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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):

Feature	NPN BJT	N-channel MOSFET (NMOS)
Terminals	Three terminals: Base, Emitter, and Collector	Three terminals: Gate, Source, and Drain
Cutoff Region	{ $V_{BE} < V_{BE(ON)}$ ; $V_{BC} < V_{BC(ON)}$ }	{ $V_{GS} < V_t$ ; $V_{GD} < V_t$ }
Forward Active Region	{ $V_{BE} >= V_{BE(ON)}$ ; $V_{BC} < V_{BC(ON)}$ }	saturation(active) region { $V_{GS} >= V_t$ ; $V_{GD} < V_t$ }
Reverse Active Region	{ $V_{BE} < V_{BE(ON)}$ ; $V_{BC} >= V_{BC(ON)}$ }	saturation(active) region { $V_{GS} < V_t$ ; $V_{GD} >= V_t$ }
Saturation Region	{ $V_{BE} >= V_{BE(ON)}$ ; $V_{BC} >= V_{BC(ON)}$ }	Triode region { $V_{GS} >= V_t$ ; $V_{GD} >= V_t$ }
Applied Voltage	$V_{BE}$ ; between base and emitter junction	$V_{GS}$ ; between Gate and Source terminals
Collector Current ( $I_c$ )	$I_c = \frac{q \cdot A \cdot D_n \cdot n_i^2}{2 \cdot W_B \cdot N_A} \cdot e^{\frac{V_{BE}}{V_T}}$	$I_D = \frac{\mu \cdot C_{ox}}{2} \cdot \frac{W}{L} \cdot (V_{GS} - V_t)^2$
Base Charge (Q)	$Q = \frac{q \cdot A \cdot W_B \cdot n_i^2}{2 \cdot N_A} \cdot e^{\frac{V_{BE}}{V_T}}$	$Q = \frac{C_{ox}}{2} \cdot WL \cdot (V_{GS} - V_t)$
Base Transit Time ( $\Delta T$ )	$\Delta T = \frac{W_B^2}{2 \cdot \mu \cdot V_T}$	$\Delta T = \frac{L^2}{2 \cdot \mu \cdot (V_{GS} - V_t)}$

Understanding the Parameters

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

$V_{BE}$ : Applied voltage across Base and Emitter
$V_{GS}$ : Applied voltage across Gate and Source
$A$ : Base-Emitter Junction Area
$W$ : Width of Channel under gate
$L$ : Length of channel under gate
$W_B$ : Width of base region
$N_A$ : Dopant density of p-type atoms in base
$C_{ox}$ : Gate Oxide capacitance per unit area
$V_t$ : Threshold voltage
$\mu$ (in NPN): Bulk mobility of electrons in the base
$\mu$ (in NMOS): surface mobility of electrons in the channel
$D_n$ : Diffusion constant of electrons in base
$n_i$ : Intrinsic carrier concentration (strongly temperature dependent)
$q$ : Electron charge

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