2. Total resistance:
The total resistance of the detector diode can be expressed using the following relation [6]:
Zd = Rd + jXd (7)
At the same time, it is worth noting, based on the equivalent diode circuit (picture 4), that the impedance of the diode depends on both the junction resistance Rj and the capacitance of the junction Cj, varying with the variation of the bias voltage, and from the parasitic parameters of the diode: the resistance of the thickness of the semiconductor RS, the capacitance of the housing CКОРП and the inductance of the supply conductors LS. With a positive bias, for example, in a positive half-period of microwave oscillations, the dominant factor affecting the operation of the device is the presence of resistance Rs. With a negative displacement due to the expansion of the spatial charge region, the capacitance of the Cj junction increases and its influence begins to prevail over the influence of the resistance of the ohmic regions. In view of this, with a negative polarity of the voltage applied to the diode, it is customary to neglect the RS resistor, and with a positive one, the capacitance Cj [6].
3. Cutoff frequency:
The cutoff frequency is the limiting frequency at which the device can maintain different conductivity when the polarity of the applied voltage changes. The value of the boundary frequency of the detector diode is calculated by the following formula [5]:
=
=
=
=
(8)
Based on the expression for the cutoff frequency of the detector diode, we conclude that preference should be given to semiconductors with high carrier mobility when choosing materials for the detector diode in weak fields and with a high level of doping ND.
4. Current sensitivity:
The conversion efficiency when operating at direct current is evaluated by a parameter called current sensitivity. The value of the current sensitivity is equal to the ratio of the rectified current increment I to the value for a given microwave power Pmicro [7]:
=
=
(9)
where I0 – current at a constant bias applied to the diode.
5. Tangential sensitivity:
To describe the concept of tangential sensitivity, consider a generalized microwave circuit:
Picture 6 – Generalized microwave circuit
The power of the generator P is supplied to the microwave detector diode. Let the signal be a high-frequency modulated rectangular pulses (meander). The detected signal is amplified by a high-frequency amplifier and displayed on the oscilloscope screen. Depending on the power level installed on the generator, three characteristic cases are possible [8]:
Picture 7 – To the definition of tangential sensitivity
In the first case, the input power level is zero, and we can observe the noise present in the circuit. In the second case, the power level is large enough, so you can easily distinguish a useful signal against the background of noise. The third case is of particular interest: the upper bound of the noise strip in the absence of a signal coincides with the lower bound of the noise strip in the presence of a signal. With a further decrease in the power of the useful signal, noise will begin to drown out the useful signal. This minimum value of the power dissipated by the diode is the tangential sensitivity. However, for the convenience of work, when determining tangential sensitivity, it is customary to use an expression in which it is measured in decibels relative to 1 mW – in dBm [8].
Thus, the tangential sensitivity of the detector diode is determined by the following expression [9]:
= 10lg (10)