12.4 Comparison of BJT and FET characteristics

Device Parameters

BJTs and FETs are the two main transistor families, and they differ in almost every electrical characteristic. BJTs are current-controlled devices: a small base current controls a larger collector current. FETs are voltage-controlled devices: a gate voltage controls the drain current, drawing almost no current itself. That single distinction drives most of the differences below.

Impedance Characteristics

Input impedance is the effective resistance (and reactance) that the input signal "sees" at the device's input terminals.

• BJTs have low input impedance (typically a few kΩ) because the base-emitter junction is forward-biased, so current flows into the base.
• FETs have high input impedance (typically several MΩ, sometimes into the GΩ range for MOSFETs) because the gate is electrically insulated from the channel by an oxide layer (MOSFET) or a reverse-biased junction (JFET). Almost no DC current enters the gate.

Output impedance is the effective resistance seen by whatever load you connect to the output.

• BJTs have relatively low output impedance (hundreds of Ω), set by the collector-emitter path.
• FETs have high output impedance (tens to hundreds of kΩ) because the drain-source path behaves more like a current source in saturation.

Transconductance ($g_m$) measures how much the output current changes per unit change in input voltage. It tells you how "sensitive" the device is to its input.

• BJTs have higher $g_m$ for a given bias current. For a BJT, $g_m = I_C / V_T$, where $V_T \approx 26 \text{ mV}$ at room temperature, so even a small collector current gives substantial transconductance.
• FETs have lower $g_m$ at comparable currents, but their transconductance tends to be more stable across a wider range of operating conditions.

Key Parameters Comparison

Because FETs combine high input resistance with low input capacitance, they're well suited for high-frequency applications like RF and microwave circuits, where you don't want the transistor loading down the signal source.

Performance Characteristics

Noise and Power Considerations

Noise performance describes how much unwanted random signal the device adds.

• BJTs tend to have lower noise at low frequencies (below about 1 MHz). Their noise is dominated by shot noise in the junctions, which is relatively predictable and manageable.
• FETs tend to have better noise performance at higher frequencies (above 1 MHz), which is another reason they're preferred in RF front-end stages.

Power consumption matters a lot for portable and battery-powered designs.

• BJTs generally consume more power because the base must continuously draw current to keep the transistor on.
• FETs consume less power because the gate draws essentially zero steady-state current (only tiny leakage). This is the main reason CMOS (complementary MOSFET) technology dominates digital ICs: millions of transistors can sit idle without burning power.

Environmental and Speed Factors

Temperature sensitivity affects how stable your circuit stays as it heats up or cools down.

• BJTs are more temperature-sensitive. Their operation depends on minority carrier injection, and parameters like $\beta$ and $V_{BE}$ shift noticeably with temperature. This can cause thermal runaway if the circuit isn't designed carefully.
• FETs are more temperature-stable overall and work across a wider temperature range. MOSFETs even have a built-in negative temperature coefficient at certain bias points, which makes them naturally resist thermal runaway.

Switching speed is how fast the device toggles between ON and OFF.

• BJTs can switch very fast in many applications because of their high $g_m$ and strong current drive.
• FETs have an advantage in high-frequency circuits despite sometimes slower raw switching, because their high input impedance means the driving stage doesn't need to supply large currents to charge and discharge the input. In digital CMOS logic, switching speeds are extremely fast at modern process nodes.