BJT VS MOSFET- Current controlled vs Voltage controlled

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.

The most important difference between a BJT and a MOSFET lies in their control mechanism. A BJT is a current-controlled device, meaning it requires a continuous base current to operate.

In contrast, a MOSFET is voltage-controlled, where applying a voltage to the gate controls the device with little to no gate current. This fundamental distinction affects how each device is used: MOSFETs are generally more power-efficient and preferred in switching applications, while BJTs are often chosen for analog circuits due to their better linearity.

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is a voltage-controlled device, while BJT (Bipolar Junction Transistor) is current-controlled. MOSFETs require a voltage across the gate-source to operate, whereas BJTs need a continuous base current. In switching applications, MOSFETs are preferred due to lower power loss and simpler drive requirements. BJTs are still widely used for analog amplification because of their linear characteristics.

In terms of construction, MOSFETs come in N-channel and P-channel types, while BJTs are either NPN or PNP. MOSFETs generally handle higher currents and offer better efficiency due to low RDS(on), while BJTs suffer from higher VCE saturation loss.

MOSFETs have defined switching characteristics (e.g., gate charge, rise and fall times), whereas BJTs switch faster but with higher conduction losses. Additionally, MOSFETs are thermally more stable and simpler to bias, making them more suitable for modern high-speed, low-loss applications, such as power supplies and motor drivers.

MOSFETs are great for fast switching and are commonly used in inverter circuits, DC-DC converters, and motor drivers — up to a few hundred volts and moderate power levels. But when you start dealing with high voltages and power (like in industrial inverters or motor drives), IGBTs are preferred. They’re not as fast as MOSFETs but handle high voltage and current better.

There are people claim that BJTs (Bipolar Junction Transistors) are obsolete, but they continue to play a vital role in analog signal amplification due to their high gain and linear characteristics.

They are especially preferred in applications like audio amplifiers and analog front-end circuits. However, for switching applications, especially in modern embedded systems and power electronics, MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are generally more practical.

Their high input impedance, faster switching speeds, and greater efficiency at handling high currents make them ideal for use in digital circuits, motor drivers, and power converters.