This particular voltage is called the knee voltage or the threshold voltage. Once the knee voltage is reached, the current may increase but the voltage does not change. c -velocity of light. h -Planck’s constant. λ -wavelength of light.
YLight emitting diodes{LED's{display a charac eristic threshold voltage. This voltage is known as the knee voltage, Vk. Befor this voltage, there is very little change in resistance through the LED. However, at Vk the resistance begins to increase exponentially with in-creasing voltage. This knee voltage is unique to each LED and varies
At a particular voltage the height of potential barrier becomes very low and the LED starts glowing, i.e., in the forward biased condition electrons crossing the junction are excited, and when they return to their normal state, energy is emitted. This particular voltage is called the knee voltage or the threshold voltage.
Increase the photocell bias voltage (Vbias) in small steps by using the rheostat. 8. Record the values of the photo current (I) on the nanoammeter as a function of the increasing photocell bias voltage, till the photo current reduces to zero.
The best way to measure a range of photocell outputs accurately is with an analog interface circuit, either using an operational amplifier, or driving a suitable analog-digital converter (ADC) directly from a photocell. Fig 14 Voltage Output Photodiode Circuits
The first such sensor was the “phototube” or “photo-electric cell” (Fig 1A) which consists of a photo-sensitive cathode in a vacuum (or, sometimes, very low-pressure gas) tube which when illuminated emits electrons which move to a positively biased anode, allowing a current to flow which is proportional to the light intensity.