Photodiodes: Characteristics, Applications, and Circuit Examples, Exams of Chemistry

Download Photodiodes: Characteristics, Applications, and Circuit Examples and more Exams Chemistry in PDF only on Docsity! 1 UDT Sensors Inc. Phone: 310-978-0516 Fax: 310-644-1727 http:\\www.udt.com PHOTODIODE CHARACTERISTICS Silicon photodiodes are semiconductor devices responsive to high- energy particles and photons. Photodiodes operate by absorption of photons or charged particles and generate a flow of current in an ex- ternal circuit, proportional to the incident power. Photodiodes can be used to detect the presence or absence of minute quantities of light and can be calibrated for extremely accurate measurements from in- tensities below 1 pW/cm2 to intensities above 100 mW/cm2. Silicon photodiodes are utilized in such diverse applications as spectroscopy, photography, analytical instrumentation, optical position sensors, beam alignment, surface characterization, laser range finders, optical com- munications, and medical imaging instruments. PLANAR DIFFUSED SILICON PHOTODIODE CONSTRUCTION Planar diffused silicon photodiodes are simply P-N junction diodes. A P-N junction can be formed by diffusing either a P-type impurity (an- ode), such as Boron, into a N-type bulk silicon wafer, or a N-type impurity, such as Phosphorous, into a P-type bulk silicon wafer. The diffused area defines the photodiode active area. To form an ohmic contact another impurity diffusion into the backside of the wafer is necessary. The impurity is an N-type for P-type active area and P-type for an N-type active area. The contact pads are deposited on the front active area on defined areas, and on the backside, completely covering the device. The active area is then deposited on with an anti-reflection coating to reduce the reflection of the light for a specific predefined wavelength. The non-active area on the top is covered with a thick layer of silicon oxide. By controlling the thickness of bulk substrate, the speed and responsivity of the photodiode can be controlled. Note that the photodiodes, when biased, must be operated in the reverse bias mode, i.e. a negative voltage applied to anode and positive volt- age to cathode. Figure 1. Planar diffused silicon photodiode PRINCIPLE OF OPERATION Silicon is a semiconductor with a band gap energy of 1.12 eV at room temperature. This is the gap between the valence band and the con- duction band. At absolute zero temperature the valence band is com- pletely filled and the conduction band is vacant. As the temperature increases, the electrons become excited and escalate from the valence band to the conduction band by thermal energy. The electrons can also be escalated to the conduction band by particles or photons with energies greater than 1.12eV, which corresponds to wavelengths shorter than 1100 nm. The resulting electrons in the conduction band are free to conduct current. Due to concentration gradient, the diffusion of electrons from the N- type region to the P-type region and the diffusion of holes from the P- type region to the N-type region, develops a built-in voltage across the junction. The inter-diffusion of electrons and holes between the N and P regions across the junction results in a region with no free carri- ers. This is the depletion region. The built-in voltage across the deple- tion region results in an electric field with maximum at the junction and no field outside of the depletion region. Any applied reverse bias adds to the built in voltage and results in a wider depletion region. The electron-hole pairs generated by light are swept away by drift in the depletion region and are collected by diffusion from the undepleted region. The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially with distance and is proportional to the absorption coefficient. The absorption coeffi- cient is very high for shorter wavelengths in the UV region and is small for longer wavelengths (Figure 2). Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon be- comes transparent to light wavelengths longer than 1200 nm. More- over, photons with energies smaller than the band gap are not absorbed at all. Figure 2. Penetration depth of light into silicon substrate for various wavelengths. ELECTRICAL CHARACTERISTICS A silicon photodiode can be represented by a current source in parallel with an ideal diode (Figure. 3). The current source represents the current generated by the incident radiation, and the diode represents N-Type Substrate P + N + Deplet ion Region AR Coat ing SiO 2 Contact Metal Act ive Area Diameter Anode (+) Cathode (-) 1E-05 1E-04 1E-03 1E-02 P en et ra tio n D ep th ( m m ) 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 Wavelength (nm) 2 UDT Sensors Inc. Phone: 310-978-0516 Fax: 310-644-1727 http:\\www.udt.com PHOTODIODE CHARACTERISTICS Figure 3. Equivalent Circuit for the silicon photodiode the p-n junction. In addition, a junction capacitance (C j ) and a shunt resistance (R SH ) are in parallel with the other components. Series re- sistance (R S ) is connected in series with all components in this model. Shunt Resistance, R SH Shunt resistance is the slope of the current-voltage curve of the photo- diode at the origin, i.e. V=0. Although an ideal photodiode should have a shunt resistance of infinite, actual values range from 10s to 1000s of Mega ohms. Experimentally it is obtained by applying ±10 mV, measuring the current and calculating the resistance. Shunt resis- tance is used to determine the noise current in the photodiode with no bias (photovoltaic mode). For best photodiode performance the high- est shunt resistance is desired. Series Resistance, R S Series resistance of a photodiode arises from the resistance of the con- tacts and the resistance of the undepleted silicon (Figure 1). It is given by: ( ) C dS S RA WW R += ( 1 ) Where W S is the thickness of the substrate, W d is the width of the depleted region, A is the diffused area of the junction, is the resistiv- ity of the substrate and R C is the contact resistance. Series resistance is used to determine the linearity of the photodiode in photovoltaic mode (no bias, V=0). Although an ideal photodiode should have no series resistance, typical values ranging from 10 to 1000 ohm is measured. Junction Capacitance, C J The boundaries of the depletion region act as the plates of a parallel plate capacitor (Figure 1). The junction capacitance is directly pro- portional to the diffused area and inversely proportional to the width of the depletion region. In addition, higher resistivity substrates have lower junction capacitance. Furthermore, the capacitance is depen- dent on the reverse bias as follows: ( )biASi Si J VV A C + = 0 0 2 ( 2 ) Figure 4. Capacitance of PIN-RD100 versus Reverse Bias Voltage where 0 = 8.854x10-14 F/cm, is the permittivity of free space, Si =11.9 is the silicon dielectric constant, µ = 1400 cm2/Vs is the mobility of the electrons at 300 ºK, is the resistivity of the silicon, V bi is the built-in voltage of silicon and V A is the applied bias. Figure 4 shows the depen- dence of the capacitance on the applied reverse bias voltage. Junction capacitance is used to determine the speed of the response of the pho- todiode. Rise/Fall Time and Frequency Response, t r / t f / f 3dB The rise time and fall time of a photodiode is defined as the time for the signal to rise or fall from 10% to 90% or 90% to 10% of the final value respectively. This parameter can be also expressed as frequency response, which is the frequency at which the photodiode output de- creases by 3dB. It is roughly approximated by: dB r f t 3 35.0= ( 3 ) There are three factors defining the response time of a photodiode: 1. t DRIFT , the charge collection time of the carriers in the depleted region of the photodiode. 2. t DIFFUSED , the charge collection time of the carriers in the undepleted region of the photodiode. 3. t RC , the RC time constant of the diode-circuit combination. t RC is determined by t RC =2.2 RC, where R, is the sum of the diode series resistance and the load resistance (R S + R L ), and C, is the sum of the photodiode junction and the stray capacitances (C j +C S ). Since the junction capacitance (C j ) is dependent on the diffused area of the pho- todiode and the applied reverse bias (Equation 2), faster rise times are obtained with smaller diffused area photodiodes, and larger applied reverse biases. In addition, stray capacitance can be minimized by 10 100 1000 C ap ac ita nc e (p F ) 1 10 100 Reverse Bias Voltage (V) Iph Cj Rsh Rs Id RL I0 + - V0 5 UDT Sensors Inc. Phone: 310-978-0516 Fax: 310-644-1727 http:\\www.udt.com PHOTODIODE CHARACTERISTICS ( ) fIIqI DPsn += 2 ( 9 ) Where q=1.6x10-19C, is the electron charge, I P is the photogenerated current, I D is the photodetector dark current and f is the noise mea- surement bandwidth. Shot noise is the dominating source when oper- ating in photoconductive (biased) mode. Thermal or Johnson Noise The shunt resistance in a photodetector has a Johnson noise associ- ated with it. This is due to the thermal generation of carriers. The magnitude of the this generated current noise is: SH B jn R fTk I = 4 ( 10 ) Where k B =1.38 x 10-23 J/ºK, is the Boltzmann Constant, T, is the abso- lute temperature in degrees Kelvin (273 ºK= 0 ºC), f is the noise measurement bandwidth and R SH , is the shunt resistance of the photo- diode. This type of noise is the dominant current noise in photovoltaic (unbiased) operation mode. Note: All resistors have a Johnson noise associated with them, in- cluding the load resistor. This additional noise current is large and adds to the Johnson noise current caused by the photodetector shunt resistance. Total Noise The total noise current generated in a photodetector is determined by: 22 jnsntn III += ( 11 ) Noise Equivalent Power (NEP) Noise Equivalent Power is the amount of incident light power on a photodetector, which generates a photocurrent equal to the noise cur- rent. NEP is defined as: R I NEP tn= ( 12 ) Where R is the responsivity in A/W and I tn is the total noise of the photodetector. NEP values can vary from 10-11 W/ Hz for large active area photodiodes down to 10-15 W / Hz for small active area photo- diodes. TEMPERATURE EFFECTS All photodiode characteristics are affected by the change in tempera- ture. They include shunt resistance, dark current, breakdown voltage, responsivity and to a lesser extent other parameters such as junction capacitance. Shunt Resistance and Dark Current: There are two major currents in a photodiode contributing to dark current and shunt resistance. Diffusion current is the dominating fac- tor in a photovoltaic (unbiased) mode of operation, which determines the shunt resistance. It varies as square of the temperature. In photo- conductive mode (reverse biased), however, the drift current becomes the dominant current (dark current) and varies directly with tempera- ture. Thus, change in temperature affects the photodetector more in photovoltaic mode than in photoconductive mode of operation. In photoconductive mode the dark current may approximately double for every 10 ºC increase change in temperature. And in photovoltaic mode, shunt resistance may approximately double for every 6 ºC de- crease in temperature. The exact change is dependent on additional parameters such as the applied reverse bias, resistivity of the substrate as well as the thickness of the substrate. BreakDown Voltage: For small active area devices, by definition breakdown voltage is de- fined as the voltage at which the dark current becomes 10 A. Since dark current increases with temperature, therefore, breakdown volt- age decreases similarly with increase in temperature. Responsivity: Effects of temperature in responsivity are discussed in the “Responsivity” section of these notes. BIASING A photodiode signal can be measured as a voltage or a current. Cur- rent measurement demonstrates far better linearity, offset, and band- width performance. The generated photocurrent is proportional to the incident light power and it requires to be converted to voltage using a transimpedance configuration. The photodiode can be oper- ated with or without an applied reverse bias depending on the applica- tion specific requirements. They are referred to as “Photoconductive” (biased) and “Photovoltaic” (unbiased) modes. Photoconductive Mode (PC) Application of a reverse bias (i.e. cathode positive, anode negative) can greatly improve the speed of response and linearity of the devices. This is due to increase in the depletion region width and consequently decrease in junction capacitance. Applying a reverse bias, however, will increase the dark and noise currents. An example of low light level / high-speed response operated in photoconductive mode is shown in figure 9. 6 UDT Sensors Inc. Phone: 310-978-0516 Fax: 310-644-1727 http:\\www.udt.com In this configuration the detector is biased to reduce junction capaci- tance thus reducing noise and rise time (t r ). A two stage amplification is used in this example since a high gain with a wide bandwidth is required. The two stages include a transimpedance pre-amp for cur- rent-to-voltage conversion and a non-inverting amplifier for voltage amplification. Gain and bandwidth (f 3dB MAX ) are directly determined by R F , per equations (13) and (14) . The gain of the second stage is approximated by 1+ R 1 / R 2 . A feedback capacitor (C F ) will limit the frequency response and avoids gain peaking. Figure 9. Photoconductive mode of operation circuit example: Low Light Level / Wide Bandwidth )(2 )(max3 AFjF dB CCCR GBP Hzf ++ = ( 13 ) Where GBP is the Gain Bandwidth Product of amplifier (A 1 ) and C A is the amplifier input capacitance. ( ) R R R R P V W VGain F OUT ÷÷+== 2 11 ( 14 ) In low speed applications, a large gain, e.g. >10M can be achieved by introducing a large value (R F ) without the need for the second stage. Typical component used in this configuration are: Amplifier: CLC-425, CLC-446, OPA-637, or similar. R F : 1 to 10 K Typical, depending on C j R 1 : 10 to 50 k R 2 : 0.5 to 10 k C F : 0.2 to 2 pF In high speed, high light level measurements, however, a different ap- proach is preferred. The most common example is pulse width mea- surements of short pulse gas lasers, solid state laser diodes, or any other similar short pulse light source. The photodiode output can be either directly connected to an oscilloscope (Figure 10) or fed to a fast response amplifier. When using an oscilloscope, the bandwidth of the scope can be adjusted to the pulse width of the light source for maxi- mum signal to noise ratio. In this application the bias voltage is large. Two opposing protection diodes should be connected to the input of the oscilloscope across the input and ground. Figure 10. Photoconductive mode of operation circuit example: High Light Level / High Speed Response To avoid ringing in the output signal, the cable between the detector and the oscilloscope should be short (i.e. < 20cm) and terminated with a 50 ohm load resistor (R L ). The photodiode should be enclosed in a metallic box, if possible, with short leads between the detector and the capacitor, and between the detector and the coaxial cable. The metal- lic box should be tied through a capacitor (C 1 ), with lead length (L) less than 2 cm, where R L C 1 > 10 ( is the pulse width in seconds). R S is chosen such that R S < V BIAS / 10 I PDC , where I PDC is the DC photocur- rent. Bandwidth is defined as 0.35 / . A minimum of 10V reverse bias is necessary for this application. Note that a bias larger than the pho- todiode maximum reverse voltage should not be applied. Photovoltaic Mode (PV) The photovoltaic mode of operation (unbiased) is preferred when a photodiode is used in low frequency applications (up to 350 kHz) as well as ultra low light level applications. In addition to offering a simple operational configuration, the photocurrents in this mode have less variations in responsivity with temperature. An example of an ultra low light level / low speed is shown in figure 11. Figure 11. Photovoltaic mode of operation circuit example: Ultra low level light / low speed - + R F C A A 1 - + A 2 R 2 R 1 V out -15V -15V +15V +15V 0.1µF -30V Coaxial Cable Osci l loscope V bias C 1 R s R L=50 L V OUT Metal Box V out - + R F=500 M C F -15V +15V C A PHOTODIODE CHARACTERISTICS C F 0.1µF 0 .1µF 0 .1µF 7 UDT Sensors Inc. Phone: 310-978-0516 Fax: 310-644-1727 http:\\www.udt.com PHOTODIODE CHARACTERISTICS In this example, a FET input operational amplifier as well as a large resistance feedback resistor (R F ) is considered. The detector is unbi- ased to eliminate any additional noise current. The total output is determined by equation (15) and the op-amp noise current is deter- mined by R F in equation (16): FPOUT RIV = ( 15 ) F rms N R kT Hz A I 4 )( = ( 16 ) where k=1.38 x 10-23 J/ºK and T is temperature in ºK. For stability, select C F such that FFAFjF CRCCCR GBP 2 1 )(2 > ++ ( 17 ) Operating bandwidth, after gain peaking compensation is: FF OP CR Hzf 2 1 )( = ( 18 ) Some recommended components for this configuration are: Amplifier: OP-15, OP-16, OP-17 or similar. R f : 500M These examples or any other configurations for single photodiodes can be applied to any of UDT Sensors' monolithic, common substrate liner array photodiodes. The output of the first stage pre-amplifiers can be connected to a sample and hold circuit and a multiplexer. Fig- ure 12 shows the block diagram for such configuration. Figure 12. Circuit example for a multi-element, common cathode array V OUT - + R f -15V +15V - + R f -15V +15V - + Rf -15V +15V - + Rf -15V +15V Vbias R s C d Samp le and hold Mult ip lexer