Transistor Specification and Parameter Definitions

- details and list of the basic transistor parameter specifications with definitions of the different parameters used in transistor datasheets.

Transistors have been widely available for over fifty years. As a result a huge variety of transistors is available. Choosing new transistors, or transistor replacements for existing circuits, it is necessary to look at the transistor parameters in the transistor specification sheets to assess the performance of the transistor.

To simplify the use of the transistor specification sheets or datasheets standard parameters are nearly always used. These enable the performance of different transistors to be compared. They also enable the transistor parameters to be defined in a way that can be easily understood.The transistor specification sheets may be available on the Internet, or they may be found in transistor manuals issued by the manufacturers over the years. Today, most of the transistor data is available on the manufacturers' Internet sites as paper based data books are expensive to produce and not as easy to disseminate.

When choosing a transistor using the transistor specification or data sheets, it is necessary to be able to understand what the different transistor specifications mean.

Transistor specification parameters

There are a number of standard parameters with abbreviations that are used to define the performance of a transistor. The definitions of these parameters are outlined in the table below:

Parameter Definition and description
Type number The type number of the device is an individual part number given to the device. Device numbers normally conform to the JEDEC (American) or Pro-Electron (European) numbering systems - see Related Articles under main left hand menu block. There is also a Japanese standard system for transistor numbering.
Case Case style - a variety of case standard case styles are available. These normally are of the form TOxx leaded devices and SOTxxx for surface mount devices. Note it is also important to check the pin connections as they are not always standard. Some transistor types may have their connections in the format EBC whereas occasionally they can be ECB, and this can cause confusion in some cases.
Material The material used for the device is important as it affects the junction forward bias and other characteristics. The most common materials used for bipolar transistors are silicon and germanium.
Polarity The polarity of the device is important. It defines the polarity of the biasing and operation of the device. The two types are NPN and PNP. NPN is the most common type. It has the higher speeds as electrons are the majority carriers and these have a greater mobility than holes. When run in common emitter configurations, the NPN circuits will use a positive rail voltage and negative common line, PNP transistors will require a negative rail and positive common voltage.
VCEO Collector emitter voltage with base open circuit
VCBO Collector base voltage with the emitter open circuit
VEBO Emitter base voltage with collector open circuit
IC Collector current
ICM Peak collector current
IBM Peak base current
PTOT Total power dissipation - this is normally for an ambient temperature of 25C. It is the maximum value of power that can safely be dissipated for that transistor with its stated package.
Tj Junction temperature - care must be taken to ensure that this figure is not exceeded otherwise the device could be damaged or long term reliability affected. Dissipation / temperature curves are often provided to facilitate calculations.
Tamb Ambient temperature
Tstg Storage temperature. This is the temperature range over which the device may be stored. Outside this range, damage may occur to the materials used in the device. The operating temperature range is normally well within the bounds of the storage temperature range.
ICBO Collector base cut-off current
IEBO Emitter base cut-off current
hFE Forward current gain
VCEsat Collector emitter saturation voltage
VBEsat Base emitter saturation voltage
Cc Collector capacitance
Ce Emitter capacitance
Ft Frequency Transition - the frequency where common emitter current gain falls to unity, i.e. the gain bandwidth product for the transistor. It is normally measured in MHz. The operating frequency of the transistor should normally be well below the transition frequency.

When using the transistor specifications and parameters, it is worth remembering that the "Absolute Maximum Ratings" detailed in a transistor datasheet are the values that if they are exceeded may result in the failure of the transistor. Maximum ratings usually include collector-to-base voltage, emitter-to-base voltage, collector current, emitter current, and collector power dissipation.

By Ian Poole

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