For a phototransistor having the characteristics of Fig. 50...

For a phototransistor having the characteristics of Fig. 50 , determine the photoinduced base current for a radiant flux density of $5 \mathrm{~mW} / \mathrm{cm}^{2}$. If $h_{f e}=40$, find $I_{\mathrm{C}}$.

Key Concepts

Phototransistor

A phototransistor is a semiconductor device that converts incident light into an electrical signal. It operates similarly to a transistor, but uses light to generate a base current that is then amplified in the collector circuit. This makes phototransistors useful in light sensing and optical switching applications, as they combine the sensitivity of a photodiode with the amplification capabilities of a transistor.

Radiant Flux Density

Radiant flux density refers to the amount of radiant energy (or light power) per unit area incident on a surface. It is typically measured in milliwatts per square centimeter (mW/cm²) and is crucial for determining the amount of light available to generate a photocurrent in light-sensitive devices. This concept is important in understanding and designing optical sensor circuits.

Photoinduced Base Current

The photoinduced base current is the current generated in the base region of a phototransistor when it is exposed to light. This current is produced as photons interact with the semiconductor material, creating charge carriers that contribute to the initial current flow at the base. It serves as the triggering current that is subsequently amplified by the transistor's inherent current gain, impacting the overall response of the device.

Current Gain (h_fe)

Current gain, commonly denoted as h_fe, is a measure of the amplification factor of a transistor. It represents the ratio of the collector current to the base current. In the context of a phototransistor, a known h_fe allows one to calculate the amplified output current from the initial photoinduced base current, thus linking the incident light intensity to the resulting electrical signal.

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