BJT and MOSFET Explained: Structure, Working, Regions, and Characteristics

A transistor is a three-terminal semiconductor device used for amplification and switching. It lets a small electrical control signal influence a larger output current or voltage.

The two most important transistor families in Analog Electronics are BJTs and MOSFETs. A BJT is treated as a current-controlled device, while a MOSFET is treated as a voltage-controlled device.

A Bipolar Junction Transistor has three regions: emitter, base, and collector. The emitter is heavily doped so it can inject carriers, the base is very thin and lightly doped, and the collector is designed to collect carriers.

BJTs are available as NPN and PNP devices. In most introductory analog circuits, NPN examples are used first because their current directions are easier to visualize with positive supply voltages.

For an NPN transistor in active region, the base-emitter junction is forward biased and the collector-base junction is reverse biased. This biasing condition lets carriers move from emitter to collector.

A small base current controls a much larger collector current. This is the central idea behind BJT amplification.

The emitter current is the sum of base current and collector current. Current gain beta tells how many times larger collector current is compared with base current.

In hand analysis, IC = beta IB is useful only when the transistor is actually in active region. In saturation, this relation no longer decides the collector current directly.

A BJT can operate in cutoff, active, or saturation. Cutoff means the device is OFF, active means it can amplify, and saturation means it is fully ON like a closed switch.

Most mistakes happen when students calculate gain before checking the region. Region identification should come before formula substitution.

BJT input characteristics relate base current to base-emitter voltage and look similar to a diode curve. Output characteristics relate collector current to collector-emitter voltage for different base currents.

In the active region, output curves are nearly flat, meaning collector current is mainly controlled by base current instead of VCE.

As an amplifier, a BJT is biased in active region so a small input variation produces a larger output variation. As a switch, it is driven between cutoff and saturation.

This distinction matters because amplifier design needs linearity, while switching design needs clear OFF and ON states.

A MOSFET has three main terminals: gate, drain, and source. The gate is insulated from the channel by an oxide layer, which gives the MOSFET very high input impedance.

In an n-channel enhancement MOSFET, applying enough positive gate-source voltage forms a conductive channel between drain and source.

With no sufficient gate voltage, the channel is absent or weak, so the MOSFET remains OFF. When VGS exceeds threshold voltage, carriers gather near the oxide interface and create a conducting channel.

This is why MOSFETs are called voltage-controlled devices. The gate voltage controls channel strength and therefore drain current.

MOSFET operation is usually divided into cutoff, linear or ohmic region, and saturation. Cutoff is OFF, linear region behaves like a voltage-controlled resistor, and saturation is used for amplification.

A common long-channel saturation approximation is ID = k(VGS - VT)^2. Here VT is threshold voltage and VGS - VT is called overdrive voltage.

MOSFET output characteristics plot ID against VDS for different VGS values. Transfer characteristics plot ID against VGS and show how threshold voltage starts conduction.

The most important visual idea is channel formation: as gate voltage increases, the channel becomes stronger and drain current rises.

BJTs and MOSFETs both amplify and switch, but their control mechanisms are different. BJT behavior is tied to base current, while MOSFET behavior is tied to gate-source voltage.

MOSFETs dominate digital ICs and power switching because of high input impedance and efficient voltage control. BJTs remain important in analog gain stages and current-controlled circuit examples.