SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

462 CHAPTER 9. FIELD EFFECT TRANSISTORS: MOSFET

The oxide areal capacitance is

Cox=

ox

dox

=

3 .9(8. 84 × 10 −^14 )

500 × 10 −^8

=6. 9 × 10 −^8 F/cm^2

The change in the threshold voltage is

ΔVT =

[

2(1. 6 × 10 −^19 )(11.9)(8. 84 × 10 −^14 )(2× 1016 )

] 1 / 2

6. 9 × 10 −^8

·

{

[2(0.367) + 1.0]^1 /^2 −[2(0.367)]^1 /^2

}

=0.54 V

9.5.3 Depletion and Enhancement MOSFETs .................

In the discussions of the MOSFET so far, we saw that as the gate voltage is increased, at some
positive valueVT, inversion occurs and the device starts conducting or turns ON. This, of course,
is not the only configuration in which the device can operate. It is possible to design devices that
are ON when no gate bias is applied or are ON when negative bias is applied. This versatility is
quite important since it gives a greater flexibility to the logic designer.
A device in which the current does not flow when the gate bias is zero, and flows only when
a positive or negative gate bias is applied, is called an enhancement-mode device. Conversely,
if the current flow occurs when the gate bias is zero and the device turns off when the gate bias
is positive or negative, the device is said to operate in the depletion mode. The device we have
discussed so far is an enhancement-mode device since, in our discussions, a positive gate bias
was needed to cause inversion and channel formation.
To produce a depletion-mode device that is ON without any gate bias, the MOSFET fabrica-
tion is altered. As shown in figure 9.18, one starts with ap-type substrate and twon+contacts
are placed. Additionally, in the depletion-mode device, one diffuses a thin layer of donors to
produce a thinn-type channel between then+contacts. The rest of the MOSFET is produced
in the normal way by placing an oxide layer and a gate. The I-V characteristics of such a device
are also shown in figure 9.18b.
The device discussed above can be fabricated inp-type orn-type substrates. In this device one
has free carriers due to the doping and therefore the device is ON even if the gate bias is zero.
The gate bias can now be used to turn the device OFF as shown.
The MOSFET can be used as a switching element in the same way as the bipolar devices or
other FETs. Regardless of whether the FET is an enhancement or a depletion device, the FET
carries current in one of the states of the switch. This causes power dissipation in the circuits.
This is of great concern when the circuits are dense and power dissipation can cause serious
heating problems. This can be avoided by using the NMOS and PMOS devices together, as will
be discussed next.