design of a programmable operational amplifier (opamp) is described. With this methodology, the opamp is programmable concerning noise and power consumption while keeping the stability for the whole operation range with a constant phase margin of phi res =70deg. The theoretical model is developed with the help of the transfer characteristics of the opamp determining the degrees of freedom. Experimental results for a 0.35-mum CMOS opamp show either ultra low-noise of 2 nV/radicHz or low-power consumption of 140 muW while keeping the opamp stable over the whole range of programmability

The inflow moments method for the description of electron transport at material interfaces

This paper introduces the inflow moments method as a general procedure for the derivation of interface or boundary conditions for advanced models of carrier transport in semiconductor devices. It is based on a general interface condition for the Boltzmann equation accounting for particle and energy balance at material interfaces, as metal‐semiconductor contacts or semiconductor heterojunctions. Interface conditions for transport models based on integrations of the Boltzmann equation are consistently derived by the corresponding integrations of the Boltzmann interface condition. The method is illustrated by a treatment of thermionic emission of hot electrons at a semiconductor heterojunction, resulting in interface conditions for the particle as well as the energy balance equation.

Physical explanation of the barrier height temperature dependence in metal-oxide-semiconductor leakage current models

A temperature dependence of the barrier height between silicon and oxide has been proposed by many authors in order to reflect experimental metal-oxide-semiconductor leakage current results. However, no satisfactory physical explanation of this dependence has yet been given. In this letter, the temperature dependence of the observed macroscopic barrier height is explained by thermal fluctuations of the microscopic local barrier height. Because of the exponential relationship between current and barrier height, the decrease of the barrier during the fluctuation has a dominating effect when compared to the increase, leading on the average to a raised leakage current and correspondingly to a lowered average barrier height.

Long-term decoding of movement force and direction with a wireless myoelectric implant

OBJECTIVE The ease of use and number of degrees of freedom of current myoelectric hand prostheses is limited by the information content and reliability of the surface electromyography (sEMG) signals used to control them. For example, cross-talk limits the capacity to pick up signals from small or deep muscles, such as the forearm muscles for distal arm amputations, or sites of targeted muscle reinnervation (TMR) for proximal amputations. Here we test if signals recorded from the fully implanted, induction-powered wireless Myoplant system allow long-term decoding of continuous as well as discrete movement parameters with better reliability than equivalent sEMG recordings. The Myoplant system uses a centralized implant to transmit broadband EMG activity from four distributed bipolar epimysial electrodes. APPROACH Two Rhesus macaques received implants in their backs, while electrodes were placed in their upper arm. One of the monkeys was trained to do a cursor task via a haptic robot, allowing us to control the forces exerted by the animal during arm movements. The second animal was trained to perform a center-out reaching task on a touchscreen. We compared the implanted system with concurrent sEMG recordings by evaluating our ability to decode time-varying force in one animal and discrete reach directions in the other from multiple features extracted from the raw EMG signals. MAIN RESULTS In both cases, data from the implant allowed a decoder trained with data from a single day to maintain an accurate decoding performance during the following months, which was not the case for concurrent surface EMG recordings conducted simultaneously over the same muscles. SIGNIFICANCE These results show that a fully implantable, centralized wireless EMG system is particularly suited for long-term stable decoding of dynamic movements in demanding applications such as advanced forelimb prosthetics in a wide range of configurations (distal amputations, TMR).