What is in this article?:
- Microwave Transistor-Parameter Trade-offs in Circuit Design: Part 3
- Output admittance (yoe) to hFE relationship
- Large-signal input impedance measurements
- Thermal effects, resistor stabilization and trade-offs
For a specific transistor process and family, a very strong relationship exists between the dc beta, hFE, and the high-frequency common-emitter current transfer ratio, hfe. This is primarily because of a basic difference between a high- and a low-beta transistor in a given process with fixed doping levels: The base width is narrower in a high-beta device; i.e., the emitter diffusion time is longer or the epitaxial thickness is less. A high-dc-beta device will also have a greater high-frequency current gain.
Fig. 1.Transistor-package circuit. This representation clearly shows that packaging has a significant effect on the performance of a transistor at microwave frequencies.
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The external hfe measurement is not necessarily the current gain of the internal device, since the measurement is made on the device in the package. A simplified model of the packaged transistor is shown in Fig. 1. Note that the capacitance distributed along the base resistance together with the other external capacitances will affect the short-circuit current gain of the device in the package. The package inductances and the resistance in the emitter structure will also affect the short-circuit current gain. Thus, the current gain of the package is not necessarily a good measure of the fT or hfe of the chip unless the reverse transfer admittance and other admittances are also measured and taken into account. This is an important consideration if the device is to be used for radiation resistance. Here a narrow base width is important rather than the effects of the package on the measurement of fT, which is used as an indication of base width.
Fig. 2. Package gain-bandwidth vs collector current for various dc betas. A high-beta transistor with a low narrow base will exhibit current crowding at low current levels.
A high-beta transistor with a narrow base will exhibit current crowding at low current levels. The relationship between current gain-bandwidth product and collector current for different hFE levels (for a resistor-stabilized device) is shown in Fig. 2. The current gain of the packaged device is considerably less than the current gain of the device itself. This is because of degeneration cause by the resistance introduced into the emitter structure. At a current level of about 1.1 A a sharp dropoff exists in hf current gain (as was true of dc current gain) and this is a result of current crowding. The maximum current rating of a transistor is specified in an area where the hf and lf current gains are still useful. This explains the 1.5-A max collector current rating for this transistor.
To keep within the most linear portion of the transistor characteristics, the average current within the device should be held no greater than 1 A. It can be seen that a low-hfe device has less percentage change in hf current gain-bandwidth product than a high-hfe device. This is also true for hFE (or dc beta). The other variables which affect hfe are primarily resistors introduced into the emitter circuits of resistor-stabilized transistors, and current distribution for equivalent betas. The transistors represented in Fig. 2 have been chosen for equivalent emitter resistance to eliminate this effect on measurement. Other important correlations to hFE further validate the concept of what is happening in the base under the emitter.