# What are the electrical characteristics of bipolar junction transistors (BJTs) ?

Each item (cut-off current, current gain, saturation voltage, transition frequency, collector output capacitance, noise figure) described in the electrical characteristics of the data sheet is explained. For the measurement methods of items 1 to 4, please refer to FAQ: Example of measurement of the key bipolar transistor characteristics.
Table. 1 shows an example of a data sheet description.

1. Collector cut-off current ICBO: The current that flows through the collector when a voltage is applied between the collector and the base under the specified measurement conditions with the emitter open. It is a measure of the maximum value of the current that flows through the collector in the cut-off state. At higher temperatures it will be higher.
2. Emitter cut-off current IEBO: Current flowing through the emitter when a voltage is applied between the emitter and base under the specified measurement conditions with the collector open. It is a guideline for the maximum value of the current that flows when the base and emitter are reverse-biased in a cut-off state. At higher temperatures it will be higher.
3. DC current gain hFE: Under specified conditions, the ratio of the collector current to the base current when the emitter is grounded.
DC current gain = Collector current / Base current
4. Collector-emitter saturation voltage VCE(sat): The collector-emitter voltage under the specified measurement conditions at which the transistor saturates.
5. Base-emitter saturation voltage VBE(sat): The voltage between the base and emitter under the specified conditions at which the transistor saturates.
6. Transition frequency fT: Frequency at which the current gain is 1 (=0 dB) with the emitter grounded. Measure the AC current gain at a high frequency such as 1 MHz and calculate it.
7. Collector output capacitance Cob: Collector-base capacitance value measured at specified collector-base voltage and frequency with emitter open.
8. Noise figure NF: The ratio of the SN ratio of the signal input to the device and the SN ratio of the signal output from the device. It is defined by the following formula.
NF = 10 × log [ (S N) in / (S N) out ]2

Depending on the product, there may be other electrical characteristics listed. The following application notes have explanations for other items. please refer.
The electrical characteristics and equivalent circuit:Bipolar Transistor Application Notes
See the application note below for details on absolute maximum ratings.
The maximum ratings:Bipolar Transistor Application Notes