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Many currently-available transistors are more limited by their packaging than by the basic capability of the chip itself. This is especially so for high-frequency power devices which have relatively-low input and output impedances. The package impedances can easily be as large. Thus, a revolution is taking place in transistor packaging, especially for devices in the microwave-frequency ranges above 400 Mc.

Package inductances and resistive losses have significant effects on circuit performance—more specifically, on bandwidth, stability, power gain and phase delay. Bandwidth is important in many communications circuits, and wide bandwidth is harder to achieve with high-power transistors than for small-signal devices.

Fig. 7. Equivalent input circuit for common-emitter configuration, showing small-signal relationships between frequency, bandwidth, base resistance and input reactance.

A simple representation of a common-emitter equivalent transistor input circuit is shown in Fig. 7, demonstrating the small-signal relationships between bandwidth, base resistance and input reactance. The large-signal rb and Ci are different from the small-signal values, and an exact quantitative analysis is impossible. However, the large-signal bandwidth at the transistor input circuit will be considerably smaller than that predicted from small-signal parameter measurements. This is primarily because the effective large-signal base resistance, rb, is lower.

The emitter package inductance, Le, reduces power gain as shown in the following approximate equation.

       

Le also reflects an impedance into the input circuit of a common-emitter configuration (this is the most widely used configuration and the one of primary concern here). The input impedance given by small-signal analysis is (See Appendix A):

    

where re is the combination of transistor emitter resistance and any external emitter resistance.

Low-inductance packaging improves both bandwidth and stability and thus ease of power matching to the transistor. A lower-inductance lower-Q circuit means less change of phase vs tuning and a wider range of stability with respect to internal feedback. An improved low-inductance package, incorporating wide-ribbon leads for the collector and base and with the emitter connected to the case for low and consistent emitter inductance, is shown in Fig. 8. This is a new concept in high-frequency packaging for transistors above 300 Mc. Still further improvement can be expected in the future.

Fig. 8. Low-inductance transistor package for 300 Mc and above. Wide ribbon leads are used for the collector and base, resulting in better stability over a wider range.

Dc parameters

If one knows a transistor’s dc parameters, he can determine what to expect of the rf parameters and of circuit performance. It is very useful to the circuit designer to understand the relationships between dc and rf parameters.

The EIA requirements for registering an rf power transistor for a 2N number are quite loose, allowing a wide latitude in parameters. All manufacturers use as wide a latitude as they can; yet, to ship good products the parameters must be controlled more closely than typical EIA registration requires. Often the parameters of distribution is tightened merely by selection of units to meet the requirements of a specific application.

Common emitter hFE and beta

DC forward current transfer ratio, hFE, is a most important control parameter for a device process. Many different rf parameters as well as circuit performance, correlate directly to hFE. It is typically measured at a low voltage and under pulse conditions so that power dissipation has no effect on it. Typically, hFE increases with junction temperature.

Dc beta is usually specified at both a low-current and a high-current level. Usually a minimum and maximum is specified at low-current levels; a minimum value is certainly most necessary at high-current levels. This goes back to the concept of active area and the fact that at high-current levels the current pinch-off effect occurs. At high current levels, dc beta will decrease quite rapidly when the current density has reached a high level. Therefore, low hFE devices, which have a wider base width and lower lateral sheet resistance in the base structure under the emitter of a transistor, will have more linear or constant hFE vs collector current. A high hFE device will have more radical percentage variations in hFE vs collector current. And as hFE increases, the device will reach a peak at a lower current level.

Typical relationships between hFE and collector current for different levels of hFE are shown in Fig. 9 for two different 400 Mc transistors. The curves are more linear for low hFE devices and this should significantly affect the saturation level at high frequencies and the power-output and modulation linearity. Many of the basic rf parameters also correlate directly to the hFE level of a given transistor for a given process.

Fig. 9. Relationship of hFE to Ic for two transistors at different hFE levels. The 5-W unit is designated ITT 2N3375; the 15-W transistor, ITT 3TE440.