Reverse Currents of Isolated Junctions as an Indicator of the Stability of Low-Power Transistors
Abstract Generated abstract
This paper examines reverse currents in isolated collector and emitter junctions as a criterion for the quality and operational stability of low-power transistors used in threshold switching devices. It distinguishes bulk junction current from surface leakage components caused by channel and ionic conductivity, then evaluates current-voltage characteristics for several transistor types, with particular attention to MP104 devices before and after electrical conditioning and temperature exposure. The results indicate that voltage-dependent reverse currents, especially linear characteristics associated with ionic leakage, can shunt junctions, alter circuit operating modes, and contribute to instability of switching levels. The paper recommends excluding such transistors from threshold circuits when possible and specifying reverse-current behavior at normal and maximum temperatures as a screening requirement.
Full Text
REVERSE CURRENTS OF ISOLATED JUNCTIONS AS AN INDICATOR OF THE STABILITY OF LOW-POWER TRANSISTORS
V. A. DOBRUSKIN
(Presented by the scientific and technical seminar of the Research Institute of AEM)
The stability of operation of transistor threshold devices is determined to a significant degree by the stability of the properties of the transistors. The criterion of the quality and stability of the properties of a transistor is the reverse currents of the isolated collector and emitter junctions \(I_{\mathrm{ko}}\) and \(I_{\mathrm{eo}}\).
The idealized equation of the current-voltage characteristic (I–V characteristic) of a p–n junction has the form
\[ I = I_0 \left(e^{U/\varphi} - 1\right), \tag{1} \]
where \(U\) is the voltage across the junction,
\(\varphi\) is the temperature potential \((\varphi = 0.025\ \mathrm{V}\) at a temperature of \(300^\circ\mathrm{K})\).
It follows from (1) that the current through a reverse-biased junction reaches the value \(I_0\) already at \(U = 0.1\text{–}0.2\ \mathrm{V}\). With a further increase in voltage, this current is equal to \(I_0\) and does not depend on the magnitude of the voltage. The current determined from the I–V characteristic is the bulk (temperature) current of the junction.
In real transistors, in addition to the bulk component, there exists a surface component of the reverse junction current (leakage current \(I_y\)). The increase in the reverse junction current due to surface conductivity is of a channel or ionic nature.
In the case of channel conductivity, moisture and impurities create space charges on the surface of the base, which in effect increase the junction area. An additional channel is formed, through which the current of minority carriers \(I_0\) increases. The I–V characteristic of the channel component has the form [1]
\[ I_{\mathrm{ky}} = \sqrt{\frac{2 I_0}{s_{\mathrm{v}}}\ln 2 \cdot U}, \tag{2} \]
where \(I_{\mathrm{ky}}\) is the channel component of the leakage current,
\(s_{\mathrm{v}}\) is a constant depending on humidity.
Like the temperature current, \(I_{\mathrm{ky}}\) depends on the polarity of the voltage across the junction. \(I_{\mathrm{ky}}\) makes the reverse current dependent on the magnitude of the voltage across the junction.
In the case of ionic conductivity, moisture and impurities create leakage along the surface, whose resistance becomes comparable with the resistance of a reverse-biased, and sometimes even a forward-biased, junction. Ionic—
component \(I_y\) is equivalent to the current through a resistance shunting the junction. In this case \(I_y\) is proportional to the voltage across the junction and does not depend on its polarity.
To clarify the reasons for the differences in the stability of the switching levels of semiconductor relays using different types of transistors, the current–voltage characteristics of the reverse-biased junction of transistors MP116B, P416B, 1T321V, and MP104 were determined experimentally. The current–voltage characteristics of the most typical transistor specimens are given in Table 1.
Table 1
| Transistor type | \(I_{\mathrm{eo}}\), μA, at \(U_{\mathrm{eb,rev}}\), V | \(I_{\mathrm{eo}}\), μA, at \(U_{\mathrm{eb,rev}}\), V | \(I_{\mathrm{eo}}\), μA, at \(U_{\mathrm{eb,rev}}\), V | \(I_{\mathrm{eo}}\), μA, at \(U_{\mathrm{eb,rev}}\), V | \(I_{\mathrm{eo}}\), μA, at \(U_{\mathrm{eb,rev}}\), V |
|---|---|---|---|---|---|
| 0.25 | 0.5 | 1.0 | 1.5 | 5.0 | |
| MP116B | 2.00 | 2.00 | 2.00 | 2.00 | 2.02 |
| P416B | 0.08 | 0.09 | 0.11 | 0.14 | — |
| 1T321V | 2.80 | 2.90 | 3.12 | 3.25 | — |
| MP104 after storage | 0 | 0 | 0 | 0 | 0 |
The greatest instability during continuous operation of the switching levels was found in relays using MP104 transistors. To study the reasons, the latter were subjected to current conditioning \((I_k = 18\ \text{mA},\ I_b = 2\ \text{mA}\) for \(20\) h).
Figure 1 gives the current–voltage characteristics of those MP104 transistors in which, during conditioning, \(I_{\mathrm{eo}}\) increased to \(2\)–\(6\ \mu\text{A}\). In them \(I_{\mathrm{eo}}\) reaches a steady value at \(4\)–\(6\) V, which indicates the presence of leakage currents many times exceeding the thermal current of the junction. The nonlinear dependence of \(I_{\mathrm{eo}}\) on \(U_{\mathrm{eb,rev}}\) indicates
Fig. 1
Fig. 2
the presence in them of a channel component of the leakage current.
Figure 2 gives the current–voltage characteristics of those MP104 transistors in which, during conditioning, \(I_{\mathrm{eo}}\) increased sharply. In them \(I_{\mathrm{eo}}\) changes over the entire range of voltage variation. The current–voltage characteristic, linear over a wide voltage range, indicates the presence of a considerable ionic component of the leakage current.
The influence of the preceding state of MP104 transistors on the current-voltage characteristic of the emitter junction is illustrated by the test results for one of the MP104 transistors (Table 2).
Table 2
| Sequential states of the transistors in which measurements were made | $I_{\text{eo}}$, μA, at $V_{\text{eb, rev}}$, V | $I_{\text{eo}}$, μA, at $V_{\text{eb, rev}}$, V | $I_{\text{eo}}$, μA, at $V_{\text{eb, rev}}$, V | $I_{\text{eo}}$, μA, at $V_{\text{eb, rev}}$, V | $I_{\text{eo}}$, μA, at $V_{\text{eb, rev}}$, V |
|---|---|---|---|---|---|
| 0.5 | 1.0 | 2.0 | 5.0 | 10.0 | |
| After conditioning at +20°C | 13 | 45 | 88 | 164 | 245 |
| Temperature +60°C | 10 | 36 | 62 | 126 | 183 |
| Temperature +100°C | 10 | 28 | 50 | 87 | 108 |
| Temperature +120°C | 2.5 | 6 | 10 | 11 | 13 |
| Cooling to +20°C | 1 | 1 | 1 | 1 | 1 |
| Repeated conditioning at +20°C | 11 | 41 | 75 | 145 | 210 |
In the tested transistor specimens, the temperature dependence of $I_{\text{eo}}$ typical of semiconductors is absent, although it is an indicator of transistor quality. After repeated conditioning, the values of $I_{\text{eo}}$ in transistors, as a rule, are not reproduced.
Similar results were also obtained for $I_{\text{co}}$.
Conclusions
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In designing circuits of threshold devices, it is necessary to avoid using transistors with elevated reverse currents of isolated junctions, dependent on voltage (at $U \geq 0.2$–$0.5$ V).
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When such transistors are used, it is necessary to take into account the shunting action of the ionic component of the leakage current, which substantially changes the operating mode of the circuit and is a source of instability in the parameters of the device.
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A reliable guarantee of the absence of the ionic component of the leakage current is the specification, in the technical requirements for transistors, of the reverse currents of isolated junctions (or of one of them) under normal conditions and at the maximum ambient temperature. Moreover, the reverse currents of the junctions under normal conditions must not differ substantially from the values calculated from the currents at the maximum temperature, taking into account the theoretical temperature dependence of the junction reverse current (a twofold decrease in $I_0$ for every 9–12°C for germanium transistors and for every 6–8.5°C for silicon transistors).
LITERATURE
I. F. Nikolaevskii. Operating Parameters and Features of the Use of Transistors. Svyazizdat, 1963.