Light Dependent Resistors

Light Dependent Resistors

Light dependent resistors, also known as photoresistors, are passive electronic components whose resistance changes in response to incident light intensity. This phenomenon arises from the photoconductivity effect, where light excites electrons within the LDR material, increasing its conductivity and decreasing its resistance. While the basic principle is straightforward, the relationship between light intensity and resistance is not always linear, leading to various non-linear responses that need to be considered in their applications.

Types of Non-Linear Responses:

1. Logarithmic response: This is the most common type of non-linearity observed in LDRs. In this case, the change in resistance with respect to light intensity follows a logarithmic curve. This means that small changes in light intensity at low light levels produce significant resistance changes, while larger changes in intensity at high light levels result in smaller resistance variations. This characteristic makes LDRs suitable for detecting low-level light changes, such as dusk/dawn sensors.

2. Exponential response: In some LDRs, particularly those made with specific materials like lead sulfide, the resistance change exhibits an exponential behavior. Here, even minute increases in light intensity can lead to substantial drops in resistance. This sensitivity makes them ideal for applications requiring high-precision light detection in low-light environments. However, their non-linearity necessitates careful circuit design to interpret the signal accurately.

3. Saturation: Under very high light intensity, the resistance of many LDRs may reach a minimum value and remain relatively constant despite further increases in light. This saturation effect limits the dynamic range of the sensor and needs to be considered when selecting an LDR for applications involving wide variations in light levels.

4. Spectral sensitivity: Different LDR materials exhibit varying sensitivities to different wavelengths of light. For example, cadmium sulfide (CdS) LDRs are most sensitive to visible and near-infrared light, while lead sulfide (PbS) LDRs are more responsive to infrared radiation. This spectral dependence adds another layer of non-linearity to the LDR response, requiring careful selection based on the desired light detection spectrum.

5. Temperature dependence: The resistance of LDRs can also change with temperature, further complicating their response. As temperature increases, the resistance generally decreases, even in darkness. This thermal sensitivity necessitates temperature compensation techniques or using LDRs in controlled temperature environments for precise measurements.

6. Voltage Influence: Applied voltage plays a minor role in most LDR applications. Within their designated voltage range, LDRs exhibit primarily ohmic behavior, meaning current linearly increases with voltage (I = V/R). Therefore, voltage changes themselves don't introduce significant non-linearities in the LDR's response.

7. Exceptions: In some specific scenarios, high applied voltages exceeding the specified limits can lead to:

   • Breakdown: Excessive voltage can cause current breakdown in the semiconductor, leading to abrupt resistance changes, but this isn't a controlled response and damages the LDR.

   • Heating: High voltages can also generate heat, affecting the temperature dependence of the LDR's resistance. However, this non-linearity arises from temperature, not directly from voltage.

Response Time

The response time of a light dependent resistor (LDR) being slow stems from several fundamental physical processes within the semiconductor material:

1. Carrier Generation: Upon light absorption, electrons within the semiconductor gain enough energy to jump from the valence band to the conduction band, becoming mobile charge carriers. This process, however, isn't instantaneous. It depends on the photon energy (wavelength) and the material's bandgap. Some photons don't have sufficient energy, and even for suitable photons, there's a probability factor involved. This probabilistic nature contributes to the delay in generating enough carriers for significant resistance change.

2. Recombination: Excited carriers don't remain indefinitely in the conduction band. They eventually "recombine" with holes in the valence band, losing their excess energy and returning to their original state. This recombination process also takes time, further contributing to the overall response time. Factors like material defects and impurities can influence the recombination rate, impacting the LDR's speed.

3. Trapping: In some materials, excited carriers can become temporarily trapped in energy states within the bandgap, known as traps. These traps delay the carriers' contribution to conductivity, slowing down the overall response. The number and type of traps within the material determine the extent of this effect.

4. Material Properties: Different LDR materials possess inherent characteristics affecting their response times. For example, materials with larger bandgaps generally require higher energy photons for excitation, leading to slower responses. Additionally, materials with higher carrier mobility tend to exhibit faster response times.

5. Temperature Dependence: Response time is also temperature-dependent. Higher temperatures generally increase carrier mobility and reduce recombination rates, leading to faster responses. However, excessively high temperatures can also increase leakage currents and negatively impact performance.

It's crucial to note that these factors often interact, making the LDR response time a complex phenomenon. While some materials like lead sulfide offer faster response times (<1ms), others like cadmium sulfide may have response times in the tens of milliseconds. Carefully considering these underlying mechanisms and their interplay is essential when selecting and using LDRs for applications requiring specific response characteristics.

Typical cadmium sulfide LDR characteristics

An optical feedback technique extends the frequency response of cadmium sulfide LDRs and renders them useful at frequencies orders of magnitude above the internal 3-dB frequency of the photoconductor. 

The optical feedback maintains, or attempts to maintain, a constant summed input and feedback light on the CdS. This maintenance of operating point is useful, since simple frequency compensation of CdS is impractical because its internal 3-dB frequency is ambient illumination dependent. The optical feed- back stabilizes the operating point and hence the effective response time of the CdS, which permits simple frequency compensation.