What is the PIN diode in a cell phone. What is it used for

On the boards of older cell phones, but not all of them, you can see a metal box near the antenna connector containing several diodes. What are they there for?

In phones, these diodes are necessary to switch the transmitter's output signal to the antenna, since the transmitter can operate in different frequency ranges, such as 900 MHz and 1800 MHz. There are multiple ranges, but only one antenna. But why aren't regular diodes used instead of PIN diodes?

The fact is that PIN diodes can operate perfectly at frequencies of, say, 900 MHz and switch very quickly, while a regular diode will likely not operate at these frequencies or will switch slowly. PIN diodes also have low resistance when closed and high resistance when open, and they have low capacitance. This is why these diodes are used in telephones due to their fast antenna switching and low resistance. These diodes have n- and p-regions like regular diodes, but also an i-region.

Let's do a simple test. I have a generator with a frequency of, say, 900 MHz. An incandescent lamp is connected to the generator output as a load, and it glows because high-frequency power is supplied to the lamp.

Between the generator and the lamp, we'll first connect a regular diode, and we'll also apply bias voltage to it so it's closed. A regular diode won't work at this frequency.

Now we do the same with the PIN diode and it works perfectly at this frequency and the lamp lights up.

By turning the bias voltage on the diodes on and off, you can switch the transmitter output to the antenna or switch the antenna

By changing the bias voltage, the output power of the RF signal entering the antenna can be adjusted.This is actually an example of an attenuator, where the input signal can be attenuated.

These are all actually simplified examples. By the way, at a frequency of 14 MHz, instead of a PIN diode, I used a 1n4007 diode, and it works as a switch and regulator of the RF signal power to the antenna. But its switching speed will probably be slower compared to a PIN diode.