AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Use in photo-conductive mode as a detector in optical systems.

    Full specification wording

    Photodiode (A-level only): Use in photo-conductive mode as a detector in optical systems.

    In photo-conductive mode the photodiode is reverse biased and its conductivity rises when light creates electron-hole pairs, so it acts as a light-controlled current source.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 2 of 3
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    In photo-conductive mode the photodiode is reverse biased and its conductivity rises when light creates electron-hole pairs, so it acts as a light-controlled current source. A load resistor or transimpedance amplifier converts the photocurrent into a measurable voltage. The detector is chosen so its spectral response matches the source wavelength, for example a silicon photodiode for near-infrared or visible light and an indium gallium arsenide device for longer wavelengths. Response time depends on junction capacitance and load resistance, so a smaller load resistor gives a faster but smaller signal. Optical systems use this arrangement in fibre-optic receivers, barcode scanners, smoke detectors and light-beam intruder alarms, where the photocurrent must track rapid changes in light intensity.

    What earns the marks

    • States that photo-conductive mode uses reverse bias, giving a wide depletion region and a strong electric field across the junction.
    • Explains that absorbed photons create electron-hole pairs, which are separated by the field and produce a photocurrent.
    • Describes the photocurrent as approximately proportional to irradiance over the intended operating range.
    • Explains that a load resistor or amplifier converts the photocurrent into a voltage that can be processed by the rest of the system.
    • Relates the choice of detector material to the spectral response matching the wavelength of the optical source.
    • Discusses response time in terms of junction capacitance and load resistance, noting the trade-off between speed and signal size.
    • Applies the detector to a named optical system, such as a fibre-optic receiver, barcode reader or beam-break alarm.

    Common mistakes and how to put them right

    • Confusing photo-conductive mode with photovoltaic mode.

      Put it right by stating that photo-conductive mode uses reverse bias and an external supply, whereas photovoltaic mode uses no external bias.

    • Assuming the output voltage is independent of the load resistor.

      Put it right by using V = IR to show that the same photocurrent gives a larger voltage across a larger load resistance.

    • Ignoring the wavelength match between source and detector.

      Put it right by checking the spectral response curve and choosing a material whose peak response covers the source wavelength.

    Exam tips

    • Name the mode explicitly and state the bias condition before describing how the photocurrent is produced.
    • Include a simple circuit diagram with the reverse bias supply, photodiode and load resistor, and mark the output voltage across the load.
    • When discussing speed, refer to the RC time constant of the junction capacitance and load resistance rather than giving a vague claim about fast response.

    Try the lesson

    The start of the Photodiode (A-level only): Use in photo-conductive mode as a detector in optical systems. lesson, made from checked teaching notes for this specification point.

    Have a go first

    In which bias condition is a photodiode operated when used in photo-conductive mode as a detector?

    Listen

    In photo-conductive mode the photodiode is reverse biased and its conductivity rises when light creates electron-hole pairs, so it acts as a light-controlled current source. A load resistor or transimpedance amplifier converts the photocurrent into a measurable voltage. The detector is chosen so its spectral response matches the source wavelength, for example a silicon photodiode for near-infrared or visible light and an indium gallium arsenide device for longer wavelengths. Response time depends on junction capacitance and load resistance, so a smaller load resistor gives a faster but smaller signal. Optical systems use this arrangement in fibre-optic receivers, barcode scanners, smoke detectors and light-beam intruder alarms, where the photocurrent must track rapid changes in light intensity. A strong answer earns marks for these points. States that photo-conductive mode uses reverse bias, giving a wide depletion region and a strong electric field across the junction. Explains that absorbed photons create electron-hole pairs, which are separated by the field and produce a photocurrent. Describes the photocurrent as approximately proportional to irradiance over the intended operating range. Explains that a load resistor or amplifier converts the photocurrent into a voltage that can be processed by the rest of the system. A common slip: Confusing photo-conductive mode with photovoltaic mode. Instead, stating that photo-conductive mode uses reverse bias and an external supply, whereas photovoltaic mode uses no external bias. Exam tip: Name the mode explicitly and state the bias condition before describing how the photocurrent is produced.

    See it

    A flowchart running from top to bottom. The first box says reverse bias applied to photodiode, which leads to a box saying wide depletion region and strong electric field. That leads to absorbed photons create electron-hole pairs, which leads to field separates pairs, producing photocurrent. This leads to photocurrent approximately proportional to irradiance, then to load resistor or amplifier converts photocurrent to voltage, and finally to voltage processed by rest of system. A separate box saying detector material chosen to match source wavelength points into the electron-hole pairs box. Another separate box saying junction capacitance and load resistance set response time points into the load resistor or amplifier box.
    It shows how a reverse-biased photodiode in photo-conductive mode turns absorbed light into a photocurrent and then into a usable voltage signal.

    Key words

    Say what each word means before you turn the card.

    Photo-conductive mode

    Try answering aloud or in your head before turning the card.

    Photocurrent

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    Spectral response

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    RC time constant

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    See every definition
    Photo-conductive mode
    Uses reverse bias and an external supply, giving a wide depletion region and a strong electric field across the junction.
    Photocurrent
    Produced when absorbed photons create electron-hole pairs, which are separated by the field; approximately proportional to irradiance over the intended operating range.
    Spectral response
    Chosen so it matches the wavelength of the optical source, for example a silicon photodiode for near-infrared or visible light.
    RC time constant
    Of the junction capacitance and load resistance determines response time, so a smaller load resistor gives a faster but smaller signal.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. The first box says reverse bias applied to photodiode, which leads to a box saying wide depletion region and strong electric field. That leads to absorbed photons create electron-hole pairs, which leads to field separates pairs, producing photocurrent. This leads to photocurrent approximately proportional to irradiance, then to load resistor or amplifier converts photocurrent to voltage, and finally to voltage processed by rest of system. A separate box saying detector material chosen to match source wavelength points into the electron-hole pairs box. Another separate box saying junction capacitance and load resistance set response time points into the load resistor or amplifier box.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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