Download MOSFET and JFET: Operation, Characteristics, and Circuits - Prof. Aurangzeb Khan and more Study notes Electrical and Electronics Engineering in PDF only on Docsity! 1 Chapter 5 Field-Effect Transistors Chapter Goals • Describe operation of MOSFETs and JFETs. • Define MOSFET characteristics in operation regions of cutoff, triode and saturation. • Discuss mathematical models for i-v characteristics of MOSFETs and JFETs. • Introduce graphical representations for output and transfer characteristic descriptions of electronic devices. • Define and contrast characteristics of enhancement-mode and depletion-mode MOFETs. • Define symbols to represent MOSFETs in circuit schematics. • Investigate circuits that bias transistors into different operating regions. • MOSFET and JFET DC circuit analysis • Explore MOSFET modeling in SPICE MOSFET as a Current Source • Ideal current source gives fixed output current regardless of voltage across it. • MOSFET behaves as as an ideal current source if biased in the pinch-off region (output current depends on gate-source terminal voltage). NMOS Current Mirror Assumption: M1 and M2 have identical VTN, Kn’, λ and W/L and are in saturation. IREF = K n 2 W L VGS1 −VTN( ) 2 1+λV DS1 IO = K n 2 W L VGS2 −VTN( ) 2 1+λV DS2 But VGS2 =VGS1 and IO = IREF 1+λV DS2( ) 1+λV DS1( ) ≅ IREF Thus, the output current precisely mirrors the reference current if VDS1 = VDS2. 2 MOS Current Mirror Ratio Kn1 = Kn ' W L 1 = 2Kn ' Kn2 = Kn ' W L 2 =10Kn ' IO = IREF W /L( )2 W /L( )1 1+λV DS2( ) 1+λV DS1( ) = 5IREF 1+λV DS2( ) 1+λV DS1( ) ∴ IO ≅ 5IREF Thus, the ratio between IO and IREF can be modified by changing the W/L ratios of the current mirror transistors (ignoring differences due to VDS mismatch)
Metal-Semiconductor FET — MESFET
® Gate is a Schottky barrier junction
@ GaAs — common semiconductor material
® Results — high majority-carrier mobility, small
transit time, and fast response
@ Enhancement-mode or depletion-mode types
@ N-channel MESFET sketch:
Rectving
mia > came
no
(semidnsulating = 0)
MESFET Symbols
N-Channel P-Channel
dD 3
fs .
cof” vn s
.
N-Channel MESFET Formulas
(same as MOSFET)
@ The transition point:
Vosisa) = Vos — Vin
Vy = n-channel threshold voltage
@ Vos > Vosieary — the saturation region
j = v4
ip = Kn(Ves — Vin)
® Vos < Vpsieary) — the nonsaturation region:
ip = K]2(Ves - Vrn)Vps - Vis!
kK, = n-channel conduction parameter
Design Example 5.20 Objective: Design a circuit with an enfancement-mode
MESFED
Consider the circuit shown in Figure §
ad 4 =0. Let Ri +2
) the drain current is wav
Solution: From Equation (5.36(a
fy = Bos — Vix = C1NOS — 0.24 = MAYA
From Figure $.59(b), the voltage at the dain is
Up UpRy = 4~ (0 07AEN6 7) = 35
Therefore, 1
Vem Vn Vs 2S -1V
The source resistance is th
rs —28— 1 ake
The voltage at the Re
Vo= Vest ¥5=03 41-139
Since the gate current is zero, the gate voltage is also given by.
2
We ster = 2 > Vos iy = 05-02 =026V
Which confiems thatthe transistor is biased inthe saturation region, as initially assumed
(a). The transistor parameters are:
0X. Design the circuit
Vp =4= (00744627
*DS.33 Consider the circuit shown in Figure 5.63 with transistor parameters Ips =
V, and Design the circuit such that Ry, = 100k, Ing = SMA, and
2V. (Ams. Ry = O.4KR, Ry — 387KR, Ry — 135K2)
BSiay
Sines thet a
x ye
Tdi at. Vp +)
rn Rat. Veo Wy adZeg ? Vese otwey
Sm SE cesens Ley re Rp
A wa fer Yee Vacve 2 ota s
Sh = Woke ng ae. ie =(22
one tag 2 Easel ', es
As ley -S10= 5 fae, yp
Sah UN ead
ee esta
Tdde Sat
DP Ve - tok = Sa,
Sapa EAU Te -
3 weqatevvece im Carp Figures.63 Gieuit er
- Exercise 533
> Cary
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Vora ay
9 tne Ven long = = rary
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