0195136047.pdf

378 SEMICONDUCTOR DEVICES

Solution

Applying Equation (7.4.11):

vGS=VP

[{

iD

IDSS

+

(

vDS

VP

) 2 }(

VP

2 vDS

)

− 1

]

= 4

[{

1

7

+

(

0. 8

4

) 2 }(

4

1. 6

)

− 1

]

=− 2 .17 V

Check:vDS= 0 .8V<vGS+VP=− 2. 17 + 4 = 1 .83 V. The ohmic-region operation is verified.

While there is no difference in the general shape of the characteristics between the depletion

and enhancement MOSFETs, the practical distinction is the gate voltage range. In particular, a

depletion MOSFET can be in the active region whenvGS=0, whereas an enhancement MOSFET

must havevGS>VT>0.

While a JFET behaves much like a depletion MOSFET, there are several minor differences

between JFETs and depletion MOSFETs. First, withvGS<0, the junction in the JFET carries a

reverse saturation gate currentiC∼=−IGSS, which is quite small (on the order of 1 nA) and can

usually be neglected. Second, any positive gate voltage above about 0.6 V would forward-bias the

junction in the JFET, resulting in a large forward gate current. Thus, enhancement-mode operation

is not possible with JFETs. On the side of advantages for JFETs, the channel in a JFET has greater

conduction than the channel in a MOSFET of the same size, and the static characteristic curves

are more nearly horizontal in the active region. Also, JFETs do not generally suffer permanent

damage from excessive gate voltage, whereas MOSFETs would be destroyed.

The transistor is operated within its linear zone and acts like a controlled source in electronic

amplifiers. It is also used in instrumentation systems as an active device. In digital computers or

other electronic switching systems, a transistor effectively becomes a switch when operated at

the extremes of its nonlinear mode.

7.5 Integrated Circuits

For the fabrication of semiconductor circuits, there are three distinct technologies employed:

1. Discrete-component technology,in which each circuit element is an individual component
   and circuit construction is completed by interconnecting the various components.


2. Monolithic technology,in which all the parts (such as transistors, resistors, capacitors, and
   diodes) needed for a complete circuit (such as an amplifier circuit) are constructed at the
   same time from one silicon wafer (which is typically 5 mils or 0.005 inch in thickness).


3. Hybrid technology,a combination of the preceding two technologies, in which various
   circuit components constructed on individual chips are connected so that the hybrid IC
   resembles a discrete circuit packaged into a single, small case.

Integrated circuits(ICs), in which several transistors, resistors, wires, and even other com-

ponents are all fabricated in a single chip of semiconductor, are ideal building blocks for

electronic systems. Space, weight, cost, and reliability considerations gave much impetus for

the development of ICs. The ability to place circuit elements closer on an IC chip helps in

extending the frequency range of the devices. Whereas the IC technology involves the use of only

solid-state devices, resistors, and capacitors, the elimination of inductors is necessitated by the