P-I-N Junctions: A Dance With Semiconductors and Photovoltaics

2024-01-19 | By Antonio Velasco

Before jumping into the world of optoelectronics, it's important to understand the fundamentals behind it. The best place to start is the fundamental building block for most things with the photo- prefix: P-I-N junctions. This is a component at the microelectronic level that makes it all possible, combining photovoltaics and semiconductors.

What is a P-I-N Junction?

P-I-N Junctions A Dance With Semiconductors and Photovoltaics

At its core, P-I-N junctions consist of three layers, making up its name!

- The "P" layer (positive) is doped with acceptor impurities, creating an excess of positively charged "holes" in the crystal lattice. These holes are essentially missing electrons.

- The "I" (intrinsic) layer is undoped or lightly doped, making it a semiconductor region with equal numbers of electrons and holes. Here, electrons can move freely.

- The "N" (negative) layer is doped with donor impurities, introducing an abundance of negatively charged electrons into the lattice.

When you put and sandwich all these layers together, charges "dance" in between them. N-region electrons may migrate toward the P-region, but the holes in the P-region (which are positively charged) move into the N-region. The positive and negative charges don't cancel out here though; rather, they create a charged boundary (or a depletion region) where an electric field is established. This depletion region/boundary prevents further flow of elections and thus prevents it from moving over--that is unless a voltage is applied, allowing the current to move forward. This is how the standard PN diode works. They act as a one-way street that opens when a traffic light turns green, essentially.

P-I-N junctions differ from PN diodes as they include an intrinsic layer, which is basically a neutral region that, in our case, allows for the photons to transfer their energy to electrons within the semiconductor material.

Silicon is extremely useful for these junctions due to its semiconductor properties and its ability to be doped copiously. Impurities can be added in to adjust its properties and thus manufacture the diode for specific roles.

Some of the Numbers Behind PIN Diodes

Now, let's delve into the mathematics of P-I-N junctions. Two essential equations govern their behavior:

The electric field within the depletion region can be calculated as:

E = (Vbi - V) / W

Where:

- E is the electric field strength.

- Vbi is the built-in voltage potential across the junction.

- V is the applied voltage.

- W is the width of the depletion region.

The current (I) through a P-I-N junction can be determined using the diode equation:

I = I0 * (e^(qV/kT) - 1)

Where:

- I0 is the reverse saturation current.

- q is the charge of an electron.

- V is the voltage across the junction.

- k is Boltzmann's constant.

- T is the temperature in kelvin.

These equations allow engineers to utilize and modify junctions as needed.

Applications for PIN Junctions

Within optoelectronics, and thus photovoltaics, PIN junctions have a ton of usage. Most notable are photodiodes--the most common application of a diode.

P-I-N Junctions A Dance With Semiconductors and Photovoltaics

I talk more about photodiodes in another article that I've linked below, but to put it simply, they act as a photodetector that generates electrical current. These are typically used in light sensors (in cameras, smartphones, etc.) or simple motion detectors.

The most popular usage is in photovoltaic cells, another aspect that I mention in an article linked below.

P-I-N Junctions A Dance With Semiconductors and Photovoltaics

They utilize the photovoltaic effect to transform sunlight into electricity--aka solar panels! Photons from the light strike the semiconductor material and thus excite electrons, producing a current flow.

PIN junctions have a ton of usage and are used in a wide variety of optoelectronics. They play a pivotal role in the light detection and energy generation technology we have today.

If you're interested in the applications and possibly using them in your projects, see the articles below!

Photodiodes: Light Meets Semiconductors

Harvesting the Sun: Photovoltaic Cells

Photoresistors: Opening a Path with Light!