Phototransistor behavior: Evaluate the claim “A phototransistor has a varying resistance from collector to emitter depending on how much light strikes it.” Decide whether this phrasing correctly describes the device operation.

Introduction / Context:
Light-sensitive devices include photoresistors (LDRs), photodiodes, and phototransistors. Each has a distinct operating principle. This question probes whether describing a phototransistor as having a “varying resistance from collector to emitter” is an accurate statement of how it works.

Given Data / Assumptions:

• A phototransistor is essentially a BJT where incident light generates base current.
• Collector current is proportional to light-induced base current times gain.
• Photoresistors (LDRs) are resistive sensors whose resistance changes with light intensity.

Concept / Approach:
A phototransistor is a current-controlled (actually light-controlled) device, not a variable resistor. Light generates carriers equivalent to base drive, causing a collector current that depends on illumination and transistor gain. While one could model the overall I–V behavior as an effective resistance in some operating region, that is not the correct nor standard description. The phrasing in the statement better matches an LDR, not a phototransistor.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets specify collector current vs. irradiance, current gain, and V_CE characteristics—not “resistance” vs. light curves as a primary parameter, which are common for LDRs.

Why Other Options Are Wrong:
“Correct” confuses phototransistors with LDRs. “Correct only for LDRs” names a different device, so it does not validate the statement for phototransistors. “Correct only in saturation” still mischaracterizes the device’s fundamental control mechanism.

Common Pitfalls:
Treating all light sensors as interchangeable. Selecting a phototransistor when a linear resistive response is required can complicate interface design; conversely, using an LDR where fast response is needed is problematic.

Final Answer:
Incorrect