Chapter 14
MOSFET DEVICE PROBLEMS
version 2.3
14-1. The plot shown is a C-V plot for an nMOSFET test device of gate dimensions W/L = 420m/4m.
From the plot, determine within 5% accuracy:
(a) VTH and VFB
(b) COX (capacitance/area in fF/m2) and tOX.
(c) CMOS(min) and 
(Hint: use the depletion approx to evaluate
CMOS(VG(tot))
Answers: 0.714 fF/m2 , 48.3nm, 0.179 fF/m2, 0.632 V
14-2. The plot shown is a C-V plot for a pMOSFET test device of gate dimensions W/L = 420m/4m. From
the plot, determine within 5% accuracy:
(a) VTH and VFB
(b) COX (in fF/m2) and tOX.
(c) CMOS(min) and 
(Hint: use the depletion approx to evaluate
CMOS(VG(tot))
14.3 For an nMOS junction construct having gate oxide thickness tOX = 45 nm and substrate doping NA = 4 x
1016 #/cm3 find:

(a)
(b)
(c)
(d)
Oxide capacitance COX in fF/m2
Debye length of substrate
Fermi potential F
Body-effect coefficient 
Answers: 0.77 fF/m2, 20.5 nm, 0.382V, 1.51 V
14-4 A silicon-gate nMOS transistor with TPG = +1 has tOX = 45nm and (p-type) substrate with impurity
doping NSUB = 4 x 1016 #/cm3 . The oxide gate layer is afflicted with trapped charge of density 1.6 x 1010
#/cm2. Determine:
(a) VFB and VTH
(b) Depletion layer thickness at VGS = VTH
(c) CMOS(min)
14-5. A silicon-gate pMOS transistor with TPG = -1 has tOX = 69nm and (p-type) substrate with impurity
doping NSUB = 1 x 1016 #/cm3 . The oxide gate layer is afflicted with trapped charge of density 1.6 x 1010
#/cm2. Determine:
(a) VFB and VTH
(b) Depletion layer thickness at VGS = VTH
(c) COX and CMOS(min)
14-6. For an nMOS transistor with VFB = -0.7V, B = 0.9V, COX = 0.682 fF/m2, CFBS = 2.4 fF/m2, GAMMA
= 0.8 V , VGS = 6.0V and VDS = 3.0V find:
(a)
(b)
(c)
(d)
(e)
VTO (= zero-bias threshold)
Inversion charge/area qI at the source
Inversion charge/area qI at the drain
Depletion charge/area QB at the source
Depletion charge/area QB at the drain
(Assume that VBS = 0 in determining each of these quantities)
14–7. Find the threshold voltage for a high–voltage (= thick oxide) nMOS transistor which has tox = 800 nm.
Assume NB = 2 x 1015 #/cm3, VFB = –0.7 V, and B = Lindner potential = 2.1F + 2.08 VT. In your analysis
identify the value of gamma.
Answers: = 6.00 V , VTH0 = 4.995 V
14–8. The gate oxide of a MOS device has field strength (breakdown field) of approximately 3.5 x 106 V/cm.
Determine the maximum drain voltage VD for an oxide of thickness (a) 20 nm (b) 800 nm, assuming gate
voltage VG = 2 V.
14–9. The figure shown represents a cross–section of a CMOS inverter fabricated using a typical p–well
CMOS process. Both the n– and p– channel inverters have a gate oxide thickness tox = 50 nm and n+
polysilicon gates. The area density of oxide fixed trapped charge is Nox = 5 x 1010 #/cm2. The n– susbstrate
has a doping level ND = 2 x 1015 #/cm3 and the p-well has doping NA = 2 x 1016 #/cm3.
a) Find the zero–bias threshold voltages, VTN and VTP for these two transistors from the above
information. Assume B = Lindner potential = 2.1F + 2.08 VT.
b) It is desired to shift the threshold voltages of the n– and p– channel transistors, as found in part (a),
using an implant so that they are equal in magnitude, that is, VTN = – VTP . To simplify processing, a ’blanket’
implant is applied, with all portions of the wafer receiving the same dose. Should a boron or a phosphorus
implant be applied? What implant dose is required?
Figure P14–9. CMOS device cross–section, p–well fabrication protocell
Answers: (a) VTN = .757V, VTP = –1.335V, (b) NI = 1.38 x 1011 #/cm2
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14–10. A high–voltage power MOSFET is constructed as shown. Note that this is a vertical transistor with an
lightly–doped (n–) layer (ND = 5 x 1014 #/cm3) in series with the drain. If this layer is 10m thick and the
breakdown voltage of Si is 3 x 105 V/cm, determine
(a) the maximum reverse VDS that this transistor can withstand.
(b) the series resistance of this layer for a transistor of gate dimensions 20m x 100m. Assume
n = 1300cm2/Vs. Note that most of the conduction takes place below the gate.
Figure 14–10 The power MOSFET: (a) cross–sections (b) ID vs VGS output characteristics, (c) ID vs
VDS output characteristics.