Photodiode: current and voltage in an illuminated Junction
Junction devices can be used to improve the speed of response and sensitivity of detectors of optical or high-energy radiation. Two-terminal devices designed to respond to photon absorption are called photodiodes. Some photodiodes have extremely high sensitivity and response speed.
Current and voltage in an illuminated Junction:
- In particular, carriers generated within the depletion region W are separated by the junction field, electrons being collected in the n region and holes in the p region.
- Also, minority carriers generated thermally within a diffusion length of each side of the junction diffuse to the depletion region and are swept to the other side by the electric field.
- If the junction is uniformly illuminated by photons with hv > Eg, an added generation rate gop (EHP/cm3-s) participates in this current (Fig. given below).
- The number of holes created per second within a diffusion length of the transition region on the n side is ALpgop
- Similarly A Lngop electrons are generated per second within Ln of xpo and AWgop carriers are generated within W.
The resulting current due to collection of these optically generated carriers by the junction is
- If we call the thermally generated current Ith, we can add the optical generation to find the total reverse current with illumination.
- Since this current is directed from n to p, the diode equation becomes
- Thus the I-V curve is lowered by an amount proportional to the generation rate (Fig.c).
- This equation can be considered in two parts —the current described by the usual diode equation, and the current due to optical generation.
- When the device is short circuited (V = 0), the terms from the diode equation cancel as expected.
- However, there is a short-circuit current from n to p equal to Iop.
Thus the I- V characteristics of Fig.c cross the I-axis at negative values proportional to gop. When there is an open circuit across the device, I = 0 and the voltage V = Voc is
Neglecting generation within W:
Actually, the term gth =pntn represents the equilibrium thermal generationrecombination rate.
The appearance of a forward voltage across an illuminated junction is known as the photovoltaic effect.