EE-4232
Review of
BJTs, JFETs and MOSFETs
0
A simplified structure of the npn transistor.
1
A simplified structure of the pnp transistor.
2
Current flow in an npn transistor biased to operate in the active mode, (Reverse current
components due to drift of thermally generated minority carriers are not shown.)
3
Current flow in an pnp transistor biased to operate in the active mode.
4
The iC-vCB characteristics for an npn transistor in the active mode.
5
(a)
Conceptual circuit for measuring the iC-vCE characteristics of the BJT.
(b) The iC-vCE characteristics of a practical BJT.
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(a) Conceptual circuit to illustrate the operation of the transistor of an amplifier.
(b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.
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Linear operation of the transistor under the small-signal condition: A small signal vbe with a
triangular waveform is superimpose din the dc voltage VBE. It gives rise to a collector signal
current ic, also of triangular waveform, superimposed on the dc current IC. Ic = gm vbe, where gm is
the slope of the ic - vBE curve at the bias point Q.
8
Two slightly different versions of the simplified hybrid-Π model for the small-signal
operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltagecontrolled current source ( a transconductance amplifier) and that in (b) represents the
BJT as a current-controlled current source (a current amplifier).
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Two slightly different versions of what is known as the T model of the BJT. The circuit
in (a) is a voltage-controlled current source representation and that in (b) is a currentcontrolled current source representation. These models explicitly show the emitter
resistance re rather than the base resistance rΠ featured in the hybrid-π model.
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Junction Field-Effect Transistors
iD
n-JFET
vGS =0
IDSS
vGS =-1
Pinch-off
vDG > -Vp
vGS =Vp
-Vp
iD
vDS
p-JFET
vSG =0
IDSS
vSG =-1
Pinch-off
vGD > Vp
vSG =-Vp
Vp
vSD
Junction Field-Effect Transistors
Triod (VCR) Region
i D = I DSS
r DS
v DS
= -------iD
v GS  v DS   v DS 2

2 1 – -------- ---------- – -------
V p   – V p  V p 
v DS = small
2I DSS 
v GS
= -------------- 1 – -------–V p 
Vp
Boundary
v DG = – V p
iD
v DS

= I DSS ------- Vp
2
Pinch-Off Region
iD
v GS 2

= I DSS 1 – -------
Vp
–1
MOSFETs
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Physical structure of the enhancement-type NMOS transistor: (a) perspective view; (b)
cross section. Typically L = 1 to 10 µm, W = 2 to 500 µm, and the thickness of the
oxide layer is in the range of 0.02 to 0.1 µm.
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The enhancement-type NMOS transistor with a positive voltage applied to the gate. An
n channel is induced at the top of the substrate beneath the gate.
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The drain current iD versus the drain-to-source voltage vDS for an enhancement-type
NMOS transistor operated with vGS > Vt.
14
Cross section of a CMOS integrated circuit. Note that the PMOS transistor is formed
in a separate n-type region, known as an n well. Another arrangement is also possible
in which an n-type body is used and the n device is formed in a p well.
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(a) An n-channel enhancement-type MOSFET with vGS and vDS applied and with the
normal directions of current flow indicated. (b) The iD - vDS characteristics for a device
with Vt = 1 V and k’n(W/L) = 0.5 mA/V2.
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The iD - vGS characteristic for an enhancement-type NMOS transistor in saturation
(Vt = 1 V and k’n(W/L) = 0.5 mA/V2).
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Large-signal equivalent circuit model of the n-channel MOSFET in saturation,
incorporating the output resistance ro. The output resistance models the linear
dependence of iD on vDS and is given by ro ≅ VA/ID.
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Conceptual circuit utilized to study the operation of the MOSFET as an amplifier.
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Small-signal operation of the enhancement MOSFET amplifier.
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Small-signal models for the MOSFET: (a) neglecting the dependence of iD on vDS in
saturation (channel-length modulation effect); and (b) including the effect of channellength modulation modeled by output resistance ro = |VA|/ID.
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(a) The CMOS inverter. (b) Simplified circuit schematic for the inverter.
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Dynamic operation of a capacitively loaded CMOS inverter: (a) circuit; (b) input and
output waveforms.
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