Terminology » MOSFET vs BJT: Key Differences Explained

MOSFET vs BJT: Key Differences Explained

mosfet bjt transistor electronics comparison

This article explores the differences between MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) and BJTs (Bipolar Junction Transistors), two fundamental types of transistors used in electronics.

What is a MOSFET?

MOSFET stands for Metal Oxide Semiconductor FET.

MOSFET

MOSFET

There are two primary types of MOSFETs:

  • n-channel MOSFET
  • p-channel MOSFET

They can also be classified as:

  • Depletion MOSFET
  • Enhancement MOSFET

What is a BJT?

BJT stands for Bipolar Junction Transistor.

BJT

BJT

There are also two main types of BJTs:

  • NPN transistor
  • PNP transistor

For a clearer comparison, let’s examine the differences between an N-Channel MOSFET and an NPN type BJT.

MOSFET vs. BJT: A Detailed Comparison

Here’s a table summarizing the key differences between NPN BJTs and N-channel MOSFETs (NMOS):

FeatureNPN BJTN-channel MOSFET (NMOS)
TerminalsThree terminals: Base, Emitter, and CollectorThree terminals: Gate, Source, and Drain
Cutoff Region{ VBE<VBE(ON)V_{BE} < V_{BE(ON)} ; VBC<VBC(ON)V_{BC} < V_{BC(ON)} }{ VGS<VtV_{GS} < V_t ; VGD<VtV_{GD} < V_t }
Forward Active Region{ VBE>=VBE(ON)V_{BE} >= V_{BE(ON)} ; VBC<VBC(ON)V_{BC} < V_{BC(ON)} }saturation(active) region { VGS>=VtV_{GS} >= V_t ; VGD<VtV_{GD} < V_t }
Reverse Active Region{ VBE<VBE(ON)V_{BE} < V_{BE(ON)} ; VBC>=VBC(ON)V_{BC} >= V_{BC(ON)} }saturation(active) region { VGS<VtV_{GS} < V_t ; VGD>=VtV_{GD} >= V_t }
Saturation Region{ VBE>=VBE(ON)V_{BE} >= V_{BE(ON)} ; VBC>=VBC(ON)V_{BC} >= V_{BC(ON)} }Triode region { VGS>=VtV_{GS} >= V_t ; VGD>=VtV_{GD} >= V_t }
Applied VoltageVBEV_{BE}; between base and emitter junctionVGSV_{GS}; between Gate and Source terminals
Collector Current (IcI_c)Ic=q⋅A⋅Dn⋅ni22⋅WB⋅NA⋅eVBEVTI_c = \frac{q \cdot A \cdot D_n \cdot n_i^2}{2 \cdot W_B \cdot N_A} \cdot e^{\frac{V_{BE}}{V_T}}ID=μ⋅Cox2⋅WL⋅(VGS−Vt)2I_D = \frac{\mu \cdot C_{ox}}{2} \cdot \frac{W}{L} \cdot (V_{GS} - V_t)^2
Base Charge (Q)Q=q⋅A⋅WB⋅ni22⋅NA⋅eVBEVTQ = \frac{q \cdot A \cdot W_B \cdot n_i^2}{2 \cdot N_A} \cdot e^{\frac{V_{BE}}{V_T}}Q=Cox2⋅WL⋅(VGS−Vt)Q = \frac{C_{ox}}{2} \cdot WL \cdot (V_{GS} - V_t)
Base Transit Time (ΔT\Delta T)ΔT=WB22⋅μ⋅VT\Delta T = \frac{W_B^2}{2 \cdot \mu \cdot V_T}ΔT=L22⋅μ⋅(VGS−Vt)\Delta T = \frac{L^2}{2 \cdot \mu \cdot (V_{GS} - V_t)}

Understanding the Parameters

Here’s a breakdown of the parameters used in the equations above:

  • VBEV_{BE}: Applied voltage across Base and Emitter
  • VGSV_{GS}: Applied voltage across Gate and Source
  • AA: Base-Emitter Junction Area
  • WW: Width of Channel under gate
  • LL: Length of channel under gate
  • WBW_B: Width of base region
  • NAN_A: Dopant density of p-type atoms in base
  • CoxC_{ox}: Gate Oxide capacitance per unit area
  • VtV_t: Threshold voltage
  • μ\mu (in NPN): Bulk mobility of electrons in the base
  • μ\mu (in NMOS): surface mobility of electrons in the channel
  • DnD_n: Diffusion constant of electrons in base
  • nin_i: Intrinsic carrier concentration (strongly temperature dependent)
  • qq: Electron charge