Photodiode Detector Characteristics

Gabrielle Baxter8368142

School of Physics and AstronomyUniversity of Manchester

Second Year Laboratory Report

March 2014

This experiment was performed in collaboration with Archie Skone

Abstract

A photodiode amplifier circuit was built and its limitations of speed and sensitivity were studied using light from an LED of various frequencies, driven by a fast pulsed generator. The linearity of the photodiode was also tested using the Law of Malus and a polarised diode laser. The output of the photodiode circuit was compared to that of a fast photodiode and limitations were found due to slew rate and the circuitry of the photodiode amplifier.

1. Introduction

The experiment aimed to study the characteristics of photodiode amplifiers, detectors that convert light energy into electric current, using a polarised diode laser and a pulse driven light emitting diode (LED). By illuminating the diode with light of various frequencies and observing the output on a Cathode Ray Oscilloscope (CRO), both a photodiode amplifier circuit built in the experiment and a fast photodiode could be studied and their characteristics compared. The linearity of the photodiode amplifier was also tested using a dichoric polariser and the Law of Malus.

Photodiodes are essential in optical fibre communication systems [1] and must operate with high sensitivity, minimal noise and especially high response speed which must be comparable to the rate of data transmission. The photodetectors in this experiment were used close to their limits of speed and sensitivity to find their limitations.

2. Theory

2.1 The photodiode

A photodiode works by converting light energy into a photocurrent via semiconductor material in the photodiode. The photodiode consists of a junction of n-type and p-type materials; when a photon with sufficient energy strikes the photodiode, it excites an electron and a free electron and positively charged hole pair are produced [2].

Figure 1 A diagram showing a p-n junction and the depletion region [3]

If the excitation occurs in the depletion layer around the junction between p type and n type materials in the photodiode, as seen in figure 1, the electrons are swept across the junction due to the potential difference across the depletion layer. The hole-electron pair must be within a diffusion length of the depletion region to be affected by the electric field of the region or else they recombine.

As the photodiode is operating in photo-conductive mode, the diode is reverse biased; the cathode is positive compared to the anode and therefore positively charged holes move towards the anode and negative electrons move towards the cathode. Photodiodes are often reverse biased as this reduces the transit time of the electrons and holes for a faster response and reduces the capacitance of the diode [1]. Although reverse bias is a faster mode of operation, it leads to more electronic noise and an increase in dark current, the small current that flows due to the random generation of electron-hole pairs in the depletion region [2].

The photodiode used in this experiment was a silicon photodiode with a capacitance of 270 pF and an active area of 25mm2.

2.2 The Photodiode Amplifier Configuration

Figure 2 The configuration of the photodiode amplifier circuit [4]

The configuration of the photodiode amplifier circuit is shown in figure 2. The photocurrent, IPD, from the photodiode passes through R1 and converted to voltage by Ohm’s Law [5]

V R1=−IPD R1

(1)

In order for the output to be measurable on the CRO, the output of the photodiode must be amplified by an operational amplifier [2]; in this experiment an OP177

operational amplifier was used. The amplifier is non-inverting, and therefore the output voltage of the amplifier is directly proportional to the voltage across R1 by [6]

V out=(1+ RfR3 )V R1(2)

and the gain of the circuit can be controlled through the experiment by Rf.

2.3 Law of Malus

The linearity of the photodiode can be tested using the law of malus [7]

I out=I 0 cos2θ

(3)where I0 is the maximum intensity of light in volts, and θ is the relative angle between the polarisation vector of the laser beam and the polariser. The output Iout

varies between a maximum, when the polariser axis aligns with the polarisation of the laser, and a minimum, when the polariser crosses with the direction of the laser polarisation.

3. Method

3.1 Constructing the photodiode circuit and LED board

After values for resistors were selected as in table 1, the circuit in figure 2 was built and soldered onto a printed copper circuit board.

R1 100kΩ ±1%R2 1kΩ ±1%R3 1kΩ ±1%

Table 1 A table displaying the various resistors used in the photodiode amplifier circuit

The ±12V inputs of the amplifier were bypassed with 2.2 F tantalum resistors, μchosen for their high frequency response. A 12V power supply was connected to the positive and negative terminals of the circuit, and the circuit was grounded appropriately. Output voltages of the circuit were checked with a multi-meter before any experiments were carried out, the circuit mounted onto the optical rail and the photodiode placed in PVC housing to shield it from extraneous light.

3.2 Testing the photodiode using a laser

Figure 3 A diagram showing the set up for the Law of Malus experiment

The sensitivity and linearity of the photodiode amplifier circuit were tested using a polarised diode laser. The laser was mounted onto an optical rail, aligned so that it travelled co-linearly with the optical rail and left to warm up for 30 minutes to ensure a stable output for the duration of the experiment. The photodiode amplifier circuit was mounted at a distance of about 30cm from the laser, and the dichoric polariser placed in between the photodiode and the laser as shown in figure 3.

The polariser was first set to =45° and the maximum intensity set using the θpotentiometer to a 2 VDC output observed on the CRO. The polariser was then set back to =0° and the output voltage on the CRO was recorded. Readings were θtaken at intervals of 5±1° and the polariser rotated through a full 360°. The readings were then repeated at an initial maximum intensity of 8 VDC.

3.3 Measuring the LED output using the fast photodiode

Figure 4 A diagram showing the set up for the fast photodiode circuit

The fast photodiode circuit, LED circuit and a short focal lens were mounted onto the optical rail as shown in figure 4, and the light from the LED focused onto the active area of the fast photodiode using the lens. The LED was connected to a pulse generator using a 50Ω BNC cable and the fast photodiode connected to the CRO using a “T” junction with a 50 Ω terminator to match the impedance of the BNC cable.

The pulse rate of the LED was set to a 5Hz square wave using the pulse generator and the shape of the output waveform on the CRO was noted. The pulse rate was then increased to 50Hz, 500Hz, 1kHz, 5kHz, 10kHz, 50kHz, 100kHz and 500kHz

and the observations of the output waveform were repeated.

3.4 Measuring the LED output using the photodiode As with the fast photodiode, the photodiode amplifier circuit was mounted on the optical rail at a distance of 200mm from the LED in a similar fashion to figure 4, the power supply connected across the positive and negative terminals, and the output connected to the CRO. The light from the LED was aligned with the active area of the photodiode and the pulse rate of the LED set to a 5Hz square wave as before.

The gain of the circuit was set so that an output of 2V was observed on the CRO and the pulse shape noted. The gain was then increased to 5V and 10V and the observations repeated. The frequency of the LED was increased to 50Hz, 500Hz, 1kHz, 5kHz, 10kHz, 50kHz, 100kHz and 500kHz and the output shape on the CRO noted at each voltage.

4. Results

4.1 Law of Malus results

Fig 5 A plot of output voltage against angle in radians. The output voltage was set at a maximum of 2V at 45°

Figure 6 A plot of output voltage against angle in radians. The output voltage was set at a maximum of 8V at 45°

After removing anomalous results, the data at 8V appears to hold better than the data at 2V. The law of malus still holds at 2V but the changes in voltage were smaller compared to those at 8V which meant that the results appeared to fit better to the theoretical results.

The flat points at the bottom of the curve are from where the data does not fit the theoretical curve as the incident light intensity dropped below background light levels, and the photodiode could not get readings below background light levels.

4.2 LED output using the fast photodiode

Figure 7 Output waveforms from the fast photodiode at 50Hz, 10kHz and 500kHz

At low frequencies, the output of the fast photodiode was square with no noticeable response time, however at higher frequencies the response was evident from the curved response as shown in figure 7.

4.3 LED output using the photodiode amplifier circuit

Figure 8 Output waveforms from the photodiode amplifier circuit at 50Hz, 10kHz and 100kHz at a gain of 2V

Figure 9 Output waveforms from the photodiode amplifier circuit at 50Hz, 10kHz and 100kHz at a gain of 10V

At low frequencies the output waveform was square, with increasing curvature at higher frequencies for 2V whereas there was a more linear, triangular output for 5V and 10V, tending towards a sinusoidal shape at around 100kHz. At 500kHz the output signal was just noise with a noticeable wave packet and at higher frequencies it was not possible to achieve output voltages of over 2V.

5. Discussion

5.1 The response of the photodiodes

The curved response of the photodiode amplifier can be attributed to the use of capacitors in the circuit; 2.2 F capacitors were chosen for their high frequency μresponse but the curved edges on the waveforms are due to the exponential shape of charging and discharging of the capacitors.

The slew rate (the maximum rate of change of output voltage per unit time) of the operational amplifier also gave rise to non-linear effects. At high frequencies the limitations due to the slew rate were more evident and higher gains were harder to achieve, and therefore this could be used to determine the maximum frequency at which the LED could be driven without a distorted output. The slew rate for the OP177 amplifier was around 0.2V sμ -1. This could also lead to a delay in response as seen in figure 10 as the output would have a finite slope.

Figure 10 The effect of slew rate on a square wave, where the solid line is the desired output and the dashed line is the actual output

5.2 Comparison of the two photodiodes

When comparing the output waveforms of the fast photodiode and the photodiode amplifier circuit it is evident that the slew rate of the fast photodiode must be greater than the photodiode circuit as it showed a more linear response and less of a response time. Differences in response of the photodiodes can also be attributed to their circuitry and the limitations stemming from the components used.

5.3 Errors

5.3.1 Errors in the Law of Malus experiment

From the results of the Law of Malus experiment it is evident that the polariser was offset by 1-5°, leading to a shift in the measurements. The scale on the polariser was very thin and hard to read without parallax errors, leading to an uncertainty in angle of ±1°. Readings of output voltage of the CRO also had an uncertainty of ±1mV.

5.3.2 Errors due to background light

The background light in the room in which the experiment was conducted in lead to interference in measurements, and an inability to achieve outputs of below about 500mV. The background lights were fluorescent, generating a roughly 100Hz waveform on the CRO. This due to the alternating nature of mains current as although mains voltage operates at 50Hz, the electrons in the fluorescent lights oscillate back and forth in the tube and are excited twice in one cycle, leading to a photocurrent of twice the frequency of the mains. Furthermore the change of background light as the day progressed would have affected measurements, and results were taken over a short period of time to minimise this effect. In future an improvement on experimental technique would be to take readings in a dark room or with minimal background light.

5.4 Limitations

5.4.1 Limitations at high frequencies

Increasing the frequency of the LED pulse reduced the gain of the circuit and it was harder to achieve higher output voltages. To improve this, a higher resistance could be used for R1 which would yield a higher input voltage due to the photocurrent, as the input voltage is directly proportional to the resistance of R1 as in equation (1).

5.4.2 Limitations of the photodiode

The range of the wavelengths able to be detected by the photodiode is 190-1100nm for a silicon photodiode [8] and therefore the photodiode cannot be used for all applications. Experiments could be carried out to study the response of the photodiode amplifier circuit with light of varying wavelength.

The depletion region of the photodiode can also have effects on its operation; the depletion region can be kept thin for the photodiode to operate at a higher speed by reducing transmit time [1]. However to increase the efficiency of the photodiode and the number of electron-hole pairs that are generated per incident photon, the depletion region must be thicker for more light to be absorbed. There is therefore a trade-off between speed and efficiency [1]. The reverse bias of the diode also leads to an increase in signal noise and dark current.

References

[1] Sze, S. M. Physics of Semiconductor Devices. New York: Wiley, 1981.

[2] Graeme, Jerald G. Photodiode Amplifiers: Op Amp Solutions. Boston, MA: McGraw Hill, 1996.

[3] P-n Junction in a Photodiode. Digital image. Electronics Tutorials. Web.

[4] Murray. A.J. 1999. Photodiode Detector Characteristics, 2nd yr lab script, Physics Department, University of Manchester

[5] Grant, I. S., and W. R. Phillips. Electromagnetism. London: Wiley, 1975.

[6] Gingrich D.M. 1999. Electronics Lecture Notes, Department of Physics, Univeristy of Alberta

[7] Hecht, Eugene. Optics. Hecht. Reading, MA: Addison-Wesley, 1998. Print.

[8] Held, Gilbert. Introduction to Light Emitting Diode Technology and Applications. Boca Raton: CRC, 2009.