6. Noise ratio:

The presence of a built-in and applied external electric field causes its noise characteristics to differ from the noise of the equivalent (differential) resistance of the diode at the operating point. Usually such an excess is characterized by a parameter – the noise ratio. It determines how many times the average square of the noise voltage from the diode < > is greater than the average square of the noise voltage < > from the resistance R_d 1/(I_d U_d) [9]:

n_noise = (11)

4.2 Pin diode

4.2.1 The functional role of the pin diode

PIN diodes are used for rectification, conversion and switching of high-frequency and pulse signals. The control action of such diodes is based on a change in their resistance with changes in the applied bias voltage or the level of microwave power in the path. Microwave PIN diodes are widely used to control the level and phase of microwave signals, to switch and stabilize microwave power in transmission lines, to protect radio equipment from accidental microwave pulses. Diodes designed for the centimeter, decimeter and meter ranges are divided into switching and limiting. The former is used in switching devices, modulators, phase shifters, attenuators, the latter – in power limiting and control devices and to protect input receivers. Diodes can be used for the same purpose in sealed hybrid modules [10].

4.2.2 Pin diode structure

Picture 8 – PIN diode structure

The main feature of the PIN diode structure is the presence of a high-resistance region of pure silicon (or germanium) between the p- and n-regions of the semiconductor 1 and 3 with 4 contacts of the high resistance region of pure silicon (or germanium) 2 with extremely low concentration of impurities (semiconductor with intrinsic conductivity of i-type). However, a semiconductor is really i-type, no matter how thoroughly it is cleaned, it is almost impossible to obtain. Therefore, really PIN structures are structures in which the i–region has weak hole conductivity in the case of Si (denoted by the p⁺ – – n⁺) and weak electronic conductivity in the case of Ge (denoted by1 the p⁺– – n⁺). For the manufacture of PIN diodes, p-type silicon is more often used. The concentration of acceptors in it is very small and amounts to 10¹² - 10¹³ cm^-3, while the concentration of donors in the n-region and acceptors in the p-region may exceed 10¹⁸ cm^-3. The area of the i-region ranges from fractions to units of a square millimeter with a thickness of 60 - 400 µm. Diodes can be placed in a housing that is most suitable for including a diode in a particular microwave path, but they can also be case-free [11].

4.2.3 Pin diode parameters

1. Volt-ampere characteristic:

In the first approximation, when theoretically considered, for the volt-ampere characteristic of PIN diodes, the relations used for detector diodes are valid, namely [12]:

Picture 9 – Comparative volt-ampere characteristic of the detector diode and PIN diode

That is, for a PIN diode, the ratio is also equivalent (6):

However, there are a number of characteristic features for switching diodes. As can be seen from the volt-ampere characteristic (picture 9), the main difference between the volt-ampere characteristic of the PIN diode is the more resistive nature of the direct branch, which is quite logical due to the presence of a low-alloy i-region. For the same reason, the increase in forward current begins at high voltages, and the breakdown occurs later.