This is about electronics components, despite this off-topic intro. Last night I watched a Frontline episode on PBS about the drug resistant superbugs that are resistant to antibiotics. They talked about one of the genes that causes this drug resistance; it produces an enzyme called klebsiella pneumoniae carbapenemase or KPC.
I searched for KPC online and Wikipedia had it, so I clicked on it. It turned out that many things are abbreviated KPC and one of these is key product characteristics. This immediately brought to my mind the data sheets used by electronics manufacturers, especially semiconductors. Wikipedia does not have a wiki about this, or key product characteristics in general, and it asked if I wanted to create one. But I don’t believe I’m qualified to write about it in general. So I decided to write a blog about KPC as I think they should apply to semiconductor specifications, especially transistors.
Historical Background
The early days of transistors were fraught with problems that caused transistors to quit working properly and die. As with any new product, the early manufacturing was expensive. The devices were assembled and tested manually and were expensive before mass production was introduced. Most of the early purchases were made by the government, mainly military and NASA, for satellites and spacecraft. Reliability was of utmost importance, since these projects cost many millions of dollars. One component failure could terminate the whole mission. So early on every component had to be tested to meet the specifications given by the gov’t agency. The industry adopted uniform specifications so that components made by various manufacturers could be assured of compatibility. The “spec’s” were given in a datasheet.
Transistors
Semiconductors such as transistors are very small and can be damaged by excessive heat. Early germanium transistors were encapsulated in a plastic package, but much of the problem was that the package allowed air and moisture to get into the transistor chip, causing it to fail. So the packages were changed to a sealed metal can, hermetically sealed by welding. This greatly improved reliability.
Datasheet
The datasheet begins with the manufacturer’s name and product name, such as 2N4401. A single specification may have 1 or a few lines starting with the name of the specification and conditions, the abbreviation of the spec, a minimum and if applicable, a maximum value, and sometimes a typical value. The last spec is usually the measurement unit, such as volts or milliamps. The current gain is dimensionless.
Power
The first page of the transistor’s datasheet gives the specs for the power dissipation, or Pd. This depends on the size of the chip and package and if it is mounted on a heat sink. For small signal transistors this is usually below 1 watt, typically 625 milliwatts. For power transistors this is from a few watts to several tens of watts for a plastic TO-220 package. For the 2N3055, in the metal ‘diamond’ TO-3 package, it is 115 watts. But the power is limited by the heat sink, which must be able to dissipate that much power without getting excessively hot.
Maximum Collector Voltage
The maximum voltage from collector to emitter, with the base not connected, or Vceo, is the highest voltage that should be applied to the transistor’s collector. Any higher, and the transistor could break down and start conducting current, which could damage the transistor. The Vceo may be less than the other Vce or Vcb voltages.
Current Gain, Beta, hFE
An important specification is the current gain, AKA beta or hFE. The collector’s current is the base current multiplied by this number. Say the base current is 1/2 milliamp and the transistor’s current gain is 200. The collector current will then be 100 milliamps. The hFE depends on the current, the voltage and the temperature, among other things. These variables are given in graphs in the datasheet.
High Frequency Performance
This is usually given by the frequency at which the current gain drops to unity. It is abbreviated fT. A typical value for silicon small signal transistors is 200 MHz.
Vce(sat)
The Vce(sat) is the voltage from collector to emitter with the transistor forced into saturation. This is done by increasing the base current until it is typically 1/10 of the collector current. This is done at a constant collector current, given by the test. The lower the voltage, the less power is wasted in the transistor. Typical values for this may be 1/4 volt or less for a current of 1 amp or less. For the 2N3055 it is 1.1V at 4 amps.
Another transistor spec usually given when the transistor is used for switching is the turn-on, turn-off, and delay times. These are typically given in nanoseconds.
After the specifications, some data sheets give graphs of some of the measurements in relation to voltage, current, temperature, etc.
Another specification often given is a drafting printout of the package dimensions. If the package has a standard name such as TO-92 or similar, then it is the same or very similar to the package used by other manufacturers. The pinouts of transistors is different for different countries. For JEDEC (U.S.A.), the typical TO-92 pinout with the writing facing you is E B C. For Japanese transistors such as 2SC945 or marked C945, it is typically E C B. For Pro Electron, used in Europe, the typical pinout is C B E. But remember there may be exceptions to these, so always check the datasheet. Most data sheets are available online with a search for the part number and the word datasheet.