24 research outputs found
Suns-V characteristics of high performance kesterite solar cells
Low open circuit voltage () has been recognized as the number one
problem in the current generation of CuZnSn(Se,S) (CZTSSe) solar
cells. We report high light intensity and low temperature Suns-
measurement in high performance CZTSSe devices. The Suns- curves
exhibit bending at high light intensity, which points to several prospective
limiting mechanisms that could impact the , even at 1 sun for
lower performing samples. These V limiting mechanisms include low bulk
conductivity (because of low hole density or low mobility), bulk or interface
defects including tail states, and a non-ohmic back contact for low carrier
density CZTSSe. The non-ohmic back contact problem can be detected by
Suns- measurements with different monochromatic illumination. These
limiting factors may also contribute to an artificially lower -
diode ideality factor.Comment: 9 pages, 9 figures, 1 supplementary materia
Efficient ConvNets for Analog Arrays
Analog arrays are a promising upcoming hardware technology with the potential
to drastically speed up deep learning. Their main advantage is that they
compute matrix-vector products in constant time, irrespective of the size of
the matrix. However, early convolution layers in ConvNets map very unfavorably
onto analog arrays, because kernel matrices are typically small and the
constant time operation needs to be sequentially iterated a large number of
times, reducing the speed up advantage for ConvNets. Here, we propose to
replicate the kernel matrix of a convolution layer on distinct analog arrays,
and randomly divide parts of the compute among them, so that multiple kernel
matrices are trained in parallel. With this modification, analog arrays execute
ConvNets with an acceleration factor that is proportional to the number of
kernel matrices used per layer (here tested 16-128). Despite having more free
parameters, we show analytically and in numerical experiments that this
convolution architecture is self-regularizing and implicitly learns similar
filters across arrays. We also report superior performance on a number of
datasets and increased robustness to adversarial attacks. Our investigation
suggests to revise the notion that mixed analog-digital hardware is not
suitable for ConvNets
Training LSTM Networks with Resistive Cross-Point Devices
In our previous work we have shown that resistive cross point devices, so
called Resistive Processing Unit (RPU) devices, can provide significant power
and speed benefits when training deep fully connected networks as well as
convolutional neural networks. In this work, we further extend the RPU concept
for training recurrent neural networks (RNNs) namely LSTMs. We show that the
mapping of recurrent layers is very similar to the mapping of fully connected
layers and therefore the RPU concept can potentially provide large acceleration
factors for RNNs as well. In addition, we study the effect of various device
imperfections and system parameters on training performance. Symmetry of
updates becomes even more crucial for RNNs; already a few percent asymmetry
results in an increase in the test error compared to the ideal case trained
with floating point numbers. Furthermore, the input signal resolution to device
arrays needs to be at least 7 bits for successful training. However, we show
that a stochastic rounding scheme can reduce the input signal resolution back
to 5 bits. Further, we find that RPU device variations and hardware noise are
enough to mitigate overfitting, so that there is less need for using dropout.
We note that the models trained here are roughly 1500 times larger than the
fully connected network trained on MNIST dataset in terms of the total number
of multiplication and summation operations performed per epoch. Thus, here we
attempt to study the validity of the RPU approach for large scale networks.Comment: 17 pages, 5 figure
Training large-scale ANNs on simulated resistive crossbar arrays
Accelerating training of artificial neural networks (ANN) with analog
resistive crossbar arrays is a promising idea. While the concept has been
verified on very small ANNs and toy data sets (such as MNIST), more
realistically sized ANNs and datasets have not yet been tackled. However, it is
to be expected that device materials and hardware design constraints, such as
noisy computations, finite number of resistive states of the device materials,
saturating weight and activation ranges, and limited precision of
analog-to-digital converters, will cause significant challenges to the
successful training of state-of-the-art ANNs. By using analog hardware aware
ANN training simulations, we here explore a number of simple algorithmic
compensatory measures to cope with analog noise and limited weight and output
ranges and resolutions, that dramatically improve the simulated training
performances on RPU arrays on intermediately to large-scale ANNs
A flexible and fast PyTorch toolkit for simulating training and inference on analog crossbar arrays
We introduce the IBM Analog Hardware Acceleration Kit, a new and first of a
kind open source toolkit to simulate analog crossbar arrays in a convenient
fashion from within PyTorch (freely available at
https://github.com/IBM/aihwkit). The toolkit is under active development and is
centered around the concept of an "analog tile" which captures the computations
performed on a crossbar array. Analog tiles are building blocks that can be
used to extend existing network modules with analog components and compose
arbitrary artificial neural networks (ANNs) using the flexibility of the
PyTorch framework. Analog tiles can be conveniently configured to emulate a
plethora of different analog hardware characteristics and their non-idealities,
such as device-to-device and cycle-to-cycle variations, resistive device
response curves, and weight and output noise. Additionally, the toolkit makes
it possible to design custom unit cell configurations and to use advanced
analog optimization algorithms such as Tiki-Taka. Moreover, the backward and
update behavior can be set to "ideal" to enable hardware-aware training
features for chips that target inference acceleration only. To evaluate the
inference accuracy of such chips over time, we provide statistical programming
noise and drift models calibrated on phase-change memory hardware. Our new
toolkit is fully GPU accelerated and can be used to conveniently estimate the
impact of material properties and non-idealities of future analog technology on
the accuracy for arbitrary ANNs.Comment: Submitted to AICAS202
Spin susceptibility and effective mass of two-dimensional electrons in MgxZn1-xO/ZnO heterostructures
We report measurements of the spin susceptibility and the electron effective
mass for two-dimensional electrons confined at the interfaces of MgxZn1-xO/ZnO
single heterostructures (x = 0.05, 0.08, and 0.11), grown by molecular-beam
epitaxy on (0001) ZnO substrates. By tuning the built-in polarization through
control of the barrier composition, the electron density was systematically
varied in the range of 5.6 x 10^11 to 1.6 x 10^12 cm^-2, corresponding to a
range of 3.1 < rs < 5.2, where rs is the average electron spacing measured in
units of the effective Bohr radius. We used the coincidence technique, where
crossings of the spin-split Landau levels occur at critical tilt angles of
magnetic field, to evaluate the spin susceptibility. In addition, we determined
the effective mass from the temperature dependence of the Shubnikov-de Haas
oscillations measured at the coincidence conditions. The susceptibility and the
effective mass both gradually increase with decreasing electron density,
reflecting the role of electron-electron interaction.Comment: 4 pages, 4figures, accepted for publication in Phys. Rev.