SCEN103 Class 19

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Introduction to MOS Transistors and MOSFET Switches

CMOS is the industry standard logic used in consumer digital electronics. The CMOS NAND gate is the building block of the digital revolution; our goal over the next week is to understand the semiconductor origin of our digital world.

Today we will examine the 'MOS' of CMOS, where MOS stands for metal-oxide-semiconductor. As studied in previous classes, the availability of n-type and p-type Silicon and compensation doping, which permits one type to be turned into the other type by adding additional dopants, is exploited to fabricate transistors.

Schematic of a MOSFET transistor
A Slide Show of MOSFET fabrication

In our previous class on binary counting, we found a way to represent numbers using just two digits, 0 and 1. One simple representation of a binary system is the electrical switch -- it is either on or off. Transistor circuits can be constructed to operate as miniature electrical switches, as we will see.

In addition, the state of a transistor switch may be controlled by the output of another transistor switch. Control of switches by other switches is the key idea behind using semiconductors in the form of transistor switches to carry out digital logic operations. The ease with which transistor switches can be used to store information in the form of binary numbers and their speed of execution in processing logic operations are essential ingredients in digital electronics.

Operation of MOSFET switch:
- Examination of circuit configuration
- Explanation of field effect in n-channel MOS transistor
- "off" state resistance of MOSFET is 10⁸ ohms; i.e. 100 megaohms!
- "on" state resistance of MOSFET is only 10³ ohms as a result of induced conducting channel
- Review of voltage divider in preparation for analysis of switch -- Analysis and Examples
- Analysis of "OFF" and "ON" states of switch
n-channel MOSFET inverter
p-channel MOSFET inverter

How Transistors Work from the Education Programs at Intel

Group Activity

Consider that the transistor had not been invented:

Attempt to describe some personal events, at least one per group member, that would have prevented you from being here today (in class, alive) except that the transistor was indeed invented and miniaturized so successfully.
Next time, we will consider the "down side" of transistors in our society...

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Last updated March 9, 1999.
Copyright George Watson, Univ. of Delaware, 1996.