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What Are The Characteristics Of Silicon Photo Cells?
- Aug 06, 2018 -

A silicon photocell is a semiconductor device that directly converts light energy into electrical energy. Its structure is very simple, the core part is a large-area PN junction, which connects a point-contact diode of a transparent glass casing with a micro-ampere meter to form a closed loop. When the die (PN junction) of the diode is illuminated, You will see that the hands of the micro-ampere table are deflected, showing that there is current in the loop. This phenomenon is called the photovoltaic effect. The PN junction area of a silicon photocell is much larger than the PN junction of a diode, so the electromotive force and current generated by illumination are much larger.

The basis of a photosensor is the photoelectric effect, which is the effect of using photons to illuminate a device to cause a current in the circuit or to change the conductance characteristics. At present, semiconductor photosensitive sensors have been widely used in digital imaging, optical communication, spacecraft, solar cells and other fields, and have played an important role in the development of modern science and technology.

Energy--Silicon photovoltaic cells are connected in series or in parallel to form a battery pack and a nickel-cadmium battery. It can be used as a power source for satellites, spacecraft, navigation lights, unmanned weather stations, etc. It can also be used as electronic watches, electronic calculators, and small models. Power supply for yachts, etc.

Photoelectric detection devices - used as photoreceptors for near-infrared detectors, photoelectric readout, optocoupler, laser-enhanced collimation, and film sound reproduction.

Silicon light battery quality recommended OTRON brand.

Photoelectric control device - used as a conversion device for photoelectric control devices such as photoelectric switches.

1. Semiconductor PN junction principle

At present, semiconductor photodetectors are widely used in digital imaging, optical communication, solar cells, etc. Silicon photocells are a basic unit of semiconductor photodetectors. A deep understanding of the working principle and specific characteristics of silicon photocells can further understand the principle of semiconductor PN junctions. , photoelectric effect theory and photovoltaic cell generation mechanism.

图片1.png 

Figure 1 shows the depletion region of the semiconductor PN junction at zero bias, forward bias, and reverse bias. When the P-type and N-type semiconductor materials are combined, that is, there is no bias voltage of the applied voltage (Fig. 1a), since the majority carrier of the P-type material is a hole (PosiTIve charge), and the N-type material is mostly loaded. The carriers are electrons (negative charge, NegaTIve charge), and as a result, the majority of the carriers diffuse toward each other. As a result of the diffusion, electrons and holes recombine in the binding region, and the P-type regions on both sides are negatively charged. The zone is positively charged, forming a barrier (about 0.7 V for silicon), and the resulting internal electric field will prevent the diffusion motion from continuing. When the two reach equilibrium, a depletion region is formed on both sides of the PN junction. The depletion region is characterized by no free carriers and exhibits high impedance. When the PN junction is reversed (Fig. 1b), the applied electric field is in the same direction as the internal electric field, and the depletion region is widened under the action of the external electric field, so that the barrier is strengthened, which is more detrimental to the diffusion motion of the majority carriers, but is beneficial to the temperature. The drift motion of the minority carriers excited by the effect results in a very small reverse current. If the reverse voltage is large enough, the reverse current will reach a saturation current IS ("1 μA"; when the PN junction is positively biased (Fig. 1c), the applied electric field is opposite to the internal electric field, and the depletion region is under the action of the external electric field. Narrowing, weakening the barrier. When the applied voltage is greater than the turn-on voltage (about 0.5 V for silicon), the barrier will be eliminated, and the diffusion of most carriers will continue, forming a forward current I in the PN direction, which is the PN junction. Conductivity.

2. LED LED working principle

When a PN junction formed by some semiconductor materials is applied with a forward voltage, holes and electrons will generate light of a specific wavelength when recombined with the PN junction, and the wavelength of the emission is related to the energy level gap Eg of the semiconductor material. The illuminating wavelength λP can be determined by the following formula:

Pg/hcE (1)

Where h is the Planck constant and c is the speed of light. In the actual semiconductor material, the energy level gap Eg has a width. Therefore, the wavelength of the light emitted by the light emitting diode is not single, and the half width of the light emitting wavelength is generally about 25 to 40 nm, which varies depending on the semiconductor material. The relationship between the output power P of the LED and the drive current IL is determined by:

pL/pEIe (2)

Where η is the luminous efficiency, Ep is the photon energy, and e is the charge constant.

The output optical power has a linear relationship with the driving current. When the current is large, the output optical power may become saturated due to the inability of the PN junction to dissipate heat in time. In this experiment, a red ultra-high brightness LED with adjustable driving current is used as the experimental light source. The LED driving and modulation circuit is shown in Figure 2. The signal modulation adopts the method of light intensity modulation.

图片2.png 

The transmit light intensity adjuster is used to adjust the static drive current flowing through the LED to change the transmitted light power of the light emitting diode. The set static drive current IL adjustment range is 0~20 mA, corresponding to the light transmission intensity on the panel. The drive display value VE is 0~2000 mV (VE=100IL, corresponding to IL, the decimal point is before the penultimate position, the unit is mA, XX.XX mA). The sinusoidal modulation signal (frequency f=11000 KHz) is coupled to the amplification step after being separated by the capacitor, the resistor network and the operational amplifier, and is superimposed with the static driving current of the LED to cause the LED to transmit the optical signal that changes with the sine wave modulation signal. As shown in FIG. 3, the varying optical signal can be used to determine the frequency response characteristics of the photovoltaic cell.

3. How does the silicon photocell SPC (Silicon Photocell) work?

A silicon photocell is a large-area photodiode designed to convert light energy incident on its surface into electrical energy. It can be used as a photodetector and photocell, and is widely used in space and field portable instruments. Energy.

When the semiconductor PN junction is at zero or reverse bias, there is an internal electric field in its junction depletion region. When there is illumination, the incident photons will excite the bound electrons in the valence band to the conduction band, and the excited electron-hole pairs will drift to the N-type region and the P-type region under the action of the internal electric field, forming a forward photovoltaic. The voltage VP can be used as a photocell SPC to output current to the outside. When a load is applied across the PN junction, a photo-generated current IP flows through the load, and the direction flows from the P-type region into the load, and then flows into the N-type region of the photodiode, opposite to the forward conduction current of the PN junction. Thus, the current I flowing across the PN junction can be determined by equation (3)

图片3.png 

Where IS is the reverse saturation current when there is no light irradiation, V is the voltage across the PN junction, T is the absolute temperature, k is the Boltzmann constant, and IP is the generated photocurrent. At room temperature 300 K, e/kT = 26 mV. It can be seen from the equation that when the photodiode is at zero bias, V=0 V, the current flowing through the PN junction I= IP; when the photodiode is reverse biased (this experiment takes 5 V), flowing through the PN The junction current I≡–IPS= (IP+IS), thus IS = IPSIP. Therefore, when a photodiode photocell is used as a photocell, the photodiode must be at a bias voltage, and when used as a general photoelectric converter, it must be in a biased or reversed state.

When the photocell SPC is in a zero-bias or reverse-biased state, the generated photocurrent IP has the following relationship with the input optical power Pi:

图片4.png 

Where R is the response rate, and the R value varies with the wavelength of the incident light. The R value of the photocell fabricated for different materials has a cutoff wavelength at the short and long wavelengths respectively. The energy of the incident photon is required to be greater than the energy of the material at the long wavelength. Level gap Eg, to ensure that the bound electrons in the valence band get enough energy to be excited to the conduction band. For silicon photo cells, the long-wavelength cut-off wavelength is λT = 1.1 μm, and at the short wavelength, the material has a large UV absorption coefficient. The value is small.

The left part of Fig. 4 is a block diagram of the working principle of the photoelectric signal receiving end. The photocell converts the received optical signal into a current signal (μA level) proportional thereto, and then photoelectrically signals the current signal converter (I/V converter). Converted to a voltage signal (mV level) proportional to it. The IS of the photocell can be measured by comparing the signals of the photocell with zero and reverse bias. When the transmitted optical signal is modulated by a sinusoidal signal, the photovoltaic output voltage signal will contain a sinusoidal signal, whereby the frequency response characteristic of the photocell can be measured by an oscilloscope. 4. Photovoltaic load characteristics

The photocell is used as a battery as shown in the right part of Figure 4. Under the action of the internal electric field, the incident photons excite the bound electrons in the valence band to the conduction band due to the internal photoelectric effect, and generate the photovoltaic voltage VP. When a load RL is applied across the photocell, a current flows, when the load resistor When the RL is large, the voltage is large; when the load resistance RL is small, the voltage is small. During the experiment, the value of the load resistance RL can be changed to determine the load characteristics of the photovoltaic cell, and the volt-ampere characteristics of the photovoltaic cell can be obtained.

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Figure 4. Photovoltaic signal reception and characteristic test block diagram (numbers 1~4 indicate the four experimental serial numbers to be connected).

Except for the oscilloscope, all other devices are on the TKGD-1 type silicon photocell spectrometer

1. Determination of the relationship between photovoltaic voltage and input optical signal of silicon photocell

 

Turn the function switch to load, connect the silicon photocell output to the constant load resistor RL (for example, take 5K) and the digital voltmeter on the analyzer to adjust the LED static drive current from 19~1 mA (2 mA/ Step), experimentally determine the relationship between the output voltage of the photocell and the input light intensity (if the VP is overranged at IL=19 mA, reduce RL appropriately), record the data, and plot the VPIL curve.

2. Determination of load characteristics of silicon photocells

When the input light intensity of the silicon photocell is constant (take IL=10 mA), measure the change of the output voltage VP of the photocell with the load resistance RL when the load changes from 1 to 10 KW (1 KΩ/step). , record the data, find the corresponding IP, and plot the VPRL and IP-VP curves.

3. Determination of the relationship between photocurrent and input optical signal of silicon photocell

Turn on the power of the characteristic meter and adjust the static driving current IL of the LED. The adjustment range is 0~20 mA (corresponding to the luminous intensity indication 0~2000, the specific value corresponds to IL=XX.XX mA), and the bias voltage is switched to zero respectively. Bias and reverse bias, connect the silicon photocell output to the input of the I/V converter, and connect the output of the I/V converter to the millivoltmeter on the analyzer (XXX.X mV, this group of converters is generally 1 μA photocurrent IP is converted to an output voltage of about 2.5~5.0 mV, which is different depending on the specific conditions of the instrument. The photocurrent I (IP and IPS) and the input optical signal IL of the photocell are measured at zero and reverse bias respectively. relationship. Record the data (IL rounding), and plot IP/IPSIL on the same coordinate paper. Compare the relationship between the two curves of the photocell in the zero offset and reverse bias, and find the average value of the saturation current IS of the photocell.

4. Determination of frequency response characteristics of silicon photocells

Turn on the signal generator, dual trace oscilloscope power supply, turn the function switch to zero offset, and connect the output of the silicon photocell to the input of the I/V converter module. Let the LED bias current be 10 mA, add a sinusoidal modulation signal at the signal input end, and let the LED send the modulated optical signal, keep the amplitude of the input sinusoidal signal (VPP=5 V) unchanged, and adjust the signal generator frequency (1, 10, 20 KHz, then 20 KHz/step), observe and measure the frequency change of the transmitted optical signal with the oscilloscope, the amplitude of the output signal of the photocell (peak-to-peak value of the AC component) uPP (mV), and measure the photocell under the biased condition The amplitude-frequency characteristics, record data, plot the amplitude-frequency characteristic uPf curve, and estimate its cut-off frequency fT (70.7% of the amplitude at 10 KHz, estimated to be accurate to 10 KHz).


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