# Photoelectron to electron hole pair doubts

• Manit
In summary: However, the drift velocity of electrons in silicon is on the order of 10 m/s, but this can vary depending on the doping level and temperature of the material. It is not a constant value and would need to be determined experimentally or through simulation.
Manit
Hi,
I have a couple of questions on photoelectrons.

When a photoelectron of about 3-eV (varies) interacts within 0.2-um depletion region of silicon, what happens?

I know, it will generate an electron-hole pair with an efficiency of 1 for 3.6-eV photoelectron. But what happens if the photoelectron energy is lower than 3.6-eV?

Now, what happens to the current following this generation? Since, this pair is in a reverse biased p-n diode it will be pulled to form a signal. The question is what would be the magnitude and how much time would they be available before being lost or filled up (sorry, I am little rusty with the terminology - have moved away from electronics, but have come across this problem).

I believe, current (I) = # electrons*charge/time

Where, # electrons are the number of electrons; charge- 1.6e-19 C; time - the amount of time the electron is present to generate/affect the signal before they are lost.

So, if 25 photoelectrons (3.6-eV) with 100% efficiency then we see a generation of 25 pairs. Let's say, they each are available for a time of 1-us. So, current will be 25*1.6e-19/1e-6 (A). Am I wrong?

I don't know what happens when the energy is lower than 3.6-eV and the time this electron-hole pair are available.

Regards,
Manit.

Manit said:
I know, it will generate an electron-hole pair with an efficiency of 1 for 3.6-eV photoelectron. But what happens if the photoelectron energy is lower than 3.6-eV?
If there is no suitable transition at the energy of the photon, it will just pass through. The threshold is not 3.6 eV, however, the bandgap of silicon is 1.1 eV. You can create an electron/hole pair with a photon with 1.2 eV. It just does not happen with a 100% efficiency. If you take efficiency into account, particle detectors usually get one pair per 3.6 eV of high-energetic electrons or photons.
Manit said:
So, if 25 photoelectrons (3.6-eV) with 100% efficiency then we see a generation of 25 pairs. Let's say, they each are available for a time of 1-us. So, current will be 25*1.6e-19/1e-6 (A). Am I wrong?
The time is related to the thickness of the sensor and the drift speed of the electrons and holes in the material. It is not as simple as distance/speed, however, as your charges start inducing charges in the electrodes before they arrive there.

Manit
Thank you very much for your reply, Mr. MFB.

So, to account/estimate the current induced, what is the best way? or any electrical software that can help in measuring the relation between number of photoelectrons (electron-hole pairs) and the current?

Regards,
M

There are software packages to simulate that (but I don't know them). Capacitances in the system are important as well.

Manit
Hi,

I believe, you are referring to this equation:

I = nevA

I - Current
n = no of electrons per unit volume,
e = charge of an electron,
v = drift speed of an electron,
A = cross sectional area of the wire

However, n will be too large in general (10^18 - semiconductors). # of electrons added from photoelectrons would be marginal. What I am trying to estimate is the current magnitude from the photoelectrons.

Also, I see, v=10 m/s in semiconductors. Is it close or in fact a standard value that I can assume for hand calculation? I mean, is 10 m/s drift speed for an electron in an electron-hole pair?

Thank you for your time and help!

Manit said:
I believe, you are referring to this equation:

I = nevA
No.
The current will depend on the electrical environment of the sensor and the readout electronics, it is not as simple as plugging in numbers into a formula.

## 1. What is a photoelectron?

A photoelectron is an electron that is emitted from a material when it absorbs a photon of sufficient energy. This process is known as the photoelectric effect and is the basis for many technologies such as solar cells and digital cameras.

## 2. What is an electron hole pair?

An electron hole pair is a phenomenon that occurs when a photon of sufficient energy is absorbed by a material, causing an electron to be excited to a higher energy level and leaving behind a positively charged "hole" in its place. This pair of particles can move independently and contribute to the flow of electric current in a material.

## 3. How are photoelectrons and electron hole pairs related?

Photoelectrons and electron hole pairs are closely related as they both result from the absorption of a photon by a material. When a photon is absorbed, it can either create a photoelectron or an electron hole pair, depending on the energy of the photon and the properties of the material.

## 4. What are some applications of photoelectron and electron hole pairs?

Photoelectron and electron hole pairs have many applications in technology. They are essential for the functioning of solar cells, which convert sunlight into electricity. They are also used in digital cameras to capture images and in photomultiplier tubes for detecting and amplifying light signals.

## 5. Can photoelectron and electron hole pairs exist in vacuum?

No, photoelectron and electron hole pairs cannot exist in vacuum as they require a material to absorb the photon and create the pair. In a vacuum, there are no materials present to interact with the photon, so photoelectron and electron hole pairs cannot be created.

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