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The dc or pulse-current breakdown voltages typically specified on data sheets are:
- BVCEO(SUS) -- This is the collector-to-emitter sustaining voltage at a specified current level when the transistor is operated in avalanche with the base lead open. At high current levels, it is called LVCEO. BVCEO is the highest open-base voltage the device has in the avalanche region (the initial point). This is a guaranteed level to which the device will sustain a collector avalanche current without secondary breakdown or complete collapse of the device voltage. It is always measured in a pulse condition. Sometimes a specified quantity of energy is applied by pulsing the transistor from an inductance which has a predetermined amount of stored energy.
- BVCER(SUS) -- This is the breakdown voltage from the collector to the emitter, with a resistor connected from the base to the emitter. The measurement is made as for BVCEO(SUS). A more common designation is LVCER, or LVCES, when RBE = 0.
- BVCBO -- This is the breakdown voltage from the collector to the base and thus is the avalanche voltage of the collector-base junction. It is an important parameter because both rf parameters and circuit operation parameters correlate to this breakdown voltage. A fairly broad range of collector-base breakdown voltage is possible for a given process depending on the resistivity variation of the silicon material used.
Typical breakdown-voltage curves for a low-resistivity thin expitaxial transistor and for a high-resistivity and thick-epitaxial device are shown in Fig. 10. Note that significantly different curve shapes are obtained by different transistor designs. The VSWR capabilities of the transistor are dependent on the sustaining region and these breakdown voltages.
Fig. 10. Breakdown voltage characteristics—typical transistors. Curves at left are for a low-resistivity thin epitaxial layer; at right, for a high-resistivity relatively thick epitaxial device.
- BVEBO -- This is the emitter-base breakdown voltage and is also specified at a given current level in the avalanche breakdown region. It is of secondary concern to the circuit designer where the transistor is to be operated at very high frequencies, because the stored charged rather than the external field controls the field in the emitter-base junction over most of the cycle. However, at low frequencies, this parameter is important and should be taken into consideration.
Output capacitance, common-base (Cob)
Cob is an important parameter because it affects the circuit tuning and the output-impedance level of the transistor. It also relates to some of the other dc parameters. In a common-emitter circuit, Cob is essentially the output capacitance, too. This is because the impedance levels at the base are quite low relative to the impedance level at the transistor output. However, the high-frequency value must definitely be considered and also the large-signal value (which can be as much as twice the small signal value).
The output capacitance of a transistor represents effectively its junction capacitance in series with a resistance. If the collector resistivity is increased, the effective output capacitance is decreased as seen from the external terminals. Also, if the resistivity is increased, the collector-base breakdown voltage, BVCBO, is also increased. Junction and epitaxial-thickness variation will cause some variation in output capacitance, too. A typical distribution of COB vs. collector-base breakdown voltage is shown in Fig. 11. COB will also vary with collector voltage, which is an important consideration in large-signal operation. A typical variation in a 400-Mc transistor is shown in Fig. 12.
Fig. 11. Typical Cob-BVCBO distribution. This was obtained on a type ITT 3TE440 transistor at f ≈ 1 Mc and VCB = 28 V.
Fig. 12. Typical Cob-Vcb curve for a 400-Mc transistor, type ITT 3TE440. This variation is important in large-signal operation.
When transistors are operated in the microwave region (above 300 Mc) low output capacitance is important for good gain and high circuit impedance. Based on the “active area” concept previously discussed, it is necessary to build a finer geometric or change the material resistivity to effectively reduce the ratio of output capacitance to power capability. Changing material resistivity, however, does reduce power-output capability somewhat.
Collector-emitter saturation voltage (VCE(SAT))
VCE(SAT) is an important parameter which is always specified at dc, and which is a very misused parameter for an rf power transistor and somewhat misunderstood. Saturation voltage is specified at given collector and base currents, thus at a forced hFE (typically at the lowest guaranteed hFE, which is usually around 8 or 10).
VCE(SAT) correlates to the collector-base breakdown voltage; i.e., the resistivity of the collector epitaxial material in the transistor. Thus, a higher breakdown-voltage device has a higher saturation-voltage level at dc, and also at rf. In large-signal rf power circuits, the transistor is driven all the way from collector saturation to cutoff. Thus, the saturation level determines the extent of the voltage swing. The relationship between dc saturation and rf saturation is partly controlled by the geometry of the device because it affects the same area.
Thermal resistance (RT)
RT is an important parameter which, along with maximum-rated power dissipation, creates the actual dissipation level limits of the power devices. It is a very misunderstood rating and requires a significant understanding of high-frequency geometries and safe operating areas. In itself, thermal resistance only gives capability, provided the transistor is operated at certain collector voltages and currents as they are translated to junction temperatures and voltage fields.