Astronomy 512 -- Spring 2006 -- Assignment 5 -- Due Tuesday Apr 18
- In the Infrared Laboratory you will find a single InSb photodiode
packaged inside an LN2 cryostat. You need to do the following:
- As a group --
- Pump the vacuum jacket to a pressure less than 60 mT.
- Fill the dewar with liquid nitrogen (from the 10 liter storage
dewar). Observing handling precautions for LN2.
- After the system has thermally stabilized. Estimate the
dewar's hold time and devise a plan to keep the system
cold for the duration of the experiment.
- Individually --
- Power up the system (when it is completely stable at LN2
temperature) and obtain a "load curve" for the
photodiode by adjusting the bias and noting the DC output.
Locate the optimal operating point for the diode.
- Estimate the frequency repsonse of the system by
looking at a "chopped" source.
- Make an estimate of the noise and Noise Equivalent
Power of the system.
- In small groups (2-3 people) --
- Using the lab's blackbody source
- Characterize the systems response vs.
wavelength (i.e. in the various filters).
- Demonstrate that the Planck equation works.
- Highly recommended (in any configuration you want).
- Use the lab breadboard to lay out a simple op amp
analog to the detector circuit in the dewar. Use a 1N4148
diode in place of the detector. Configure the amplifier for a
transimpedance gain of 10V/milliamp. Construct a load curve
for the 1N4148 and compare it with expected ideal diode
- Consider an 0.5mm diameter intrinsic semiconductor
photoconductor made of Germanium. Assume the detector operates
at a bias of 50mV (See (better yet read) Rieke Chapter 2).
- a) What is the bandgap in eV and the longest detectable wavelength?
- b) Suggest a thickness for this detector to provide for good
quantum efficiency. Estimate the quantum efficiency and
photoconductive gain for your choice of thickness.
- c) What will the responsivity be (in amps/watt) for this configuration?
- d) At an operating temperature of 200K, what is the estimated
density of conduction band carriers in the material?
Given this density of carriers and the detector's other material
properties -- what is the resistance of the detector.
- e) How will the resistance change if the detector is cooled to 100K?
By how much will the detector's Johnson noise contribution decrease?
- f) The detector (operating once again at 200K) is attached to
a 1.5-meter telescope and pointed at a K=12.0 magnitude star. If the
filter is the standard astronomical Ks band (2.00-2.32um).
At what SNR will the star be detected in a 5-second integration assuming
detector Johnson noise and photon statistics are the only considerations.
- Suppose the germanium detector above is doped with Gallium
with an acceptor concentration of 5.0x1014.
- a) What is the wavelength cutoff of this material.
- b) How thick would the detector have to be to achieve a resonable
- c) At what temperature would you recommend operating this detector?