LAB $12$: THE TRANSISTOR

COMPARISON OF TWO BASIC TYPES: MOS AND BIPOLAR

$1.$ OBJECTIVES

Familiarity with MOSFET, the most used type of transistor today and its comparison with BJT$.$ Demonstrating extremely high dc impedance of the MOSFET gate.$$ Exploring linear characteristics and switching behavior of the transistors.$$ MOSFET as voltage controlled device and BJT as current controlled device.$$ MOS analog switch.

$2.$ INTRODUCTION

There are two most common transistor types today: the$$ Metal $-$Oxide $-$ Semiconductor or MOS and the Bipolar Junction Transistor or BJT$.$ The MOS is also designated as MOSFET because it is a field effect transistor $($FET$).$ A great majority of both types are made from silicon $($Si$)$ and a small fraction $($about $2\%)$ from gallium arsenide $($GaAs$).$ The BJT dominated the market initially but now most of the transistors, particularly in integrated circuits, are of the MOS type.$$ The BJT$,$ still holds its own, particularly in some analog and high power circuits.$$ While most transistors of any type are made today as elements of integrated circuits $($ICs$),$ which can contain millions of circuit elements, single or discrete transistors are still useful in many applications like high frequency or power units.$$

In this laboratory we concentrate on the MOS transistor and compare it with the BJT$.$ The important difference between the gate impedance of MOS and the base impedance of BJT is emphasized..$$ You will experiment with an N$-$channel enhancement mode power MOSFET and an npn type BJT$.$

$3.$ PRELAB

A MOSFET is characterized by a very high input $($gate$)$ resistance.$$ Does it mean that no appreciable current ever flows to the gate?$$ Explain.

Draw schematics of circuits for experiments with a MOSFET described in sections $1,2,$ and $3,$ below.