Quick Q&A
Question: Why are BJTs referred to as "Normally Off" Devices?


Bipolar junction transistors (BJTs) are referred to as "Normally Off" devices because when you connect a BJT to a circuit, the BJT does not conduct across the collector to emitter, unless a current is applied to the base.

In other words, the BJT does not conduct current without a current applied to the base. It is normally off, in the absence of any base current.

BJT Normally Off Device

This is in contrast to junction field effect transistors (JFETs), which are referred to as "normally on" devices. JFETs conduct current across the drain to source without any voltage applied to the gate of the transsitor. When sufficient voltage is applied to the gate terminal, this turns the JFET off.

A BJT has the opposite configuration in which a base current turns it on. And no base current turns it off.

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