9.5. MOSFET OPERATION 465
Thus thep-channel is ON and then-channel is OFF so that the output voltage isVout=V.No
current flows in the devices since they are connected in series.
When a positive gate bias is applied, then-channel device is ON, while thep-channel device
is OFF. The output voltageVout= 0. Once again no current flow occurs since the devices are in
series and one of them is OFF. As can be seen, for input of 1 (High) or 0 (Low), one of the devices
remains OFF. Thus the CMOS does not consume power when it is holding the information
state. Only during switching is there a current flow. This low power consumption property
of the CMOS makes it very attractive for high density applications, such as for semiconductor
memories and processors. However, it must be noted that the device is much more complex
to fabricate. Also, since thep-type transport is much poorer than then-type transport, one has
to take special care to design the two devices to have similar performances. In Chapter 10 we
discuss applications of CMOS in digital and analog circuits.
Example 9.11Ann-channel MOSFET is formed in ap-type substrate with a substrate
doping ofNa=10^14 cm−^3. The oxide thickness is 500Aand ̊ φms=− 0. 83 V. C a l c u l a t e
the threshold voltage and check whether the device is an enhancement- or depletion-mode
device. If the device threshold voltage is to be changed by 0.5 V by ion implanting the
channel by dopants, calculate the density of dopants needed. Assume that the dopant
charge is all placed near the Si-SiO 2 interface within a thickness of 0.1μm. Temperature
is 300 K.
The position of the Fermi level is given byφF=0.026 ln(
1014
1. 5 × 1010
)
=0.228 V
The threshold voltage isVT = φms+2φF+[4esNa|φF|]^1 /^2
Cox
= − 0 .83 + 0. 456+[
4 ×(1. 6 × 10 −^19 )(11. 9 × 8. 85 × 10 −^14 )(10^14 )(0.228)
] 1 / 2
(5× 10 −^6 )
(3. 9 × 8. 85 × 10 −^14 )
= − 0 .318 V
In this device there is an inversion layer formed even at zero gate bias and the device is in
the depletion mode. To increase the threshold voltage by + 0.5 V, i.e., to convert the device
into an enhancement-mode device, we need to place more negative charge in the channel.
If we assume that the excess acceptors are placed close to the semiconductor-oxide region
(i.e., within the distanceWmax), the shift in threshold voltage is simply (Na^2 Dis the areal
density of the acceptors implanted)ΔVT=
eNa^2 D
Cox