What is the "collector light current" of a phototransistor and how does it relate to the collector current?

Question

I found the "collector light current" term in this datasheet (Vishay TEFT4300) but I can't find a definition for it on the internet nor figure out how it relates to the collector current.

I am particularly interested in this graph below:

Answer 1

"I'm interested in estimating Ic based on the Irradiance I calculated (~0.5mW/cm^2)"

Ic mainly depends on:

• How precise is the estimated value (mW/cm^2) or what is expected range
• The value of the resistor connected to the collector (Rc)
• The DC voltage source connected to the collector resistor (Vcc)

Assuming Vcc is given by your circuit, by increasing the Rc the operating point will move towards the origin (lower Ic and Vce), eventually leaving the active region and entering the saturation.

You could use the current for the active region on the graph as a lower estimate for saturation:

Example 1: Vcc = 5V, Ic (graph) = 130uA -> Rc = 38.4kOhm

This resistor would result in Vce = 0V (if that was possible), so you know the transistor left the active region and entered the saturation. The larger the resistor, the more you force it into saturation but the "dark current" would also limit this increase (since it would avoid the transistor turning off: Vce ~= Vcc).

Conversely, lower values will move the transistor into the active region, but this operating point would vary with temperature, for each different device you use and the real mW/cm^2.

Example 2: Vce = 2.5V (Vcc/2), Ic (graph) = 130uA -> Rc = 19.2kOhm

E.g., if the real mW/cm^2 is 1.0 instead of 0.5, the transistor saturates.

Answer 2

Ica (what they are the "collector light current") is the current through the collector and out the emitter vs. Vce (collector to emitter voltage) at different intensity of light impinging on the device.

So, for example, if you had a Vce of 10V and shined a light of 0.1 mW/cm2 on it, you should expect an Ica of about 0.3 mA (more or less).