Like all electrical and electronic components, transistors are each subject to voltage and current limitations before they can be damaged. Since transistors are more complex than some other components you're used to seeing at this point, these components tend to have a wider variety of ratings. Below is a blow-by-blow explanation of some typical transistor ratings.

Power consumption

When a transistor conducts current between its collector and emitter, it also reduces the voltage between these two points. At any given time, the power dissipated by a transistor is equal to the product of the collector current and the collector-emitter voltage. Just like resistors, a transistor's power rating is the amount of power each transistor can safely dissipate without suffering damage.

Heat is the enemy of all semiconductor devices, and bipolar transistors are more susceptible to thermal damage than most transistors. Power ratings are always based on the temperature of the surrounding air. When transistors are used in high-temperature environments above 25°C, their power ratings must be derated to avoid shortened service life.

Reverse voltage

Just like diodes, bipolar transistors are rated for the maximum allowable reverse bias voltage across the PN junction. This includes the rated voltage EB of the emitter-base junction V EB the collector-base junction V civil band, and from the collector to the emitter.

VEB For some small signal transistors, the maximum reverse voltage from emitter to base is about 7V. Some circuit designers use discrete BJTs as 7V Zener diodes with a series current limiting resistor. Transistor inputs to analog integrated circuits also have a VEB rating, exceeding this value will cause damage and do not allow zenering of the input.

The rating of the maximum collector-emitter voltage V Church of England can be considered to be the maximum voltage it can withstand in cutoff mode (no base current). This rating is especially important when using bipolar transistors as switches. Typical values for small signal transistors are 60 to 80V. In power transistors, this value can reach 1000V, such as the horizontal deflection transistors in cathode ray tube displays.

collector current

The manufacturer will give the maximum value of the collector current IC in amperes. Typical values are tens to hundreds of mA for small signal transistors and tens of A for power transistors. Be aware that this maximum value assumes saturation (minimum collector-emitter voltage drop). If the transistor is unsaturated and the voltage between collector and emitter drops significantly, the maximum power dissipation rating may be exceeded before the maximum collector current rating.

saturation voltage

Ideally, a saturating transistor acts as a closed switching contact between the collector and emitter, dropping zero voltage at full collector current. In fact, this is never true. The manufacturer will specify the maximum voltage drop across the transistor in saturation, including the voltage drop between collector and emitter and between base and emitter (forward voltage drop across the PN junction).

The collector-emitter voltage drop at saturation is generally expected to be 0.3 volts or less, but this number of course depends on the specific type of transistor. Low voltage transistors, low voltage Anglicans exhibit lower saturation voltages. For higher base drive current, the saturation voltage is also lower. It is not surprising that the base-emitter forward voltage drop, V exists similar to the equivalent diode ≅ 0.7V.

Beta

The ratio of collector current to base current, β, is a basic parameter characterizing the amplification capability of a bipolar transistor. In circuit calculations, it is often assumed that β is a constant, but unfortunately, this is far from the case. Therefore, manufacturers provide a set of beta (or "h (UK) CME") values for a given transistor under various operating conditions, usually expressed in the form of maximum/minimum/typical ratings.

How much does β vary within normal operating limits? The 2N3903 is a popular small-signal transistor advertised as having a beta range of 15 to 150, depending on the amount of collector current. In general, β is highest at moderate collector currents and decreases at very low and very high collector currents. h (UK) Continuing Education is small signal AC gain; hFE is large AC signal gain or DC gain.

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The ratio of collector current to emitter current, α=IC/IE. α can be derived from β, that is, α=β/(β+1). Bipolar transistors come in a variety of physical packages. Package type depends primarily on the power dissipation required of the transistor, much like a resistor: the greater the maximum power dissipation, the larger the device must be to keep cool.

In addition to bipolar transistors, which have three connection points, there are many other semiconductor devices. The pinout of plastic transistors can vary within the same package type, such as TO-92. It is not possible to unambiguously identify a three-terminal semiconductor device without referring to the part number printed on it, or subjecting it to a series of electrical tests.

Small plastic transistor packages like the TO-92 can dissipate hundreds of milliwatts of power. TO-18 and TO-39 metal cans can dissipate more energy, hundreds of milliwatts. Plastic power transistor packages such as TO-220 and TO-247 consume well over 100 watts and are close to the power consumption of all-metal TO-3. Most power transistors are rated at half the listed power or less. For actual ratings, refer to the specific device data sheet. Semiconductor chips in TO-220 and TO-247 plastic packages are mounted on a thermally conductive metal block that transfers heat from the back of the package to a metal heat sink, not shown.

Before mounting the transistor to the heat sink, apply a thin layer of thermal grease to the metal. Since the TO-220 and TO-247 moldings and TO-3 housing are connected to the collector, it is sometimes necessary to electrically insulate them from the grounded heat sink by inserting mica or polymer washers. Datasheet ratings for power packs are only valid when mounted to a heat sink. Without a heatsink, the TO-220's safe power dissipation in free air is about 1 watt.

The maximum power consumption ratings in the data sheet are difficult to achieve in practice. Maximum power dissipation is based on the heatsink keeping the transistor case at a temperature not exceeding 25°C, which is difficult with air-cooled heatsinks. Allowed power dissipation decreases with increasing temperature. This is called derating.

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