šŸ”§ Transistor Technology Comparison

Session 4: BJT vs FET

šŸ”Œ

Bipolar Junction Transistor (BJT)

Uses both electrons and holes as charge carriers

Current-Controlled

Types

NPN
PNP

Terminals

  • Emitter (E)
  • Base (B)
  • Collector (C)

Key Specifications

Input Impedance: Low (typically few kĪ©)

Switching Speed: Moderate

Power Consumption: Higher - requires continuous base current

Gain: Current gain (Ī² = Ic/Ib)

Operation Principle

Small base current controls large collector current. Both electrons and holes participate in conduction.

Active Mode: Base-emitter forward biased, collector-base reverse biased

āœ“ Advantages

  • High current gain
  • Good for low voltage applications
  • Lower cost
  • Better performance at low frequencies

āš  Disadvantages

  • Current-controlled (requires base current)
  • Lower input impedance
  • Slower switching speed
  • Temperature sensitive

šŸš— Applications in Electric Vehicles

1
Motor driver circuits (low power)
2
Signal amplification in sensors
3
Battery protection circuits
4
Audio amplifiers in infotainment
āš”

Metal-Oxide-Semiconductor FET (MOSFET)

Uses electric field to control current flow

Voltage-Controlled

Types

N-Channel (NMOS)
P-Channel (PMOS)

Terminals

  • Source (S)
  • Gate (G)
  • Drain (D)

Key Specifications

Input Impedance: Very High (MĪ© range)

Switching Speed: Very Fast (nanoseconds)

Power Consumption: Very Low - minimal gate current

Gain: Transconductance (gm)

Operation Principle

Gate voltage creates electric field that controls channel conductivity. No gate current flows.

Enhancement Mode: Normally off, turns on with gate voltage

āœ“ Advantages

  • High input impedance
  • Fast switching speed
  • Low on-resistance (RDS(on))
  • Excellent efficiency

āš  Disadvantages

  • Gate oxide can be damaged by static
  • More expensive than BJTs
  • Can have higher switching losses at very high frequencies

šŸš— Applications in Electric Vehicles

1
Inverter circuits for motor control
2
Battery management system switching
3
DC-DC converter circuits
4
Regenerative braking systems
šŸ”‹

Junction Field-Effect Transistor (JFET)

Depletion region controls channel width

Voltage-Controlled

Types

N-Channel
P-Channel

Terminals

  • Source (S)
  • Gate (G)
  • Drain (D)

Key Specifications

Input Impedance: Very High (MĪ© range)

Switching Speed: Fast

Power Consumption: Low - minimal gate current

Gain: Transconductance (gm)

Operation Principle

Reverse-biased gate-channel junction creates depletion region that controls channel width.

Depletion Mode: Normally on, turns off with negative gate voltage

āœ“ Advantages

  • High input impedance
  • Low noise characteristics
  • Thermally stable
  • Simple gate drive

āš  Disadvantages

  • Lower transconductance than MOSFET
  • Normally-on device (depletion mode)
  • Limited availability
  • Lower power handling

šŸš— Applications in Electric Vehicles

1
Gate drivers for power MOSFETs
2
Sensor signal conditioning
3
Low-noise amplifiers
4
Current source circuits

BJT vs MOSFET vs JFET Comparison

Feature BJT MOSFET JFET
Control Type Current-controlled Voltage-controlled Voltage-controlled
Input Impedance Low (kĪ©) Very High (MĪ©) Very High (MĪ©)
Switching Speed Moderate Very Fast Fast
Power Consumption Higher Very Low Low
Cost Low Moderate-High Moderate
Temperature Stability Sensitive Good Very Good
Preferred Use in EV Low-power control Power electronics Signal processing

* Highlighted cells indicate best-in-class performance

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