Page 46 - DSP Integrated Circuits

VLSI CIRCUIT

TECHNOLOGIES

2.1 INTRODUCTION

There are two classes of silicon devices: bipolar and unipolar transistors. They dif-
fer in that both majority and minority carriers participate in the bipolar transistor
action while only minority carriers participate in the unipolar transistor. The main
bipolar circuit techniques are ECL/CML (emitter-coupled logic/current mode
logic), \?L (integrated injection logic), and TTL (transistor-transistor logic). These
circuit techniques are for various reasons not suitable for large integrated circuits.
There are several types of unipolar devices, e.g., MOS (metal oxide semicon-
ductor) transistor, JFET (junction field-effect transistor), and MESFET (metal-
semiconductor field-effect transistor). In practice only MOS transistors; are used
for VLSI circuits. The main unipolar circuit technique is CMOS (complementary
metal oxide semiconductor) circuits, but numerous alternatives exist [2, 23—26].
CMOS circuits are often augmented with a few bipolar devices, so-called BiCMOS,
to achieve higher speed in critical parts.
Gallium arsenide (GaAs)-based VLSI circuits have recently become available.
They are important because of their high speed and compatibility with optical
components, for example, lasers. GaAs-based circuits are therefore interesting
candidates for many DSP applications. However, there is a widespread consensus
that no other technology will effectively compete with CMOS and BiCMOS for sev-
eral years to come.

2.2 MOS TRANSISTORS

There are two basic types of MOS devices: n-channel and p-channel transistors.
Figure 2.1 illustrates the structure of an MOS transistor [10,12, 14, 15, 21]. An n-
channel (nMOS) transistor has two islands of n-type diffusion embedded in a sub-
strate of p-type. A thin layer of silicon dioxide (SiC^) is formed on top of the sur-
face between these islands. The gate is formed by depositing a conducting material
on the top of this layer. The gate was made of metal in now outdated technologies,
a fact that explains the name of the device (metal oxide semiconductor transistor).

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