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Photo-Gate LED test

This study investigates the response of a photo-gate in Sector 5 to LED pulses, comparing it to a standard diode in Sector 1. The results indicate a significant difference in the photo-gate's response, potentially due to surface states at the epi-oxide interface. Further testing is required to understand the observed effects.

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Photo-Gate LED test

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  1. Photo-Gate LED test Wieman/Bieser 10-Sept-04

  2. Background • DC Measurements of Photo-gate/Drain gave (Sector 5) • Slow response • Fe55 x-ray signal showed large spread in amplitude • Problem blamed on surface states at the epi-oxide interface beneath the photo gate • Support of this notion by Janesick who observed similar photo-gate behavior using TSMC process, but made a good photo-gate using the JAZZ foundry with a buried channel

  3. New photo-gate test Sector 5 • Used a single pixel in sector 5 (simple structure with photo-gate and small drain on the edge) • Photo-gate voltage 1 volt • Drain voltage set by full reset voltage • Test sequence • Reset all pixels • Clock row and column shift registers to select a single pixel • Inject 2 red LED pulses • Observe output voltage with oscilloscope throughout sequence • Also did same test with sector 1 ( the standard diode) for comparison drain Photo-gate

  4. Pulse 1 Pulse 2 Measured output voltage • Features to note • Both sectors exposed to same LED pulses, but light attenuated for sector 1 • Sector 5 response to 1st LED pulse much smaller than to 2nd • Sector 5 leakage current ramp increases after 1st LED pulse • Response difference for the two pulses in sector 5 is directly related to gate, since sector 1 shows that readout structures are not affected differently by the two pulses • Reducing the pulse separation did not change effect

  5. Pulse 1 Pulse 2 Photo-gate time response to two pulses the same, only amplitude difference Adjust amplitude and overlay Signal fits: + plus leakage slope   204 s

  6. What does it mean? • Don’t know yet • Is this what should happen if surface states are saturated by the first pulse? Would have expected time response to change • Need more tests varying the size of the two pulses to see what effects second signal size. Also explore over wider range the effects of pulse time separation

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