What you'll learn in this topic:

• How a photodiode operates in photo-conductive mode (reverse bias).
• How to interpret photodiode characteristic curves (I-V graphs) and spectral response curves.
• How photodiodes are used as fast-acting detectors in optical systems.
• How pairing a photodiode with a scintillator allows us to detect high-energy atomic particles.

The Photodiode and Photo-conductive Mode

You already know that standard diodes only allow current to flow in one direction (forward bias). If you connect them the "wrong way" round (reverse bias), they block the current almost entirely.

A photodiode is a special type of semiconductor diode designed specifically to take advantage of this reverse-bias behaviour when exposed to light.

When we use a photodiode to measure light intensity or detect light pulses, we operate it in photo-conductive mode. This simply means we connect the photodiode in reverse bias within a circuit.

In complete darkness, the reverse-biased photodiode blocks almost all current. However, a tiny leakage current still flows, known as the dark current.

When light hits the P-N junction of the photodiode, the photons transfer their energy to electrons in the semiconductor. If the photons have enough energy, they free these electrons, creating electron-hole pairs. Because the diode is in reverse bias, the built-in electric field instantly sweeps these free charges away, creating a measurable current.

Characteristic Curves (I-V Graphs)

To fully understand a photodiode, you need to look at its characteristic curve—a graph of current against voltage.

Because we operate photodiodes in reverse bias, the interesting part of the graph is in the third quadrant (negative voltage, negative current).

Notice the following key features:

• The y-axis: Represents the reverse current (often called the photocurrent). In reality, this is usually in microamps ($\mu A$) or milliamps ($mA$).
• Dark Current: The top line shows the tiny current that flows when there is zero illumination.
• Spacing of the lines: As light intensity (illuminance) increases, the reverse current increases. For a constant reverse voltage, doubling the light intensity will roughly double the photocurrent.

Spectral Response Curves

Not all light is equal. A photodiode does not react to all wavelengths of light with the same sensitivity. The energy of an incoming photon is given by $E=\frac{hc}{\lambda }$. If a photon's wavelength $\lambda$ is too long (and thus its energy is too low), it won't have enough energy to free an electron in the semiconductor, meaning zero current is produced.

A spectral response curve shows how the sensitivity of the photodiode changes with the wavelength of the incident light. The y-axis is usually given as responsivity, measured in Amperes per Watt (${A\; W}^{-1}$). This tells you how much current is produced for every watt of light power hitting the detector.

For a typical silicon photodiode:

• It does not respond to far-infrared light (wavelengths longer than roughly $1100nm$) because the photons lack the energy to free electrons.
• The peak sensitivity is usually in the near-infrared region (around $900nm$).
• Sensitivity drops off for ultraviolet light because high-energy photons are absorbed too close to the surface of the semiconductor, before they can reach the active junction.

Use in Optical Systems

Because photodiodes react to changes in light intensity almost instantaneously, they are fantastic detectors in optical systems.

Common uses include:

• Fibre-optic communications: Pulses of infrared light travel down an optical fibre. A photodiode at the receiving end converts these incredibly fast light flashes back into high-speed electrical signals.
• Remote controls: When you press a button on your TV remote, it flashes an infrared LED. A photodiode on your TV detects these specific flashes and converts them into an electrical signal the TV's processor can decode.

Detecting Atomic Particles (Scintillators)

Photodiodes are great at detecting visible and infrared light, but what if we want to detect high-energy atomic particles (like alpha or beta particles) or gamma radiation?

These particles carry so much energy that they often pass straight through a thin semiconductor without depositing their energy usefully, or they damage the delicate crystal structure.

The solution is to use a scintillator.

By coupling a scintillator block directly to a photodiode, we create a highly effective particle detector.

Here is the step-by-step process of how this combined detector works:

1. An invisible, high-energy atomic particle enters the scintillator crystal.
2. The particle collides with atoms in the crystal, exciting them. As they de-excite, they release their energy as flashes of visible light photons (scintillation).
3. These visible photons travel out of the crystal and hit the attached photodiode.
4. The photodiode absorbs the visible photons and generates a pulse of electric current.
5. This current pulse is passed to an amplifier and a counter to record the detection of the original particle.

This method is incredibly useful in medical imaging (like PET scanners) and nuclear physics experiments. The amount of light produced in the scintillator—and therefore the size of the current pulse from the photodiode—is often proportional to the energy of the original incoming particle, allowing physicists to not only count particles but also measure their energies.