# What is the advantage of P-Intrinsic-N photodiodes with respect to a single intrinsic semiconductor with a bias voltage?

P-I-N structures are an improvement from simpler PN photodiodes. You can get higher responsivity values with them since there is more area for light absorption.

But that made me think... why do you need the P and N doped semiconductors in the first place? For the built-in electric field? That can be compensated with an external bias voltage (Which is used anyway in P-I-N photodiodes).

• How does a PN or PIN diode junction have a build-in electric field? There are energy band differences but that is not the same as an electric field. Intrinsic (non or lightly doped) Silicon is not a good electric conductor and therefore not a very useful material to make semiconductor components from which can conduct usable currents. – Bimpelrekkie Sep 19 '17 at 15:05
• The built-in Potential and field is due to the space charge region at the electrode surface, created by doping a semiconductor , normally an insulator. Lower doping increases speed and bandwidth but degrades other factors. – Tony Stewart Sunnyskyguy EE75 Sep 19 '17 at 15:17
• With just a piece of silicon and a bias voltage you will see current all the time. And then more current with photo-excited carriers. – George Herold Sep 19 '17 at 15:42
• @Bimpelrekkie Diodes have a built in electric field caused by differences in the energy bands of the materials. That is what it is called and that is what it is. Place a positive test charge in the region and it will move in the direction of the built in field. – Matt Sep 19 '17 at 16:24
• @George Herold. And how is that different from the intrinsic material where the absorption takes place in a PIN photodiode? There is also "current all the time" due to the spontaneous e-h generation being swept away (dark current), with more current when light of the right wavelength hits the device. – Frilance Sep 19 '17 at 17:57

Your question asks about 3 different types of light sensing devices, a photoconductor, a p-i-n photodiode, and a p-n photodiode. There are pros and cons when using each of these

Photoconductor:
This is a piece of semiconductor with ohmic contacts on the ends. When illuminated electron hole pairs are generated and, if a bias is applied to the terminals, contributes to a photocurrent in the device. The photocurrent gain of such a device can be given by:

$$G_a = \frac{(\mu_n+\mu_p)\tau\mathcal{E}}{L}$$

where $\mu_n$ and $\mu_p$ are the electron and hole mobilities. $\tau$ is the carrier lifetime, $\mathcal{E}$ is the applied electric field, and $L$ is the length of the photoconductor.