On the Application of Silicon Zener Diodes in Limiting Circuits
Abstract Generated abstract
This paper examines the suitability of several silicon Zener diode types for pulse limiting in measuring and pulse circuits, focusing on the knee region of the reverse current voltage characteristic, temperature dependence, dynamic resistance, and intrinsic noise. Using stabilized measurement circuits and thermostat testing, the authors compare D-808, D-810, D-811, D-813, and temperature compensated D-818 devices, finding the sharpest reverse characteristic bends in D-810 and D-811 diodes and noting that matched breakdown voltage does not imply matched dynamic resistance. The study also reports that temperature affects breakdown behavior and dynamic resistance in the transition region, especially for temperature compensated devices, and that Zener noise can significantly distort limited pulses at microampere currents, so it must be considered in limiter design.
Full Text
ON THE APPLICATION OF SILICON ZENER DIODES IN LIMITING CIRCUITS
G. I. Zaidman, S. A. Zaidman
(Presented by the scientific-technical seminar of the Department of Radio Engineering)
In measuring and pulse technology, nonlinear elements having a sharp bend in the current–voltage characteristic are used for limiting pulses.
Among the known characteristics of elements used for limiting purposes (vacuum diode, triode, semiconductor diode, triode, thermistors, etc.), the reverse branch of silicon Zener diodes is of considerable interest.
In the literature we have not found quantitative data on the current–voltage characteristic of Zener diodes in the region of the bend and on its variation with temperature precisely in this region. In the present work, some experimental investigations are carried out on the question raised.
Measuring apparatus and method for measuring current–voltage characteristics
We investigated the Zener diodes most widely used in limiting circuits, of types D-808, D-810, D-811, D-813, as well as domestic temperature-compensated Zener diodes of type D-818E.
For a quantitative evaluation of the course of the bend of the reverse branches of Zener diodes, a specially assembled measuring circuit was used, shown in Fig. 1.
Fig. 1
As the power supply, a stabilized electronic rectifier VS-12 was used. A parametric voltage stabilizer using resistors \(R_3\) and \(R_4\) and a type D-813 Zener diode was connected to the rectifier. The magnitude of the voltage ripple at the output of the parametric voltage stabilizer was approximately \(0.2\ \mathrm{mV}\).
The voltage across the Zener diode was calculated as
\[ U_{\mathrm{st}} = U_{\mathrm{in}} - U_{R5}, \]
where
\[ U_{\mathrm{in}} \text{ — voltage on the divider,} \]
\[ U_{R5} \text{ — voltage on the standard resistor } R_5 . \]
During the measurements, in order to eliminate instability of the power supply (a parametric voltage regulator), simultaneous readings of the voltage across resistors \(R_2\) and \(R_5\) were taken using PPTV-1 potentiometers. To eliminate a methodological error that may arise when two standard cells are used as reference voltage sources, only one saturated standard cell was used to set the operating current of each potentiometer. To investigate the temperature behavior of the initial portion of the reverse branch of the Zener diode, thermostat No. 3 was used. In Fig. 2 the temperature behavior of the bend in the current–voltage characteristic of the Zener diodes is shown by the dash-dot line.
From Fig. 2 (curve 1) it can be seen that the sharpest bend of the reverse branch of the current–voltage characteristic corresponds to Zener diodes D-810. This is apparently explained by the predominance of the avalanche nature of breakdown.
Fig. 2
With increasing temperature, as is evident from Fig. 2, the reverse current in the prebreakdown region increases. The breakdown voltage also changes; in this case the dynamic resistance of the reverse branch of the Zener diode changes hardly at all.
The change in breakdown voltage with temperature is probably caused by a change in the mean free path of the charge carriers. As the temperature increases, the mean free path decreases.
In order that, with a shorter mean free path, a carrier acquire energy sufficient for ionization of valence bonds, the field in the junction must have a larger magnitude. Consequently, the breakdown voltage will increase with increasing temperature [1].
The parallel behavior of the reverse branches of the current–voltage characteristics of Zener diodes with increasing temperature can be explained as follows. Apparently, the increase in the width of the \(p\)—\(n\) junction, caused by the large reverse voltage as temperature rises, leads to a decrease in the base thickness. Therefore, although the number of free charge carriers in the base increases with temperature, the dynamic resistance of the reverse branch of the Zener diode changes only weakly (see Fig. 2, curve 1).
Specially conducted investigations showed that selected Zener diodes of the same type, matched by breakdown voltage at one and the same operating current, have different values of dynamic resistance. This is explained by the presence of a spread in parameters during their manufacture.
In Fig. 2 (curve III) the reverse branch of a type D-808 Zener diode is presented at two different temperatures. It is evident that the breakdown voltage in the knee region at different temperatures is not sharply defined, as a result of which the dynamic resistance does not change abruptly. With increasing temperature, the angle of inclination of the reverse branch of the Zener diode in the knee region decreases. In type D-808 Zener diodes, in contrast to type D-810—D-813 Zener diodes, the dynamic resistance increases with rising temperature.
A study of domestic temperature-compensated D-818 Zener diodes (Fig. 2, curve II) showed that the prebreakdown regions and the beginning of the reverse branch of the Zener diodes are subject to a strong temperature effect. This is explained by the fact that, for temperature compensation of the positive temperature coefficient of voltage (TCV), silicon diodes connected in back-to-back series with a silicon Zener diode are used. As is evident from Fig. 2 (curve II), temperature-compensated Zener diodes also do not have a clearly expressed transition to the breakdown region. The dynamic resistance in this region is increased. This is explained by the fact that the forward branch of the compensating silicon diode, connected in series with the Zener diode, has a fairly large flat portion.
Experimentally investigating the forward branch of uncompensated Zener diodes, in particular a type D-811 Zener diode, it was found that the current-voltage characteristic has a different form depending on the injection level.
As is known, in the general case
\[ I_{\mathrm{f}} = I_s \left(e^{\frac{qV}{mkT}} - 1\right), \]
where \(m\) is a number determined both by the injection level and by the structure of the semiconductor. In our case it turned out that for very small currents \(m = 2.56\), for currents of medium magnitude \(m = 1\), and, finally, for large currents \(m = 1.6\). Consequently, the form of the current-voltage characteristic changes with increasing current.
Intrinsic noise of Zener diodes
When using Zener diodes in measuring circuits as amplitude limiters, it is very important to take their intrinsic noise into account.
In those cases where the currents of the clipped part of the pulse (limiting from above) are measured in tens or hundreds of microamperes, the intrinsic noise of Zener diodes leads to a strong distortion of the shape of the limited pulse. Therefore, accounting for the noise amplitude is extremely important.
Experimental investigation of noise was carried out by us according to the circuit shown in Fig. 3. As the power source, an electronic voltage regulator with a ripple amplitude of less than \(0.3\) mV was used. The dc voltage, in magnitude equal to the voltage
Fig. 3
breakdown was applied to the circuit: the zener diode under study—resistor \(R_1\). As measurements carried out with an S1-19 oscilloscope showed, the noise of zener diodes has a white spectrum, and its amplitude is not constant. Thus, for example, for type D-810 zener diodes, noise appears at currents through the zener diode on the order of \(35\text{–}40\,\mu\text{A}\) and reaches its maximum value at currents of \(120\text{–}130\,\mu\text{A}\); then the noise amplitude decreases. At currents through the zener diode of several milliamperes, noise is practically not observed. It was noted that the noise amplitude of zener diodes decreases as the breakdown voltage decreases.
Conclusions
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Analyzing the experimental results, the following may be said: the sharpest bend in the reverse characteristics is observed in type D-810 and D-811 zener diodes.
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The dynamic resistance and breakdown voltage of the transition regions of the current–voltage characteristics of temperature-compensated zener diodes vary with temperature.
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When using zener diodes in measuring circuits as amplitude limiters, their intrinsic noise should be taken into account.
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Selecting zener diodes by breakdown voltage does not give an unambiguous answer as to the equality of their dynamic resistances.
Literature
- Ya. A. Fedotov. Fundamentals of the Physics of Semiconductor Devices. “Sovetskoe Radio,” 1963, p. 148.