MOSFET vs BJT | difference between MOSFET and BJT
This page on MOSFET vs BJT describes difference between MOSFET and BJT transistor types.
MOSFET
MOSFET stands for Metal Oxide Semiconductor FET. There are two types of MOSFET viz. n-channel MOSFET and p-channel MOSFET. It can also be classified as Depletion MOSFET and Enhancement MOSFET.
BJT
BJT stands for Bipolar Junction Transistor. There are two types of BJT viz. NPN transistor and PNP transistor.
Let us take N-Channel MOSFET and NPN type BJT for comparing the both. Following table illustrates the difference between MOSFET and BJT.
| NPN BJT | N-channel MOSFET (NMOS) |
|---|---|
| It has three terminals viz. base, emitter and collector | It has three terminals viz. Gate, Source and Drain |
| Cutoff region { <VBE(ON) ; <VBC(ON) } | Cutoff region { VGS<Vt ; VGD<Vt } |
| Forward Active region { >=VBE(ON) ; <VBC(ON) } | saturation(active) region { VGS>=Vt ; VGD<Vt } |
| Reverse Active region { <VBE(ON) ; >=VBC(ON) } | saturation(active) region { VGS<Vt ; VGD>=Vt } |
| Saturation region { >=VBE(ON) ; >=VBC(ON) } | Triode region { VGS>=Vt ; VGD>=Vt } |
| Applied voltage is VBE; between base and emitter junction | Applied voltage is VGS; between Gate and Source terminals |
| Current in collector terminal: Ic = { ( q*A*Dn*ni2 )/(2*WB*NA) }* e(VBE/VT) |
Current in drain terminal: ID = { ( μ*Cox )/2 }* {W/L}* (VGS - Vt) 2 |
| Charge in base: Q = { ( q*A*WB*ni2 )/(2*NA) }* e(VBE/VT) |
Charge in channel: Q = { ( Cox )/2 }* {WL}* (VGS - Vt) |
| Transit time through the base: ΔT = { (WB2)/(2*μ*VT) } |
Transit time through the channel: ΔT = { (L2)/(2*μ*(VGS-Vt)) } |
The parameters used in the above equations are as follows:
VBE : Applied voltage across Base and Emitter
VGS : Applied voltage across Gate and Source
A : Base-Emitter Junction Area
W: Width of Channel under gate
L: Length of channel under gate
WB : Width of base region
NA : Dopant density of p-type atoms in base
Cox : Gate Oxide capacitance per unit area
Vt : Threshold voltage
μ in NPN : Bulk mobility of electrons in the base
μ in NMOS : surface mobility of electrons in the channel
Dn : Diffusion constant of electrons in base
ni : Intrinsic carrier concentration (strongly temperature dependent )
q : Electron charge
Dn :
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