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Reversed biased LED (again.. sorry) Thinking out loud

Discussion in 'Electronic Design' started by George Herold, Apr 30, 2013.

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  1. Hi guys, I hope this is the last time for the following reversed
    biased LED as Spad thread. (We've got to do a newsletter then manual,
    so I need to at least wave my hands at the details.) I should also
    apologize as this is mostly just my ‘thinking out loud’ to the SED.
    So again here's the circuit.

    | |
    (+) |
    Vbias - LED
    (-) ^
    | |
    | +-----opa314-->out
    | | buffer
    GND 100k

    Vbias is ~24V (device dependent)
    When light is shinned at the LED I see pulses.
    the number of which changes linearly with light intensity.

    There are two details for me to understand.
    First I've been thinking about the data in terms of 'breakdown
    channels' in the LED.
    I assume they are independent, though I can imagine that some samples
    might have
    overlapping channels.
    As any one channel breaks down it starts to discharges the entire LED
    capacitance. Also each channel has a different threshold voltage.
    When the voltage across the LED falls below that threshold then the
    breakdown stops.
    Does this make sense geometrically? The depletion width is much less
    than the square root of the area. (Is depletion width the right

    The data for this model is manifold. First here's a 'scope
    'histogram' of single photon events, 1 second persistence. ~5kHz count
    with the trigger set down near ground.

    I sent the same data into a comparator with adjustable reference
    voltage and get a stair case type distribution of count rate vs
    (data posted on request)

    If you trigger up high, then you just see one channel. The peak
    height is approximately equal to the bias voltage minus some
    'Vt' (threshold voltage). Here's a bunch of 'scope shots with
    different bias volatges.
    Vt =~23.1V

    I’m not sure how to explain why each channel has a different threshold

    The other thing I don’t understand is the turn on waveform. At low
    bias voltage and for early times with higher bias voltage the turn on
    looks linear with a slope that is related to the voltage peak... it
    takes about 50 – 100ns to turn on. (See the data above.) Now what’s
    weird is this turn on time seems to be independent of circuit
    following the LED. It’s independent of the quenching resistor value
    (100k ohm for the circuit shown) And also independent of the
    capacitance to ground. The R and C loading of the LED has an effect
    at later times and with higher bias voltages.
    Here’s a screen shot where I added 12pF to ground in parallel with the
    100k ohm resistor.
    If I do the same with lower bias voltage then there is almost no
    change in the waveform.
    I was expecting a slower ramp. The opa134 opamp that is used as a
    buffer has a input C of ~5pF.

    Anyway thanks for reading and any thoughts are most welcome.

    George H.
  2. miso

    miso Guest

    Most LEDS have a very low reverse bias abs max. I've seen as bad as 3V,
    though usually they spec 6v for operation in a matrix. (5VDC operation
    plus margin.]

    Under high reverse bias, you can get them to glow, but it is due to
    impact ionization rather than normal LED light output.

    So I assume this is for scientific interest and not a practical circuit.
  3. Hmm, OK that's interesting. I've been mostly trying to think about
    what happens in the avalnche region. So I had this impression that
    once the charge carriers reached the neutral (undepeleted) region
    (where they are majority carriers) that their image charges appeared
    on the contacts and there was not much time delay.
    I did a little reading about diffuison in photodiodes (S. Donati
    "Photodetectors" pgs 129,130). And all he said was that charge
    carriers generated in the undeplected region would have to diffuse

    Say if there is some time delay getting charges out of the LED, could
    I look for the same time delay getting charges into it? (Hit it with
    a voltage/current step and see how long it takes till I see light.
    (I'm not sure I have a fast enough PD... but hacking something would
    be fun!)

    Yeah this is a weird LED. 700 nm GaP (which should be green) but it's
    heavy doped with Zn. The manufacturer has what looks like a similar
    LED, 700nm GaP with a clear lens and a bit more light. But that one
    doesn't break down till 125 Volts!

    George H.
  4. Hi miso, Well this is a of scientific interest but (hopefully) will
    help sell
    our counter/ timer. (It will be a ~$200 box) The whole thing started
    with a question on SEB about the 5V reverse voltage of LEDs. So I
    went and tried one.... turns out you can revese bias to a lot more
    than 5V. 25 volts is the lowest I've found with other voltages
    between 50 and 120.

    George H.
  5. miso

    miso Guest

    Well yes, you can put a lot of reverse bias on a LED, but there is no
    guarantee there is no damage to the device or all devices will have the
    same breakdown.

    In the days when the LED reverse bias spec was 3V, I had a bit of a
    quandary designing display driver chips since that does involve a 5V
    reverse bias. The story I got from "I can't say" was if you ruined a LED
    at 5V reserve bias, it would have problems at normal forward bias. That
    is, the physics behind a failure at a low reverse bias meant the forward
    operation would have problems as well. That was when I brought up that
    the physics were different between forward and reverse. But since the
    rest of the industry has been selling display drivers that did the same
    thing, the discussion was moot. [I don't remotely consider myself a
    semiconductor physics expert. I only know enough to be dangerous.]

    I can say that any time I have caused a diode to fail during ESD
    testing, it has always been in reverse bias. You get a large field
    potential, and something pops at a weak spot in the crystal lattice.
    Forward bias failures tend to be related to electromigration, somewhat
    unintuitive since you would think the diode would be weaker than the
    metal. [Metal hasn't been plain metal for a long time. The connection
    layers themselves can have complicated physics.]

    But impact ionization isn't exactly an unknown phenomena. It is quite
    possible some LED manufacturers tweak the junction profiles to make
    their parts less likely to fail under that condition. [Much like the
    multi-step doping they do on drains in "regular" CMOS to reduce the

    I'm willing to bet if you could find the person in a semi that does the
    "production test" on wafers, they would have a spec on how much reverse
    bias they put on the LEDs during that part of testing. "Production test"
    isn't a standardized term in the industry. It can be called "acceptance
    test" for example. But basically when you are buying processed wafers
    from a fab or simply getting wafers from your captive fab, the
    individual devices are characterized on a test pattern. This is prior to
    wafer testing the actual product. [I never worked where they made
    discrete semis, so this step could be just part of product testing for
    discretes.] The acceptance test generates reams (well if you actually
    printed them) of data on a per wafer basis. Somewhere in that spec is
    how much reverse bias the factory considers acceptable and the current
    limit. This may not be tested in production. There are arguments as to
    whether devices should be stressed under production testing or just at
    this wafer acceptance level.

    This is why I say you never know exactly how specs are assured on a
    datasheet. You need intimate knowledge of the test flow. Note also that
    on a complex process, the customer can be shipped devices that fail
    acceptance testing IF a chain of responsible people deem the test as not
    relevant. The most common situation is when a product doesn't use the
    device that is failing the acceptance test. For example, the product is
    done on a bicmos process, but the device in question doesn't use any
    bipolar devices.

    For a basic semiconductor, testing merely means that if the part does
    not meet specifications, then the manufacturer will give you a new part.
    Kind of lame, but when you have lawyers, stuff like that happens.
  6. Grin... I'm far from any expert. As a post doc I did some FIR-IR
    spectroscopy of GaAs/Al hetero junction devices.
    Yeah, these LED's always have 100k in series
    (well with a switch down to ~30k, if the resistance is too low the
    avalanche takes longer to stop... heading to always on.)
    Yeah, I sent an email to purdy electronics in Sunnyvale.
    Hey if anyone knows someone...
    The LED is a AND114 from newark

    I'll send them something again after the newsletter is written.
    We'll have to make a life time buy. maybe 5-10k... at ~$0.1 ea.
    Would someone 'notice' a question with a kilobuck attached?

    But basically when you are buying processed wafers
    Thanks miso, most of the time I just feel like a pimple on a flea
    sucking on some dog that is part of consumer electronics.
    But I'm a happy pimple! life would stink w/o the dog.
  7. So where do you think the diffusion limited current might be
    (some transition region between netrual and depleted sections?)

    I remember distinctly hooking up a x10 (16pF) scope probe to the non-
    inverting node,
    and the signal didn't change... well only a little.
    (It's like the dog that didn't bark)

    George H.
  8. miso

    miso Guest

    I guess I wasn't too clear about the wafer acceptance. It is a step in
    the test flow for every product manufactured. It doesn't matter if the
    fab is captive or outside foundry.

    When you, the end user, buy a semiconductor device, the first step in
    the chain to bring you the magic is this "acceptance testing" of the
    wafer. A test pattern of basic devices is evaluated. If the wafer is
    deemed good, it goes to wafer test (terrible terminology here since that
    sounds redundant) that actually checks the product dice. Again, the
    acceptance testing only looks at the test pattern. But that is where the
    devices are usually stressed since nobody will be buying the devices on
    the test pattern. The idea behind this acceptance testing is to avoid
    shipping a lot of bad wafers to the next step in the test flow. That is,
    usually the acceptance testing is done at the fab. The wafers might not
    even leave the fab if they really suck. Rather the fab notifies the next
    person in the food chain that their wafer run bombed so they better
    start another. Then parts go on hold and end users start to bitch.

    Now back to the acceptance testing. Say you were buying a sports car.
    Would you want the sports car you will be buying to be driven by some
    maniac, beating the crap out of the engine and doing panic stops that
    could warp a rotor. Or do you want to know that the factory occasionally
    takes a vehicle and abuses it, but never sells the abused vehicles to
    the public. Well it is similar for chips, though you never really know
    the nitty gritty unless someone on the inside tells you. That is, the
    semi may just stress the parts on the test pattern, or your actual part
    may be stressed. I've seen it done both ways.

    Once wafer tested, the good dice are mounted in the package. The
    packaged part is tested. Often at high temperature and room temp, with a
    QA sample at cold. Parts tested at temperature need a soak time, so you
    usually do the room temperature test first, reject parts, then test the
    rest at hot. A test flow could do 100% at cold, but that will cost you.
    Cold testing is a pain. The parts handler jams due to ice or condensation.

    So somewhere deep in the bowels of the company is the person that knows
    the acceptance parameter for the reverse bias of your LED. And a
    different person will have the history of that reverse bias test. There
    are a lot of buzzwords, but probably "statistical process control" is
    the most popular. Every test parameter from the testing the test pattern
    is logged. The fab monitors all the test data to see if some parameter
    is drifting. Maybe some machine is drifting and will be out of
    calibration soon, etc.

    This wafer acceptance is tricky business. Like I said, wafers that fail
    may go into production if enough people sign off. If you scrap a lot,
    people don't get parts. If you sign off on the failed parameters, the
    parts may not be reliable. Some companies may go an extra step by
    running a "factorial" during the initial wafer run. That is, they will
    vary processing parameters in a carefully designed test matrix to
    determine how sensitive a particular part is to a particular parameter.
    Then if the a parameter is out of spec, you have actual test data to
    show if it is significant or not.
  9. Bill Sloman

    Bill Sloman Guest

    As I've posted here before, the one time I saw anybody run a LED reversed biased, the authors were trying to make a very narrow optical pulse at a well-defined instant to test a constant fraction discriminator.

    Lo CC and Leskovar B IEEE Trans. Nucl. Sci. NS 93-105 (1974).
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