Transistor ratings and packages

Chapter 4.15 Transistor ratings and packages

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 4.15. TRANSISTOR RATINGS AND PACKAGES2714.15Transistor ratings and packagesLike all electrical and electronic components, transistors are limited in the amounts of volt-age and current each one can handle without sustaining damage. Since transistors are morecomplex than some of the other components you’re used to seeing at this point, these tend tohave more kinds of ratings. What follows is an itemized description of some typical transistorratings.Power dissipation: When a transistor conducts current between collector and emitter, it alsodrops voltage between those two points. At any given time, the power dissipated by a transis-tor is equal to the product (multiplication) of collector current and collector-emitter voltage.Just like resistors, transistors are rated for how many watts each can safely dissipate with-out sustaining damage. High temperature is the mortal enemy of all semiconductor devices,and bipolar transistors tend to be more susceptible to thermal damage than most. Power rat-ings are always referenced to the temperature of ambient (surrounding) air. When transistorsare to be used in hotter environments (>25o, their power ratings must be deratedto avoid ashortened service life.Reverse voltages: As with diodes, bipolar transistors are rated for maximum allowablereverse-bias voltage across their PN junctions. This includes voltage ratings for the emitter-base junction VEB , collector-base junction VCB , and also from collector to emitter VCE .VEB , the maximum reverse voltage from emitter to base is approximately 7 V for somesmall signal transistors. Some circuit designers use discrete BJTs as 7 V zener diodes witha series current limiting resistor. Transistor inputs to analog integrated circuits also have aVEB rating, which if exceeded will cause damage, no zenering of the inputs is allowed.The rating for maximum collector-emitter voltage VCE can be thought of as the maximumvoltage it can withstand while in full-cutoff mode (no base current). This rating is of particularimportance when using a bipolar transistor as a switch. A typical value for a small signal tran-sistor is 60 to 80 V. In power transistors, this could range to 1000 V, for example, a horizontaldeflection transistor in a cathode ray tube display.Collector current: A maximum value for collector current IC will be given by the manufac-turer in amps. Typical values for small signal transistors are 10s to 100s of mA, 10s of A forpower transistors. Understand that this maximum figure assumes a saturated state (mini-mum collector-emitter voltage drop). If the transistor is notsaturated, and in fact is droppingsubstantial voltage between collector and emitter, the maximum power dissipation rating willprobably be exceeded before the maximum collector current rating. Just something to keep inmind when designing a transistor circuit!Saturation voltages: Ideally, a saturated transistor acts as a closed switch contact betweencollector and emitter, dropping zero voltage at full collector current. In reality this is nevertrue. Manufacturers will specify the maximum voltage drop of a transistor at saturation, bothbetween the collector and emitter, and also between base and emitter (forward voltage dropof that PN junction). Collector-emitter voltage drop at saturation is generally expected to be0.3 volts or less, but this figure is of course dependent on the specific type of transistor. Lowvoltage transistors, low VCE , show lower saturation voltages. The saturation voltage is alsolower for higher base drive current.Base-emitter forward voltage drop, kVBE , is similar to that of an equivalent diode, ∼=0.7 V,which should come as no surprise.Beta: The ratio of collector current to base current, β is the fundamental parameter char-