4.3.2 Similarities of the detector diode and pin diode

However, it is possible to distinguish a certain similarity in these diodes, namely, the PIN diode can also be used to detect signals, because the PIN diode has the ability to detect optical signals modulated by gigahertz (GHz) frequencies, which is especially important for high–frequency devices for receiving and processing information.

5. Description of circuit models of microwave diodes with positive dynamic resistance

The following diodes with positive dynamic resistance were selected: PIN diode and mixer diode.

5.1 Description of the pin diode circuit model

b

a

Picture 11 – Equivalent PIN diode circuit in open (а) and closed (b) states [14]

With forward bias at sufficiently high frequencies, the diffusion capacitance of the p⁺- i - and n⁺ - i - junctions of the diode completely shunts them, thus the equivalent circuit is reduced to picture 11, а, where r_ПР – base resistance determined by forward current. Usually, the r_ПР values in the operating mode are close in magnitude to 1 [14].

With reverse bias, the equivalent PIN diode circuit is represented as picture 11, b, where r_ОБР - resistance of the i-region.

Thus, when operating in the microwave range, the PIN diode (without taking into account the parasitic parameters of the housing) is a nonlinear resistor, whose resistance at forward bias is significantly less than at reverse, while the value of the resistance r_ПР depends on forward current [14].

Here is an equivalent PIN diode circuit taking into account the housing:

Picture 12 – Equivalent PIN diode circuit taking into account the housing [14]

Picture 12 shows an equivalent PIN diode circuit that takes into account the state of the diode with both forward and reverse bias. In this circuit, the r_S resistance is caused by losses in the semiconductor and the diode terminals.

5.2 Description of the mixer diode circuit model

Picture 13 – Equivalent mixer diode circuit [15]

The transition in this diagram is represented by differential parameters: the transition resistance r_ПЕР and barrier capacity C_БАР. Losses in the diode base, ohmic junctions and terminals are displayed by the serial loss resistance r_Б, inductance of terminals and contact spring – L_К, constructive capacitance between the terminals in the absence of contact with the diode structure – C_К.

Due to the voltage drop on r_Б the voltage applied to the junction turns out to be less than that supplied to the diode, and the capacitance C_K shunts it. These parameters are called parasitic.

In case-free diodes, the values of C_К and L_K are an order of magnitude smaller, so their efficiency is higher. The value of the differential resistance can vary widely depending on the position of the operating point of the volt-ampere characteristic of the diode. The circuit parameters can be determined by measurements at low frequencies or approximately based on the rectification process [15].

An equivalent circuit is used to calculate the characteristics of the mixer at high frequencies.

Answer:

1. Current sensitivity:

= 10,81 V^-1

2. Increment of current when applying microwave power to the diode:

= 1,622 mA

3. Tangential sensitivity:

µ = 99,84 dBm

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