EEWeb Pulse - Issue 71

INTERVIEW

EEWeb Issue 71

November 6, 2012

Jess Lee President & CEO InVisage

TECHNICAL ARTICLE

Medical Body Area Networks PROJECT

Quantum-dot Photodetector Technology

Electrical Engineering Community

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EEWeb PULSE

TABLE OF CONTENTS

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Jess Lee INVISAGE Interview with Jess Lee - President and CEO

Featured Products

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Fast, Sensitive and Spectrally Tuneable Colloidal-Quantum-Dot Photodetectors

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BY JASON P. CLIFFORD, GERASIMOS KONSTANTATOS, KEITH W. JOHNSON, SJOERD HOOGLAND, LARISSA LEVINA & EDWARD SARGENT, WITH INVISAGE A look into the back-end technology that goes into InVisage’s groundbreaking quantum-dot photo technology.

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Medical Body Area Networks: High Level Overview

BY LS RESEARCH How MBANs provide a cost-effective way to monitor every patient in a healthcare institution, so clinicians can provide realtime and accurate data, allowing them to intervene and save lives.

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RTZ - Return to Zero Comic

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EEWeb | Electrical Engineering Community

INTERVIEW

InVisage Technologies, Inc. is a venturebacked, fabless semiconductor company based in Menlo Park, California. We spoke with their CEO, Jess Lee, about how they are revolutionizing digital photography with their amazing QuantumFilm technology and how they are putting the innovation back into the CMOS image sensor field.

Jess Lee InVisage

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EEWeb PULSE How did you get into electrical engineering?

I received my bachelor’s degree in Electrical Engineering at a school in Toronto and moved to Silicon Valley to begin my career. Soon after that, I got into working with startups, and then landed at SGI. After that, I joined a product marketing group at Creative Labs, where I had the chance to see products from a slightly different perspective as opposed to directly laying out the gates and writing the software.

At the time, I felt it was a lot more interesting because you had a wider canvas to paint on, so to speak. This is when I started working on my first camera. You may remember the days where there were blackand-white cameras that looked like a little round eyeball. At Creative Labs, we made a follow-on to that—instead of black and white,

the digital photography revolution.

Digital photography became a huge hobby and interest for me and I’ve been working in it ever since.

How did you get from your early work with Creative Labs to InVisage?

I took a break from cameras to work on connectivity technology at a start-up to gain some experience for wireless devices. Soon after that, around 2002, the team at OmniVision convinced me that they had a great opportunity ahead of them and that they really needed help with this new idea to put cameras in cellphones. The concept was crazy enough for me, so I joined the company. If you remember back then, camera phones weren’t yet popular, and companies were just starting to produce cameras that you could attach to the body of the

low-cost. I like to think that we were successful in doing that.

Our product at InVisage is a whole new type of image sensor that integrates QuantumFilm, a very dark, thin and highly absorbent layer of film on top of silicon that captures light. Early on in the development of our product I spoke to some outside engineers who didn’t how the silicon could absorb light. I had to explain to them that it was actually the film that captured the light, and then translated it into electrons. There is a silicon device underneath the film that actually stores the captive electrons, so it is, in a very traditional sense, a sensor, except it’s doing half the job that a CMOS sensor would do, which is reading out the electrons. This is the crux in our sensor technology—we don’t use silicon to absorb the light, we use film.

“There is a silicon device underneath the film that actually stores the captive electrons, so it is, in a very traditional sense, a sensor, except it’s doing half the job that a CMOS sensor would do, which is reading out the electrons.” we used color and CMOS. We very quickly evolved that product into somewhat of a hybrid, which was a camera that you could connect to your computer. It had a little bit of storage so you could unplug it to take pictures but had no LCD panel so the design was very simple. It was essentially a portable webcam, which came at the beginning of

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cellphone that took decent pictures.

That’s when I first got into the world of camera phones.

I worked with a really amazing team at OmniVision to bring CMOS image sensors into the mainstream— mainly trying to make these digital cameras much easier to use and more complete while remaining

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How did you come up with this product?

The original invention was actually out of our CTO Ted Sargent’s lab at the University of Toronto. There, he was making transmitters and receivers for all sorts of wavelengths, and he discovered that one of his receivers was really sensitive.

INTERVIEW

When he got to the bottom of it, what he discovered was something that was groundbreaking and he determined he should really try to commercialize it. Ted ended up getting funded in the early days to bring this to market. Starting out, the primary question was which market was most well suited to address with this technology. When I got together with Ted during this time, I

put it out there—there is this rather large market for mobile cameras that is quite demanding and has very high needs for high quality, yet is challenging from a design standpoint. You need a device that extremely sensitive in the smallest possible size. Sensors don’t really follow Moore’s law—you make it smaller and it doesn’t get two times cheaper or faster. The problem is in

traditional CMOS image sensors, the performance does not go up two times, it usually goes down. So I knew this was a great market to enter and we decided to focus the entire company on this space with this technology.

The material is unique—it’s made out of quantum dots that are composed into a matrix film material that lies really flat onto8 or 12-inch wafers. This film is optically very thin and absorbent—it has to be in order to capture all of the photons that hit it and convert those into electrons.

The quantum dot is already a semiconductor, and what we do is modify that semiconductor by changing its size. By shrinking this material, you actually change its fundamental properties, which is the source of our innovation.

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EEWeb PULSE Do you license your technology to other companies?

As a small, venture-backed startup, we have to be very focused with our resources. Our first goal is to get our product to market as quickly as possible, because we see very strong demand from a lot of the handset folks we worked with. This is how we will establish this technology in its own right.

In the longer term, especially for certain adjacent markets that we’re interested in but lower priority for us, there could easily be a discussion about partnership for licensing. My experience has been that whatever technology you produce for the market you really want to control its roll out, its production and all the other parts because it’s possible that you can throw too many parties into the mix.

Can you tell us a little about your team and where you are based?

We have around 30 plus employees now and the team is awesome —we have a great combination of folks here. Everyone is deeply technical and falls into one of two categories.

The first group are sensor- and silicon-design guys from a very traditional background who are designing a very different type of sensor now. The second group is our materials team, which is comprised of a group of chemists, scientists, process engineers and material scientists to scale us with a fab environment.

What’s the goal for InVisage?

We love cameras, we love imaging and want to shake things up.

There’s very little innovation today in traditional CMOS image sensors, so digital image quality and performance aren’t nearly as

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“The material is unique—it’s made out of quantum dots that are composed into a matrix film material that lies really flat on to 8 or 12-inch wafers. This film is optically very thin and absorbent—it has to be in order to capture all of the photons that hit it and convert those into electrons.” good as they can be. We are going revolutionize this space, and bring back the gorgeous image quality of traditional film. Back then, if you thought about your old Kodak film, you never thought twice about resolution. You just knew it was going to be extremely high. This is our vision…that you will ultimately be able to capture the true reality of the scene. QuantumFilm, at the nanoscale resolution, utilizing tens of billions of quantum dots per image sensor, is what will make this possible. ■

To the right: A close-up of the QuantumFilm sensor

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INTERVIEW

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Fast, se

and spectrall

COLLOIDAL-Q PHOTODETECT Jason P. Clifford, Gerasimos Konstantatos, Keith W. Johnston, Sjoerd Hoogland, Larissa Levina and Edward H. Sargent Department of Electrical and Computer Engineering University of Toronto

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PROJECT

ensitive

ly tuneable

QUANTUM-DOT TORS

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EEWeb PULSE Solution-processed semiconductors are compatible with a range of substrates, which enables their direct integration with organic circuits1,2, microfluidics3,4, optical circuitry1,5 and commercial microelectronics.

Ultrasensitive photodetectors based on solutionprocess colloidal quantum dots operating in both the visible and infrared have been demonstrated6,7, but these devices have poor response times (on the scale of seconds) to changes in illumination, and rapidresponse devices based on a photodiode architecture suffer from low sensitivity8. Here, we show that the temporal response of these devices is determined by two components—electron drift, which is a fast process, and electron diffusion, which is a slow process. By building devices that exclude the diffusion component, we are able to demonstrate a >1,000-fold improvement in the sensitivity bandwidth product of tuneable colloidal-quantum-dot photodiodes operating in the visible and infrared6–8.

Colloidal quantum dots (CQDs) combine quantum size effect tuning with the practical advantages of solution processing. Quantum size effect tuning allows the energy bandgap and absorption onset of CQDs to be varied over a wide range of wavelengths in the visible and infrared (IR)9. Operation at IR wavelengths allows the detection of light transmitted through atmospheric10, biological11,12 and other materials absorption windows, dramatically increasing the range of potential applications. The ability to limit spectral absorption is also important for photodetector applications; the bandgap should only be as small as is necessary to absorb photons of interest, while rejecting background photons and minimizing internal noise that would otherwise limit sensitivity.

Photoconductive CQD photodetectors have recently been reported with remarkable sensitivities up to 1 x 1013 Jones (refs 7, 8) (see Supplementary Information, Section 1, for a definition of photodetector sensitivity).

As a result, slow modulation response and narrow response bandwidth (<20 Hz) have been tolerated in these devices, but these limitations severely curtail potential applications.

An alternative photodetector architecture, the photodiode, offers the potential of significantly higher response speed. However, the only report of a fast CQD detector (50 kHz bandwidth) demonstrated a sensitivity of approximately 1 x 107 Jones (ref.

8)â&#x20AC;&#x201D;five orders of magnitude lower than crystalline semiconductor photodetectors.

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Figure 1 | Photodiode structure, energy bands and photocurrent components. a, A schematic representation of the photodiode device architecture. b, The Schottky barrier at the Al/PbS (Q1) interface forms a depletion region in the CQD film with a biasdependent width (wDR) and a built-in potential (Vbi). The remaining width of the CQD film (wQNR) is unaffected by the Schottky barrier and is designated as a quasi-neutral region. Electrons and holes generated in the depletion region (GDR) drift under the influence of the electric field to the aluminium contact and the edge of the quasi-neutral region, respectively. Electrons generated in the quasi-neutral region (GQNR) must diffuse to the edge of the depletion region, where they are rapidly swept away by the electric field. The movement of holes in the quasi-neutral region is governed by the relaxation of the p-type semiconductor.

In this work, we took the view that reliance on the longlived minority carrier traps essential to photoconduction would necessarily lead to slow response speeds. We therefore pursued the charge separating photodiode architecture, recognizing that an orders-of-magnitude improvement in sensitivity would be required.

CQD films offer a number of chemically tuneable degrees of freedom that provide control over their macroscopic electronic properties. The spacing between individual CQDs is controlled by the length of the organic ligands used to passivate their surfaces and has been shown to be a determining factor in the charge carrier mobility and conductivity of CQD films8,13,14. The consistency of surface passivation is also important, as this limits oxidation and other chemical modification of the CQD surface. Oxidation provides a route to dope CQD films14,15; however, uncontrolled variation in the CQD surface leads to

PROJECT substrate generates electrons and holes in the CQD film that are collected at the aluminium and ITO contacts, respectively. The energy band diagram in Fig. 1b shows the Schottky barrier formed at the Al/ PbS CQD interface16, and the built-in potential derived from the difference in work function between the CQDs and the metal contact. A depletion region in the CQD film forms at the metalâ&#x20AC;&#x201C;CQD interface, whereas the remaining volume of CQD film is a quasi-neutral region of p-type semiconductor16. The large potential barrier in the valence band limits majority carrier (hole) injection from the aluminium contact, resulting in highly rectifying dark Iâ&#x20AC;&#x201C;V characteristics16.

We synthesized PbS CQDs with a diameter of ~6 nm, increasing the effective bandgap from the bulk PbS value of 0.42 eV to 0.86 eV through the quantum size effect9. This effective bandgap corresponds to a ground state excitonic absorption feature at 1,450 nm.

A three-stage CQD surface modification strategy was used to create films of densely packed CQDs with stable benzenedithiol (BDT) surface passivation and controlled effective doping (see Figure 2 | Photodiode spectral response, illumination response and frequency response. a, Supplementary Information, External quantum efficiency (at 295 K) and normalized detectivity (at 250 K) as a function of Section 2). BDT treatment wavelength. b, Photocurrent density as a function of irradiance at 1,550 nm. c, Frequency dependence of photocurrent at zero bias and 17.9 mWcm22 irradiance at 1,550 nm. d, of the CQD films increased Frequency dependence (3 dB) on irradiance. e, Frequency dependence (3 dB) on bias. photodiode lifetime from ~4 h to >2 months and dramatically reduced short-circuit dark current densities from ~100 interface states and a reduction of the built-in potential -2 to below 0.1 nA cm . The 1,000-fold decrease in shortwhen forming metallurgical junctions. circuit dark current is attributed to the elimination of primary butylamine ligands in the CQD film and their In diodes, the energetic barrier associated with the interaction with the aluminium contact. The noise metallurgical junction defines the effective shunt associated with this electrochemical dark current resistance (ROA), which, in turn, determines noise previously limited the detectivity of CQD photodiodes performance. to ~1 x 1010 Jones.

We fabricated photodiodes based on a Schottky Figure 2a shows external quantum efficiency (EQE) barrier at the interface between a PbS CQD film and (at 295 K) and normalized detectivity (at 250 K) as a an aluminium contact (Fig. 1a). A planar, transparent function of wavelength in the CQD photodiode. The indium tin oxide (ITO) thin film formed the opposing shape of the EQE and D* spectra follows the absorption ohmic contact. Light incident through the glass Visit www.eeweb.com

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EEWeb PULSE spectrum of the CQD film (see Supplementary Information, Section 3). The peak in the absorption and EQE at 1,450 nm corresponds to the ground-state CQD excitonic absorption feature, whereas peaks in EQE at shorter wavelengths are the result of Fabry– Perot interference in the thin CQD film. Figure 2b shows photocurrent density as a function of irradiance.

The photoresponse is linear within 6% over 4 decades of irradiance. (See Supplementary Information, Section 4, for current–voltage characteristics of the photodetector.)

The dramatic increase in EQE compared to previous CQD photodiode photodetectors was achieved by creating a large (0.20 V) built-in potential for photogenerated charge carrier separation. Controlled oxidization increased the doping of the CQD film (after treatment with a reducing agent, BDT) and the height of the Schottky barrier at the Al/PbS CQD metallurgical junction. The optimal duration of the oxidation process was determined empirically to provide the largest built-in potential before growth of interfacial states at the metallurgical junction began to degrade EQE. Figure 2c shows normalized photocurrent in the photodiode as a function of illumination modulation frequency. The poles (onset of an exponential decay in response) at 1 and 50 kHz correspond to the time required to reach the steady state in the quasineutral region and the transit time of the depletion region, ~500 and ~10 μs, respectively, as shown in Fig. 3a,b and discussed below. Figure 2d,e shows an exponential dependence of the 3 dB frequency on irradiance and a sublinear dependence of the 3 dB frequency on reverse bias.

Figure 3 | Photodiode transient response and quantum efficiency as a function of bias and irradiance. a, Measured (noisy line) and simulated (smooth line) photocurrent transient response, as a function of bias, to a 500-μs-square illumination pulse at 17.9 μW cm-2 at 1,550 nm. Note that the measured and simulated lines are coincident for 0.0 V. b, Measured and simulated photocurrent transient response (normalized) as a function of irradiance at zero bias. The simulated response is shifted for clarity. c, Measured and simulated EQE as a function of irradiance at 1,550 nm.

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Under zero-bias conditions and in the absence of illumination, noise current in photodiodes is proportional to RoA, the product of the zero-bias effective shunt resistance and the device area. In the CQD photodiode, RoA increases with decreasing temperature from 1 x 105 V cm2 at 300 K to just under 1 x 107 Ω cm2 at 235 K (see Supplementary Information, Section 5)—values comparable to highly optimized crystalline semiconductor photodiodes.

The magnitude of RoA (and the dark short-circuit current) is typically dependent on the bandgap of the semiconductor, as a larger bandgap enables a larger Schottky barrier. We maximized RoA through effective CQD surface passivation, which limits surface states at the Al/PbS CQD metallurgical junction. Despite

PROJECT working with an IR-sensitive semiconductor, we achieved a 1,000-fold reduction in short-circuit noise current and a 100-fold reduction in short-circuit dark current compared to recent reports on visible CQD photodetectors based on wider bandgap semiconductors8.

The speed and sensitivity of the CQD photodiode represents a 3,300-fold improvement in response speed and an 11 orders-of-magnitude reduction in dark current density over the most sensitive CQD photodetector reported6. D* values of 1 x 1012 Jones represent sensitivity on the order of commercially available photodetectors fabricated with crystalline semiconductors such as silicon and InGaAs. Equally important, this represents a 100,000-fold improvement in sensitivity and response speeds up to 20% greater than the fastest CQD photodetector reported8.

We used the transient response to stepwise changes in illumination, combined with a physical model, to establish the operation of the CQD photodiode.

Figure 3a shows the photocurrent response to a 500-μs-square illumination pulse at biases of 0.0, -0.5 and -1.0 V. At each bias, the transient response is composed of two components: an initial, fast, linearly increasing component, and a slower component that exponentially settles to a steady state. The rise-and-fall characteristics of the photocurrent are symmetric. The fast component is attributed to carriers generated in the depletion region (GDR in Fig. 1b) and swept out as a drift current proportional to the built-in electric field (E).

The rise time of this component is the time required to transit the depletion region (ttr = wDR/[µdriftE]), where µdrift is the drift mobility, approximately the same for both electrons and holes according to measured drift mobilities (see Supplementary Information, Section 9). The slower component is attributed to electrons generated in the quasi-neutral region (GQNR in Fig. 1b) that must diffuse to the depletion region. The rise time of this component is the time required for generation, diffusion and recombination in the quasi-neutral region to reach a steady state, and is dependent on the width of the quasi-neutral region (wQNR) and inversely dependent on the electron diffusivity (De).

Measurement of photodiodes fabricated from polydispersed CQD films indicated that exciton diffusion does not play a significant role in photodiode operation (see Supplementary Information, Section 6).

We have developed a numerical model that solves

Figure 4 | Fully depleted photodiode spectral response and frequency response. a, Photocurrent transient response, as a function of bias, to a 1-μs-square illumination pulse at 40 mW cm-2 and 1,550 nm. b, Frequency dependence of photocurrent at zero bias and 40 mW cm-2 irradiance at 1,550 nm. c, Frequency dependence (3 dB) on bias.

the continuity equations for electrons and holes in the quasi-neutral and depleted regions of the CQD photodiode based on measured materials and device parameters (see Supplementary Information, Sections 7 to 11). The model accurately depicts the dependence of the photocurrent transient on bias and irradiance, as

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EEWeb PULSE shown in Fig. 3a,b. Using the measured carrier lifetime dependence on irradiance, the numerical model also quantitatively predicts the dependence of photodiode EQE on irradiance, as shown in Fig. 3c. Photodiode EQE is independent of irradiance at irradiances, <1 x 10-5 W cm-2, but begins to decrease at irradiances >1 x 10-5 W cm-2. This transition corresponds to the minimum carrier lifetime required for photocarriers generated in the quasineutral region to diffuse to the edge of the depletion region.

The EQE of CQD photodiodes is determined by three processes, each characterized by an efficiency ranging from 0 to 1:

ηexternal = ηabs • ηdiss • ηextr ηabs is the fraction of the incident photon flux absorbed by the CQD film, ηdiss represents the probability of photogenerated excitons dissociating into individual charge carriers, and ηextr quantifies the efficiency with which these charge carriers are transported, through drift and diffusion, to the contacts. We determined ηabs to be ~37% at the excitonic absorption feature (1,450 nm). ηextr approaches unity below irradiances of 1 x 10-5 W cm-2, because the drift length and diffusion lengths exceed the depletion region and quasi-neutral region depths, respectively (see Supplementary Information, Section 12). ηdiss therefore lies in the range 40–60%, depending on bias, to account for the observed external EQE of 17 to 25% at 1,450 nm over the bias range 0.0 to -1.0 V. The model of CQD photodiode operation pointed to a clear conclusion: an efficient and much faster CQD photodiode could be developed if photocarriers could be transported by drift alone. By reducing the thickness of the CQD film to equal the width of the depletion region (180 nm), we fabricated a fully depleted photodiode in which all photogenerated electrons and holes are swept directly to the contacts by the electric field resulting from the built-in potential of the junction and any externally applied bias. Figure 4a shows that transient photocurrent response to a 1-ms-square illumination pulse is indeed limited only by the drift transit time (~300 ns).

The drift transit time of the CQD photodiode was further reduced by increasing the mobility of the CQD film through a more complete ligand exchange than that used in the initial CQD photodiode development (majority carrier hole mobility increased from 1 x 10-4

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to 6 x 10-4 cm2 V-1 s-1). The frequency response of the fully depleted photodiode is shown in Fig. 4b. The 3 dB frequency increases with reverse bias, as shown in Fig. 4c, due to a reduction in carrier transit time with increasing electric field in the depletion region. The device remains linear within 10% over six decades of irradiance. The short-circuit dark current density was unchanged from the previous device at <0.1 nA cm-2.

The response speed of the fully depleted CQD photodiode represents a significant improvement over all reported CQD photodetectors: a 170,000fold improvement in response speed over the most sensitive CQD photodetector reported6, and a 60-fold improvement in response speed over the fastest CQD photodetector reported8. With D* > 1 x 1011 Jones at room temperature, this device maintains a 10,000fold improvement in sensitivity over the fastest CQD photodetector reported8.

The combination of speed and sensitivity demonstrated by the fully depleted CQD photodiode represents a >1,000-fold improvement in sensitivity–bandwidth product relative to all previous CQD photodetectors.

This improvement in device performance was achieved by designing a photodiode architecture to take advantage of insights into charge carrier transport and by tailoring the CQD film passivation to support both high carrier mobilities and a large builtin potential at a metallurgical junction.

References 1. Xue, J. & Forrest, S. R. Organic optical bistable switch. Appl. Phys. Lett. 82, 136–138 (2003).

2. Kymissis, I., Sodini, C. G., Akinwande, A. I. & Bulovic, V. An organic semiconductor based process for photodetecting applications. 2004 IEDM Tech.

Dig., 377–380 (2004).

3. Hofmann, O. et al. Thin-film organic photodiodes as integrated detectors for microscale chemiluminescence assays. Sens. Actuators B 106, 878–884 (2005).

4. Wang, X. Integrated thin-film polymer/ fullerene photodetectors for on-chip microfluidic chemiluminescence detection. Lab. Chip 7, 58–63 (2007).

5. Morimune, T., Kajii, H. & Ohmori, Y. Semitransparent organic photodetectors utilizing sputter-deposited

PROJECT indium tin oxide for top contact electrode. Jpn J. Appl.

Phys. 44, 2815–2817 (2005).

6. Konstantatos, G. et al. Ultrasensitive solution-cast quantum dot photodetectors. Nature 442, 180–183 (2006).

7. Konstantatos, G., Clifford, J. P., Levina, L. & Sargent, E. H. Sensitive solution-processed visible-wavelength photodetectors. Nature Photon. 1, 531–534 (2007).

8. Oertel, D. C., Bawendi, M. G., Arango, A. C. & Bulovic, V. Photodetectors based on treated CdSe quantum-dot films. Appl. Phys. Lett. 87, 2135051 (2005).

9. Wise, F. W. Lead salt quantum dots: the limit of strong quantum confinement. Acc. Chem. Res. 33, 773–780 (2000).

first step of the CQD surface modification strategy.

S.H. and J.P.C. co-developed the second step of the CQD surface modification strategy. L.L. synthesized all CQDs used to fabricate the devices. E.H.S. assisted in interpretation of the results, commented on the device model, and commented on the manuscript. All authors discussed the results and the capacity of the model to describe the underlying physics of device operation.

Additional information Supplementary Information accompanies this paper at www.nature.com/naturenanotechnology. Reprints and permission information is available online at http://npg.nature.com/reprintsandpermissions/.

Correspondence and requests for materials should be addressed to E.H.S.

10. Jones, A. V. The infrared spectrum of the airglow.

Space Science Rev. 15, 355–400 (1973).

11. Sargent, E. H. Infrared quantum dots. Adv. Mater.

17, 515–522 (2005).

12. Kim, S. et al. Near-infrared fluorescent type II quantum dots for sentinel lymph node mapping.

Nature Biotechnol. 22, 93–97 (2004).

13. Jarosz, M. V., Porter, V. J., Fisher, B. R., Kastner, M. A. & Bawendi, M. G. Photoconductivity studies of treated CdSe quantum dot films exhibiting increased exciton ionization efficiency. Phys. Rev. B 70, 195327 (2004).

14. Luther, J. M. et al. Structural, optical and electrical properties of self-assembled films of PbSe nanocrystals treated with 1,2-ethanedithiol. ACS Nano 2, 271–280 (2008).

15. Talapin, D. V. & Murray, C. B. PbSe nanocrystal solids for n- and p-channel thin film field-effect transistors. Science 310, 86–89 (2005).

16. Clifford, J. P., Johnston, K. W., Levina, L. & Sargent, E. H. Schottky barriers to colloidal quantum dot films.

Appl. Phys. Lett. 91, 2531171 (2007).

Author contributions J.P.C conceived and fabricated the CQD photodiodes, performed all device performance characterization, and conceived and implemented the CQD photodiode device model. G.K. coordinated and interpreted the XPS measurements. K.W.J. and J.P.C co-developed the

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bestartech.com | sales@bestartech.com | 520.439.9204 QS9000 • TS/ISO16949 • IS O 14001 • IS O 13485 • IS O 9001

EEWeb PULSE

Medic Area

High L Robert Buczkiewicz

Director of Hardware Developmen LS Research

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EEWeb EEWeb||Electrical ElectricalEngineering EngineeringCommunity Community

TECH ARTICLE

cal Body Networks:

Level Overview

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The Federal Communications Commission has advanced its wireless health care agenda by adopting rules that will enable Medical Body Area Networks (MBAN), low-power wideband networks consisting of multiple body-worn sensors that transmit a variety of patient data to a control device. MBAN devices free patients from cumbersome cables that tether them to their hospital beds. MBAN devices provide a cost effective way to monitor every patient in a healthcare institution, so clinicians can provide real-time and accurate data, allowing them to intervene and save lives. Wireless devices that operate on an MBAN spectrum can be used to actively monitor a patientâ&#x20AC;&#x2122;s health, including blood glucose and pressure monitoring, delivery of electrocardiogram readings, and even neonatal monitoring systems. MBAN devices will be designed to be deployed widely within a hospital setting and will make use of inexpensive disposable body-worn sensors. MBAN technology will also make it easier to move patients to different parts of the health care facility for treatment and can dramatically improve the quality of patient care by giving health care providers the chance to identify life-threatening problems or events before they reach critical levels.

Visit www.eeweb.com Visit www.eeweb.com

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EEWeb PULSE Currently almost 50% of all patients in U.S. hospitals are not monitored.

FCC Chairman Julius Genachowski stated that MBAN systems will enable patient monitoring in real time that is both accurate and costeffective. “A monitored patient has a 48% chance of surviving a cardiac arrest.

Unmonitored patients have a 6% chance of survival.” By allowing for continuous monitoring of patients, MBAN enabled devices can help doctors respond more quickly in emergency situations and improve overall in-home patient care as well. According to the FCC, this action “represents an improvement over traditional medical monitoring devices,” as it makes it easier to move a patient, allows for increased patient comfort, and could represent annual savings of $1.2 billion in expenses accrued after relocating patients to different clinics and departments.

This final ruling sets aside 40 MHz of protected spectrum in the 2360-2400 MHz band specifically for wireless medical devices. Dedicated spectrum should help alleviate the interference problems normally associated with Wi-Fi and other high-powered devices used in hospitals. The 2360-2390MHz frequency range is available on a secondary basis. The FCC will expand the existing Medical Device Radiocommunication (MedRadio) Service in Part 95 of its rules. MBAN devices using the band will operate under a ‘licenseby-rule’ basis, which eliminates the need to apply for individual transmitter licenses. Usage of the 23602390MHz frequencies are restricted to indoor operation at health-care facilities and are subject to registration and site approval by coordinators to protect aeronautical telemetry primary usage. Operation in the 2390-2400 MHz band is not subject to registration or coordination and may be used in all areas including residential.

The MBANs Advantage Small, wearable sensors monitor patients’ vital signs and collect real-time clinical information such as temperature, blood glucose levels, blood pressure, pulse and respiratory function, and aggregate it over an MBAN at a nearby device for local processing and forwarding to centralized displays and electric medical

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Figure 1: MBAN Device (courtesy FCC photostream)

records. Body sensors can also be used to deliver medical therapy to particular areas of the body.

Key advantages include: • Allowing patient monitoring throughout the entire hospital which increases comfort and mobility • Patients can be monitored before they reach the hospital and after they are sent home • Enhanced patient safety, care and comfort by eliminating the need for cables that restrict patients by tethering them to hospital beds • Ease of transport: allowing patients to be easily relocated in different clinics • Medical-equipment makers employing wireless monitoring should rid hospital rooms of nests of cables and let patients move out of expensive intensive-care wards • Infection control: eliminating wires could help reduce the risk of infection

EEWeb | Electrical Engineering Community

Figure 2: (Right) MBAN Devices (courtesy of GE Healthcare)

TECH ARTICLE

Regulatory Status • Beginning October 1, 2012 devices are permitted under the FCC’s “license by rule” standards under which healthcare providers must register their devices and coordinate how and when they are used.

• The FCC is expected to appoint a frequency coordinator by next June to determine how two users can share the spectrum without interfering with one another. Both GE Healthcare and Philips Healthcare Systems are recommending that this process be expedited. American Society for Healthcare Engineering (ASHE), which is now the WMTS coordinator, has expressed its interest in being the MBAN coordinator as well.

• MBAN devices will need to receive FCC and Food and Drug Administration (FDA) approval before they can be used in hospitals. The FCC and FDA, which has regulatory control over mobile medical devices, are working together to streamline the approval process.

The process could call for the FCC to review the technical aspects of a device, while the FDA would review its medical features.

Figure 3: Wireless Spectrum (courtesy of Modern Mobile Apps)

• The FCC’s approval makes the U.S. the first country to allocate spectrum for MBANs. • The 2360- 2400 MHz band holds potential for international harmonization. Both Texas Instruments and GE Healthcare note that the band is included in the technical requirements developed by a European standards group studying medical devices.

Technical Overview • 40 MHz of protected spectrum in the 2360-2400 MHz band • The 2360-2390 MHz portion of the band is rangerestricted to indoor use at health care facilities and will be subject to registration with an MBAN coordinator and additional coordination if warranted by location.

The maximum transmit power is 1 mW measured over 1 MHz bandwidth • The 2390-2400 MHz band will not require registration and coordination, and may be used in any location – including in-home residential settings. The maximum transmit power is 20 mW measured over 5 MHz bandwidth • Use is on a shared, secondary, non-interference basis with Aeronautical Mobile Telemetry (AMT) or flight test radios occupying the 2360-2395 MHz band • MBANs require a clear channel and the Wireless Visit www.eeweb.com

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EEWeb PULSE Medical Telemetry Service (WMTS) does not have the spectrum to accommodate them • Expected Transmission Range: short range, about one hospital room, similar to Bluetooth • Limited Data rate: <2 Mbps

What is an MBAN network?

The FCC defines a Medical Body Area Network (MBAN) as a low power network consisting of a MedRadio programmer/control transmitter and multiple medical body-worn devices all of which transmit or receive nonvoice data or related device control commands for the purpose of measuring and recording physiological parameters and other patient information or performing diagnostic or therapeutic functions via radiated bi- or uni-directional electromagnetic signals.

How does an MBAN system operate?

A typical MBAN consists of a master programmer/control transmitter (“hub device”), which is included in a device close to the patient, and one or more client transmitters (“body sensors”), which are worn on the body and only transmit while maintaining communication with the hub that controls the transmissions. The hub conveys data messages to the body-worn sensors to specify, for example, the transmit frequency that should be used.

The hub and sensor devices will transmit in the 23602400 MHz band.

To protect AMT operations from harmful interference, MBAN and AMT frequency coordinators will work together to coordinate MBAN operations in the 23602390 MHz band. The control point serves as the interface between the MBAN coordinator and the MBAN master transmitters to control MBAN operation in the 2360-2390 MHz band. The control point will receive an electronic key which is a data message that specifies and enables use of specific frequencies by the MBAN devices. The control point, in turn, will generate a beacon or control message to convey a data message to the hub via the facility’s LAN that specifies the authorized frequencies and other operational conditions for that specific MBAN.

What’s Next?

As of October 1, 2012 devices are permitted under the FCC’s “license by rule” standards under which healthcare providers must register their devices and coordinate how and when they are used. However, the FCC is not expected to appoint a frequency coordinator until June, 2013 and according to GE Healthcare, “MBAN devices are expected to hit the market in about 2 years.”

From a wireless product development, several of the major semiconductor manufacturers have indicated that their current 2400-2483.5 MHz ISM band chipsets used for ZigBee/Bluetooth can easily be modified or adapted.

Simple RF design changes to cover the 2360-2400 MHz MBAN spectrum combined with a new antenna design should allow for rapid deployment of the technology.

Figure 4: MBAN Hub/Sensor Nodes

The hub aggregates patient data from the body-worn sensors under its control and transmits that information, using the health care facility’s local area network (LAN) (which could be, for example, Ethernet, WMTS or Wi-Fi links), to locations where health care professionals monitor patient data. The hub also connects via the facility’s LAN to a central control point that will be used to manage all MBAN operations within the health care facility. Neither body sensors nor programmer/control transmitters may communicate directly with each other.

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EEWeb | Electrical Engineering Community

TECH ARTICLE Get the Datasheet and Order Samples http://www.intersil.com

Low Voltage ORing FET Controller ISL6146

Features

The ISL6146 represents a family of ORing MOSFET controllers capable of ORing voltages from 1V to 18V. Together with suitably sized N-channel power MOSFETs, the ISL6146 increases power distribution efficiency when replacing a power ORing diode in high current applications. It provides gate drive voltage for the MOSFET(s) with a fully integrated charge pump.

• ORing Down to 1V and Up to 20V with ISL6146A, ISL6146B, ISL6146D and ISL6146E

The ISL6146 allows users to adjust with external resistor(s) the VOUT - VIN trip point, which adjusts the control sensitivity to system power supply noise. An open drain FAULT pin will indicate if a conditional or FET fault has occurred.

The ISL6146A and ISL6146B are optimized for very low voltage operation, down to 1V with an additional independent bias of 3V or greater.

The ISL6146C provides a voltage compliant mode of operation down to 3V with programmable Undervoltage Lock Out and Overvoltage Protection threshold levels The ISL6146D and ISL6146E are like the ISL6146A and ISL6146B respectively but do not have conduction state reporting via the fault output.

TABLE 1. KEY DIFFERENCES BETWEEN PARTS IN FAMILY PART NUMBER

KEY DIFFERENCES

ISL6146A

Separate BIAS and VIN with Active High Enable

ISL6146B

Separate BIAS and VIN with Active Low Enable

ISL6146C

VIN with OVP/UVLO Inputs

ISL6146D

ISL6146A wo Conduction Monitor & Reporting

ISL6146E

ISL6146B wo Conduction Monitor & Reporting

+

VOLTAGE DC/DC (3V - 20V)

VIN GATE VOUT BIAS ADJ ISL6146B FLT GND

EN

Q2 +

VOLTAGE DC/DC (3V - 20V)

VIN GATE VOUT VOUT BIAS ADJ ISL6146B FLT GND

EN

-

FIGURE 1. TYPICAL APPLICATION

October 5, 2012 FN7667.3

• VIN Hot Swap Transient Protection Rating to +24V • High Speed Comparator Provides Fast <0.3µs Turn-off in Response to Shorts on Sourcing Supply • Fastest Reverse Current Fault Isolation with 6A Turn-off Current • Very Smooth Switching Transition • Internal Charge Pump to Drive N-channel MOSFET • User Programmable VIN - VOUT Vth for Noise Immunity • Open Drain FAULT Output with Delay - Short between any two of the ORing FET Terminals - GATE Voltage and Excessive FET VDS - Power-Good Indicator (ISL6146C) • MSOP and DFN Package Options

Applications • N+1 Industrial and Telecom Power Distribution Systems • Uninterruptable Power Supplies • Low Voltage Processor and Memory • Storage and Datacom Systems

+

Q1

• Programmable Voltage Compliant Operation with ISL6146C

C O M M O N P O W E R

GATE FAST OFF, ~200ns FALL TIME ~70ns FROM 20V TO 12.6V ACROSS 57nF GATE OUTPUT SINKING ~ 6A

B U S +C O M M O N P O W E R B U S

FIGURE 2. ISL6146 GATE HIGH CURRENT PULL-DOWN

Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2011, 2012 All Rights Reserved. All other trademarks mentioned are the property of their respective owners.

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