Transistor ratings and packages
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Like all electrical and electronic
components, transistors are limited in the amounts of voltage and
current they can handle without sustaining damage. Since transistors are
a bit more complex than some of the other components you're used to
seeing at this point, they tend to have more kinds of ratings. What
follows is an itemized description of some typical transistor ratings.
Power dissipation:
When a transistor conducts current between collector and emitter, it
also drops voltage between those two points. At any given time, the
power dissipated by a transistor is equal to the product
(multiplication) of collector current and collector-emitter voltage.
Just like resistors, transistors are rated in terms of how many watts
they can safely dissipate without 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 ratings
are always given in reference to the temperature of ambient
(surrounding) air. When transistors are to be used in hotter-than-normal
environments, their power ratings must be derated to avoid a
shortened service life.
Reverse voltages:
As with diodes, bipolar transistors are rated for maximum allowable
reverse-bias voltage across their PN junctions. This includes voltage
ratings for the base-emitter junction, base-collector junction, and also
from collector to emitter. The rating for maximum collector-emitter
voltage can be thought of in terms of the maximum voltage it can
withstand while in full-cutoff mode (no base current). This rating is of
particular importance when using a bipolar transistor as a switch.
Collector current:
A maximum value for collector current will be given by the manufacturer
in amps. Understand that this maximum figure assumes a saturated state
(minimum collector-emitter voltage drop). If the transistor is not
saturated, and in fact is dropping substantial voltage between collector
and emitter, the maximum power dissipation rating will probably be
exceeded before the maximum collector current rating will. Just
something to keep in mind when designing a transistor circuit!
Saturation voltages:
Ideally, a saturated transistor acts as a closed switch contact between
collector and emitter, dropping zero voltage at full collector current.
In reality this is never true. Manufacturers will specify the
maximum voltage drop of a transistor at saturation, both between the
collector and emitter, and also between base and emitter (forward
voltage drop of that PN junction). Collector-emitter voltage drop at
saturation is generally expected to be 0.3 volts or less, but this
figure is of course dependent on the specific type of transistor.
Base-emitter forward voltage drop is very similar to that of an
equivalent diode, which should come as no surprise.
Beta:
The ratio of collector current to base current, β is the fundamental
parameter characterizing the amplifying ability of a bipolar transistor.
β is usually assumed to be a constant figure in circuit calculations,
but unfortunately this is far from true in practice. As such,
manufacturers provide a set of β (or "hfe") figures for a
given transistor over a wide range of operating conditions, usually in
the form of maximum/minimum/typical ratings. It may surprise you to see
just how widely β can be expected to vary within normal operating
limits. One popular small-signal transistor, the 2N3903, is advertised
as having a β ranging from 15 to 150 depending on the amount of
collector current. Generally, β is highest for medium collector
currents, decreasing for very low and very high collector currents.
Alpha:
the ratio of collector current to emitter current, α may be derived from
β, being equal to β/(β+1).
Bipolar transistors come in a wide
variety of physical packages. Package type is primarily dependent upon
the power dissipation of the transistor, much like resistors: the
greater the maximum power dissipation, the larger the device has to be
to stay cool. There are several standardized package types for
three-terminal semiconductor devices, any of which may be used to house
a bipolar transistor. This is an important fact to consider: there are
many other semiconductor devices other than bipolar transistors which
have three connection points. It is impossible to positively
identify a three-terminal semiconductor device without referencing the
part number printed on it, and/or subjecting it to a set of electrical
tests.
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