Question

Illuminate Si PIN Photodiode with a flashlight in reverse, zero and forward bias conditions. State what you observed. Compare the amount of change in the current. Which bias condition is best to use Si PIN photodiodes to generate electricity from light (solar cell)?Explain

Answer 1

A PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts. The wide intrinsic region is in contrast to an ordinary p–n diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes it suitable for attenuators, fast switches, photodetectors, and high voltage power electronics applications. A PIN diode operates under what is known as high-level injection. In other words, the intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. The diode will conduct current once the flooded electrons and holes reach an equilibrium point, where the number of electrons is equal to the number of holes in the intrinsic region. When the diode is forward biased, the injected carrier concentration is typically several orders of magnitude higher than the intrinsic carrier concentration. Due to this high level injection, which in turn is due to the depletion process, the electric field extends deeply (almost the entire length) into the region. This electric field holds the diode open in fast switching circuits as the circuit switches faster than the field can dissipate - making it a suitable device for high frequency transmissions through the diode.

                                         

A photodiode is an active component that converts light into an electrical voltage (photovoltaic effect) or photocurrent. The p-n junction in the silicon semiconductor serves as the physical basis for this process. When photons with sufficient energy are absorbed by the detector, this results in the formation of charge carriers (electron-hole pairs), which are separated in the space-charge region and thus generate the photocurrent.While the charge separation also occurs without the application of an external voltage, the process can be accelerated by such a reverse voltage. The photocurrent remains linear to the absorbed light volume across many orders of magnitude if the diode is not operated in a state of saturation.

Depending on the external connections, we differentiate between two different operating states: element and diode. In the case of element operation, the diode is connected directly to the consumer without the use of an external voltage source. No dark current flows in this operating state, which facilitates the detection of minimal intensities.

During diode operation, an external voltage supply is connected with the consumer in series, whereby the voltage is applied in reverse direction. This operating mode is ideal for applications in which a rapid signal response is required. The main disadvantage is the dark current, which grows exponentially with the temperature.

A PIN diode comprises a near-intrinsic semiconductor region – usually the space-charge region – sandwiched between a p-type diode and an n-type substrate. However, the term is also used for components with inverse conductivity, provided that no other non-linear effects are utilized in the component.

A normal PN junction photodiode is made by sandwiching a P type semiconductor into N type semiconductor. All the sides of PN junction diode is enclosed in metallic case or painted black except for one side on which radiation is allowed to fall.

The photodiode is operated under a moderate reverse bias. This keeps the depletion layer free of any carriers and normally no current will flow. However when a light photon enters the intrinsic region it can strike an atom in the crystal lattice and dislodge an electron. In this way a hole-electron pair is generated. The hole and electron will then migrate in opposite directions under the action of the electric field across the intrinsic region and a small current can be seen to flow. It is found that the size of the current is proportional to the amount of light entering the intrinsic region. The more light, the greater the numbers of hole electron pairs that are generated and the greater the current flowing.

Operating diodes under reverse bias increases the sensitivity as it widens the depletion layer where the photo action occurs. In this way increasing the reverse bias has the effect of increasing the active area of the photodiode and strengthens what may be termed as the photocurrent.

It is also possible to operate photodiodes under zero bias conditions in what is termed as a photovoltaic mode. In zero bias, light falling on the diode causes a current across the device, leading to forward bias which in turn induces "dark current" in the opposite direction to the photocurrent. This is called the photovoltaic effect, and is the basis for solar cells. It is therefore possible to construct a solar cell using a large number of individual photodiodes. Also when photodiodes are used in a solar cell, the diodes are made larger so that there is a larger active area, and they are able to handle higher currents. For those used for data applications, speed is normally very important and the diode junctions are smaller to reduce the effects of capacitance.

When not exposed to light the photodiode follows a normal V-I characteristic expected of a diode. In the reverse direction virtually no current flows, but in the forward direction it steadily increases, especially after the knee or turn on voltage is reached. This is modified in the presence of light. When used as a photodiode it can be seen that the greatest effect is seen in the reverse direction. Here the largest changes are noticed, and the normal forward current does not mask the effects due to the light.