ECE 482/582 - Lab #2 - Photodetectors
Reading: Kuhn, Laser Engineering , Section 8.4, pp 261-268
Preparatory Questions:
Three of the most common semiconductor photodetectors are 1) the CdS photoconductor or “ photocell,” 2) the Si PIN photodiode , and 3) the Si phototransistor .
From your reading in Kuhn –
1. Which of these three types of detectors have gain?
2. Which detector acts as a “light-variable resistor?”
3. Which two act as current sources?
The following PIN photodiode diagram is one of the detector setups used in the lab:
V bias
The equivalent circuit of a photodiode is shown in the figure below.

Fundamentally, a photodiode is a current generator. The junction capacitance of the photodiode depends on the depletion layer depth and hence bias voltage. The value of the shunt resistance is usually very high
(megohms) and is assumed to be infinite. The series resistance is low – usually assumed to be 0 ohms.
Increased incident light on the PIN detector causes an increase in generated photocurrent around the circuit.
Based on the circuit diagrams above and your knowledge of the I-V behavior of the PIN photodiode, answer the following questions about these important considerations of any detector circuit.
4. "Loading" - How does V(sig) change if the load resistor R
L
is reduced? [This is done by turning the
“R
L
” switch on the detector box which adds a smaller resistance in parallel with the 1 M 
input resistance of the oscilloscope.]
5. "Saturation" - What happens to V(sig) at high intensities with a large load resistor? What is the maximum possible value of V(sig)?
6. "Speed" - How does the detector response time change with changing load? Is there a tradeoff to consider between V(sig) and speed? [Note: t r
= 2.19 RC where R = R
L
//R scope
and C = C det
//C scope
.
Lab #1: Photodetectors: Laboratory Work:
Learning Objectives: At the end of this lab students should be able to --
1. Sketch the circuit diagrams and the I-V behavior of the Si PIN photodiode in a photovoltaic (PV) and a photoconductive (PC) mode and of the Si phototransistor in a PC (or biased) mode.
2. Describe the effects of the load resistor on these detectors in terms of their amplitude and frequency response characteristics.
3. Compare and contrast the Si PIN and phototransistor responsivity and bandwidth used in a photoconductive mode.
4. Describe the behavior of the CdS photoconductor with varying light intensity, noting its speed and sensitivity.
5. Use photodetectors correctly to characterize the temporal behavior of four “unknown” optical sources (an incandescent lamp, the room fluorescent lights, an infrared TV remote control unit, and a photoflash from a disposable camera).
Discussion:
This lab examines three commonly used solid state quantum photodetectors: 1) the Si PIN photodiode, the Si phototransistor, and the CdS photoconductor. [It is especially important to understand the Si PIN since you will primarily be using this detector in most of the subsequent labs.]
Characterization of a detector involves measuring its response as a function of 1) the light intensity ( amplitude response ), 2) the wavelength of the light ( spectral response ), and 3) how it responds to a modulated signal in terms of speed and amplitude ( frequency response ). In this lab you will only look at the amplitude and frequency responses of these detectors.

Procedure:
We will use the optical chopper chopping a focused HeNe laser beam at the focus in order to obtain an optical signal with a sub-microsecond rise time. We will use a second lens to collimate the expanded HeNe beam to give a fairly uniform optical intensity over each detector.
The aluminum box contains two photodetectors, a Si PIN photodiode [3 mm x 3 mm in area with an integrated plastic lens on front] and a Si phototransistor [1 mm x 1 mm in area with no lens or a molded plastic lens.] It also contains a switch to switch between the two detectors, a switch to change load resistors from 1 MΩ to 50

, and a switch to put a 22 V battery in series with the detector for PC mode operation or to replace the battery by a short circuit for PV mode operation.
PIN photo diodes can be used in either the photoconductive mode (biased by a DC reverse voltage) or in the photovoltaic mode with no external bias. Here you will characterize and compare the 3 mm x 3 mm Si PIN diode used in both configurations.
Si PIN photodiode: PC mode
Put the Si PIN detector ( switches in "PC" and "Si PIN" positions) at the center of the expanded HeNe laser beam and sketch the output signal (taken across the load resistor) shown on the oscilloscope for 4 load resistors
50, 1k, 100k and 1 MΩ. [Be sure to note the voltage scale for each of your scope sketches.] Measure and record the 10% - 90% rise time, t r
, and the signal amplitude for each load resistor. [Recalling that t (rise) =
2.19 R
L
C
Det
, you should be able to calculate the detector capacitance from these measurements.]
Now measure the detector linearity with varying light incident on the detector We will do this by using a linear polarizer to vary the power level of a polarized He-Ne laser (wavelength = 632.8 nm)
Using the power meter in the lab (calibrate for 633 nm wavelength), determine the optical power of the linearly polarized He-Ne laser passing through the rotatable polarizer positioned for maximum transmission. Vary the incident intensity by rotating the polarizer and record the transmitted laser power as a function of angle of the polarizer. [Plot this variation for every 10 degrees. Does it make sense?]
Now, for the 1 MΩ and 100 kΩ load resistors, record and plot the signal amplitude vs. optical power for 100%,
80%, 60%, 40%, 20%, and 0% input power. You should notice the detector becoming non-linear with power at the high load resistances with high optical power. What is the voltage limit you measure? What is this voltage from?
Si PIN photodiode: PV mode
Move the switch on the box to the "PV" position . Now the battery has been replaced by a short circuit so the
PIN photodiode is creating its own forward photovoltage. Repeat your measurements using the same 4 different load resistors and measuring rise time and amplitude for each load resistor. [Consider different R
L
"load lines" (-1/R
L
lines on the I-V plot) for both PC and PV configurations. What can you say about the
“linearity” of the detector signals? Do your readings make sense?]
Si phototransistor: PC mode
Now switch the detector to the "Si phototransistor" , again positioning the detector in the center of the expanded HeNe laser beam. Place the "PV/PC" switch back to the PC position. [Sketch the circuit in your lab notebook.] Repeat your measurements for this detector. How do the sensitivity and speed compare with the PIN diode? [Note the difference in detector areas between the two.] What is different?

CdS Photoconductor (common light-meter photodetector for photography because its spectral response is very close to the eye photoptic response; also used for triggering outdoor lights to come on at dusk and go off at dawn.)
1. Measure the dark resistance of the photoconductor by removing all light incident upon the device. [Be careful! Light can come in from the back through the white plastic package, too!] Do you notice anything unusual when measuring the dark resistance? If so, what is it?
2 Now measure the resistance with varying light incident on the detector You can do this by using a linear polarizer to vary the power level of a polarized He-Ne laser (wavelength = 632.8 nm) as before.
Plot the photoconductor resistance vs. input optical power (say, from 100% to 0% by 20% intervals). Is the relationship linear? Should it be?
3. Try to estimate (quickly) how fast the resistance recovers when the laser light is blocked. You can use a scope to measure it or just count and make a crude estimate. This recombination time determines the
"bandwidth" of the CdS photocell. As you can see, they are pretty slow.
Now you will use the detector of your choice to try to accurately determine the I(t) behavior of 4 common optical sources: the room fluorescent lights, a tungsten lamp, a photoflash from a disposable camera, and a TV remote control unit.
1. Check out the room fluorescent light output using a suitable detector:
Use either the Si PIN, in either PV or PC modes (you decide which is best), or the phototransistor that you characterized earlier to examine the optical output of the room fluorescent lights.
Sketch the output of optical intensity (I ph
) vs. time (t) in your lab notebook. Note to scale the DC and AC components of the light. [You might want to try different load resistors to be sure you are looking at the real signal.] Does the waveform change if you point at different lights? You can use a lens to collect more light and increase your signal if needed.
What is the AC frequency of the light emitted? Explain why it is at this frequency. [Think about how these lights really operate. Maybe you have never done this before. ]
2. Repeat the above measurements for an incandescent lamp source.
Is the output constant? Describe what you see. Why do you think this is happening?
3. Now measure the single shot flashlamp signal from a disposable camera (careful of the high voltage!)
If you turn the small cogged wheel on the back of the camera with your finger until it stops turning, this will reset the shutter release. Then hold down the “charge” button on the front of the camera for about 10-15 seconds while the capacitor charges to its high voltage value. Push the shutter release to fire the optical flash which is pointed toward your detector of choice. The goal is to measure the “true” time behavior of the optical flash – rise time, fall time, and pulse width. [You will have to assure yourself that what you are measuring is really the pulse shape and not limited by load resistor, saturation, detector internal response, etc. Measure the flash using both the PIN photodiode and the phototransistor. Sketch your pulse shapes in your lab notebook with measured values on it.

4. Measure the signal from the TV remote control unit.
Use any detector to determine the “true” signal the remote uses for several buttons, say, power on, and numbers
1-3. Hint: The digital pulses representing the different keys are sent on an optical carrier of 30-50 kHz.
Determine both the carrier frequency and the digital patterns for the keys.
Answer the following questions in your writeup:
1. What is “saturation” of the detector signal? What are two ways to know if your signal is “saturated.” Did you notice any “saturation” effects? Describe them.
2. For each detector, how is the signal amplitude affected by the load resistor value?
3. How is the signal bandwidth (ie, rise time and fall time) affected by the load resistor value?
4. How would you know if your measured detector response is bandwidth-limited by the source, detector, and/or the oscilloscope? Why is this important?
5. How does the response of the PIN photodiode differ between PV and PC modes of operation? Which mode is more “linear” in response?
6. Sketch the “real” temporal behaviors of the fluorescent lights, the incandescent bulb, and the photoflash.
How do you know you are looking at the real, correct signal?
7. What is the measured recombination lifetime of the CdS photoconductor?
8. What is the TV remote signal sent for a “1” and a “9”? Can you see the difference? What is the submodulation frequency of the remote signal?
WHAT DID YOU LEARN ABOUT MAKING SURE YOU ARE LOOKING AT THE “REAL”
SIGNAL? IN GENERAL, HOW DOES THE LOAD RESISTANCE AFFECT THE SIGNAL FOR
PHOTODETECTORS?
Revised Oct. 5, 2009 by T.K. Plant