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
Thermal effects, resistor stabilization and trade-offs
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The concept of resistor stabilization was introduced over two years ago for the express purpose of making a large-area hf silicon transistor operate uniformly and maintain a safe operating area. The concept that a high-power, hf transistor is nothing more than many small-signal transistors in parallel is a good premise to begin from. If this is true, and if differences exist in thermal dissipation of the device area plus differences in base width (and thus gain of some small transistors), a significant non-uniform distribution of current and power density over the large area of the high-power device exists. This is accentuated by the fact that a hf device has a high density of small transistors in parallel.
It is a well-known fact that if a bipolar transistor is heated up its gain will increase. Thus, the chip area that carries the most current will tend to heat up more than the surrounding areas and carry more of the load until it will blow itself out in that small area or “hot spot.” An obvious and direct method to equalize the current uniformity and power density is to put a small emitter resistor in series with each transistor. Such a large-area “resistor-stabilized” device operates more uniformly.
An increasing number of resistor-stabilized transistors are being introduced by semiconductor manufacturers. Thus, it is important to understand the trade-offs involved between resistor stabilization, thermal capability and operating characteristics of such devices. Since resistor stabilization is a form of emitter degeneration, power gain is degenerated as well. This is one of the more important trade-offs between thermal capability, safe-operating area and power gain. The higher the resistor stabilization, the lower the power gain. Thus, for a given power density and area, it sometimes is necessary to put a higher-gain device in for the same amount of resistor stabilization to maintain a useful over-all power gain. The more resistor stabilization is introduced, the higher the VCE(SAT) will be and the lower the saturated power output. This is a trade-off which is controlled by the manufacturer rather than the circuit designer, but it is an important one for the circuit designer to understand.
If device thermal resistances are compared, it is found that for a similar device type those with lower resistor stabilization will have a higher thermal resistance and less thermal dissipation capability. The significant trade-off here is one which relates power gain to saturated power output and VSWR capability (improved by resistor stabilization). Future devices will have improved forms of resistor stabilization structures built in, making these trade-offs less critical than at present.
Another plus feature of resistor stabilization is improved dc bias capability. This is combined with improved intermodulation distortion in single-sideband applications.
Thermal resistance and how to measure it
Contrary to the normal circuit-design concept, there is no such thing as a fixed thermal resistance for a transistor. Thermal resistance is normally defined as the temperature rise between the junction and the case per watt of dissipation. Because current uniformity (and thus power density) is a function of the current level as well as the collector supply voltage (or thus the power level in the transistor), one would expect thermal resistance to be a nonlinear function of these circuit parameters. Typically, current distribution is more uniform at low-current levels and is also more uniform at lower voltages. Thus, for a given transistor type, the low-voltage, low-current area will give the lowest value of thermal resistance. It is also to be expected that where one small area of the device tends to go into thermal runaway (i.e., has a negative slope of the base-emitter voltage vs collector current for a fixed collector voltage), that runaway will cause current from one area to suddenly shift to the area which is trying to carry more of the load. At this point, the active area of the transistor is effectively reduced resulting in high thermal resistance. This shows up as a sudden shift in a thermal resistance plot, as shown in Fig. 17. This plot of thermal resistance vs collector current as a function of dc collector-emitter voltage, is for a microwave transistor, type 2N3375. It substantiates that thermal resistance is a nonlinear function of these parameters, is much lower at low-current, low-voltage levels, and has discontinuities associated with sudden shifting of current and power density in the device. These effects, which are measured at dc, correlate reasonably well with the rf-power capabilities as well. Thus, it is important that the circuit designer be aware that thermal resistance is a function of the measurement conditions under which it is specified. Unfortunately, the conditions are very seldom specified on data sheet.
Fig. 17. How thermal resistance varies with collector current. The effect is nonlinear and much lower at low-current, low-voltage levels.
Because thermal resistance is a function of VCE and IC, it would be expected that the safe operating area under pulsed conditions would be a function of these parameters as well as the thermal time-constant of the device itself. If the safe-operating area under pulsed conditions is defined as that point at which some part of the junction area of the transistor reaches 200° C and pulsed measurements are then taken, safe operating curves are shown in Fig. 18, result.
Fig. 18. Safe-operating pulse conditions for a typical microwave transistor. These curves are based on the transistor junction area not exceeding 200° C.
VSWR capability, or the ability of a transistor to withstand a high VSWR load (i.e., approaching or equal to an open- or short-circuit condition) is an important consideration and correlates to some of the other transistor parameters. High-VSWR capability can relate closely to a transistor’s dissipation capability. Through part of the VSWR phase angle, the transistor must dissipate more power than in matched condition. Thus, a resistor-stabilized device, or one with a higher safe-operating area, can withstand this VSWR load better. The other trade-offs to obtain this capability are already apparent. Also, a device with a lower saturated-power output tends to limit its peak current more. Thus its total power dissipation is somewhat less. This is another direct trade-off between VSWR capability and the device itself. In the other phase of VSWR, a high-voltage, low-current condition exists on the collector. Here, a high-voltage break-down requirement and a sustained-high-current capability in avalanche are important. The trade-offs of high-voltage capability have already been discussed. However, high-avalanche sustaining-current capability has been difficult to relate directly to other basic device parameters that a circuit designer measures. It must be designed into the transistor.
Fig. 19. Basic VSWR test circuit. Using this test setup one can measure the variation in transistor power dissipating, power output and collector current as a function of phase length between output circuit and load.
A typical variation in transistor power dissipation, power output, and collector current as a function of phase length between the transistor-output circuit and the load can be measured as shown in Fig. 19, with results as shown in Fig. 20 when adjusted for line losses. These curves illustrate how the phase between the output circuit and the VSWR load is important in determining what is happening at the transistor.
Fig. 20. Test results from setup of Fig. 19 show how the phase between output circuit and load is important in determining events at the transistor itself.