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BJT VS MOSFET- Current controlled vs Voltage controlled

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I have read so many times that BJT is a current-controlled device: the value of the collector current depends on the amount of base current flowing. And MOSFET is a voltage-controlled device: the voltage value at the Gate of the MOSFET controls the Drain current.

But can someone explain this practically with the help of an actual circuit. What's the real difference?

1 Answer

Maybe this explanation will help:

BJT has three regions: emitter, base, and collector, with the emitter and collector, doped to create majority carriers (electrons for NPN, holes for PNP). Applying a forward bias to the base-emitter junction allows majority of carriers to diffuse into the base.

Due to the thin and lightly doped base, most carriers don't reach the collector; some recombine within the base, generating a small base current (Ib), while the remainder is injected into the collector, forming the collector current (Ic).

While voltage plays a role in creating the forward bias at the base-emitter junction, it's not the direct control factor. The voltage drop across this junction is relatively constant. It's the current injected due to this voltage that ultimately controls the collector current.

Even a small change in base current (Ib) significantly influences the number of carriers injected into the collector, resulting in a current gain (beta, β), hence BJTs are termed current-controlled devices.

MOSFET has an insulated gate separated by a thin oxide layer from the channel. When voltage is applied to the gate, it induces an electric field across the oxide, influencing charge carriers in the channel (electrons for NMOS, holes for PMOS), thereby establishing a conductive or resistive region.

High gate voltage prompts a strong electric field, creating a low-resistance channel for high current flow, whereas low gate voltage yields a weaker field, resulting in reduced current flow. Voltage modulation of the gate controls the electric field strength, subsequently regulating channel resistance and drain current, with minimal current flow between gate and channel due to insulation. Hence they are voltage controlled devices.

This post was modified 2 months ago by Tech Geek