2. Transmission and locking losses:
Signal control in microwave circuits is characterized by transmission losses Ltrans (the switch is in the transmission state) and locking losses Llock (the switch is in the locking state) [12]:
(12)
(13)
where PIN and POUT – input and output signal power, respectively.
Transmission and locking losses are expressed in decibels. It is obvious that in the circuit of parallel switching on of the switch, the transmission state is provided by the «diode closed» mode, and the locking state is provided by the «diode open» mode. For the sequential switching circuit, the corresponding modes of operation of the diode are reversed. The lower the transmission loss and the greater the locking loss, the more perfect the switching diode.
3. Quality coefficient:
The efficiency of the switching device is characterized by a quality coefficient, which is determined by the formula [12]:
(14)
4. Turn-on time of the pin diode:
The response time (turn-on time) of the PIN diode can be calculated using the following formula [13]:
=
(15)
where W – thickness of the i-region; vs – saturation rate.
Picture 10 – To determine the turn-on time [13]
Let the bias voltage on the PIN diode included in the microwave generator circuit change from reverse to forward at some point in time.
At the reverse bias voltage, the diode current is equal to the thermal one. The resistance of the diode is high, and it has little effect on the level of microwave power. At the moment of switching, a forward current will start flowing in the diode circuit Iпр, since holes and electrons will be injected into the i-region of the diode. The final resistance will result in the reflection of part of the power. After the reverse switching, all the movable holes and electrons will move and either recombine or leave, respectively, in the p- and n-regions. During this entire process, a current will flow in the diode circuit Iобр in the opposite direction. The time from the moment of switching to the moment when the forward current reaches a steady value is called the turn-on time [11].
5. Cutoff frequency:
The cutoff frequency of the PIN diode is determined by the following ratio [5]:
= (16)
where Cj = eS/W – the capacitance of a flat capacitor formed by the structure of the device.
4.3 Similarities and differences of the detector diode and pin diode
4.3.1 Differences between the detector diode and pin diode
Analyzing the functional role of each of the diodes, as well as their parameters, we conclude that the main difference between these devices lies in their functional role, namely: the detector diode is used to detect microwave signals, while the PIN diode is used in switching devices, modulators, phase shifters and attenuators, as well as in restriction and control devices power, protection of input receivers.
The differences also lie in the structures of the diodes: the PIN diode consists of three regions, as the name implies, while the detector diode consists of two regions (n and n+).
Analyzing the volt-ampere characteristic of the detector and PIN diodes (picture 9), we conclude that the volt-ampere characteristic of the PIN diode has a more resistive character of the direct branch associated with the presence of a low-alloy i-region. And that is why the increase in forward current begins at high voltages, and the breakdown, respectively, comes later.