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4

MOS Capacitors

4.1

INTRODUCTION

The primary reason to study the Metal-Oxide-Silicon (MOS) capacitor is to understand the principle of operation as well as the detailed analysis of the Metal-Oxide-Silicon Field

Effect Transistor (MOSFET). In this chapter, we introduce the MOS structure and its four different modes of operation, namely accumulation, flatband, depletion and inversion. We then consider the flatband voltage in more detail and present the MOS analysis based on the full depletion approximation. Finally, we analyze and discuss the MOS capacitance.

4.2

STRUCTURE AND PRINCIPLE OF OPERATION

The MOS capacitor consists of a Metal-Oxide-Semiconductor structure as illustrated by

Fig. 4.1. Shown in the semiconductor substrate with a thin oxide layer and a top metal contact, referred to as the gate. A second metal layer forms an Ohmic contact to the back of the semiconductor and is called the bulk contact. The structure shown has a p-type substrate. We will refer to this as an n-type MOS or nMOS capacitor since the inversion layer—as discussed in section 4.6.4—contains electrons. nMOS : Metal-Oxide-Semiconductor

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INTRODUCTION

TO

ASIC

11

Fig. 1.12

1.9 LOOK-UP TABLES

The way logic functions are implemented in a FPGA is another key feature.

Logic blocks that carry out logical functions are look-up tables (LUTs), implemented as memory, or multiplexer and memory.

Figure 1.13 shows these aternatives, together with an example of memory contents for some basic operations.

A 2n – 1 ROM can implement any n-bit function. Typical sizes for n are 2, 3, 4 or 5.

In Fig. 1.13 (a), an n-bit LUT is implemented as a 2n – 1 memory; the input address selects one of 2n memory locations. The memory locations (latches) are normally loaded with values from user’s configuration bit-stream.

In Fig. 1.13 (b), the multiplexer control inputs are the LUT inputs. The result is a generalpurpose “logic gate.” An n-LUT can implement any n-bit function. An n-LUT is a direct implementation of a function truth table.

Each latch location holds the value of the function corresponding to one input combination.

(a)

(b)

(c)

Fig. 1.13. Look-up table implemented as (a) memory or (b) multiplexer and memory

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MOS Field-EffectTransistors

5.1

INTRODUCTION

The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (nMOSFET) consists of a source and a drain, two highly conducting n-type semiconductor regions, which are isolated from the p-type substrate by reversed-biased p-n diodes. A metal or polycrystalline gate covers the region between source and drain. The gate is separated from the semiconductor by the gate oxide. The basic structure of an n-type MOSFET and the corresponding circuit symbol are shown in Fig. 5.1.

VDS

DRAIN

VGS

GATE n-SOURCE

VBS

n-DRAIN

DEPLETION LAYER

GATE OXIDE

INVERSION LAYER

GATE

SUBSTRATE

p-SUBSTRATE

BACK CONTACT

SOURCE

Fig. 5.1

Cross-section and circuit symbol of an n-type Metal-Oxide-SemiconductorField-Effect-Transistor (MOSFET)

As can be seen on the Figure, the source and drain regions are identical. It is the applied voltages, which determine which n-type region provides the electrons and becomes the source, while the other n-type region receives the electrons and becomes the drain. The voltages applied to the drain and gate electrode as well as to the substrate, by means of a back contact, are referred to the source potential, as also indicated in

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Semiconductor

Fundamentals

1.1

INTRODUCTION

To understand the fundamental concepts of semiconductors, one must apply modern physics to solid materials. More specifically, we are interested in semiconductor crystals.

Crystals are solid materials consisting of atoms, which are placed in a highly ordered structure called a lattice. Such a structure yields a periodic potential throughout the material, which results in some remarkable properties.

Two properties of crystals are of particular interest, since they are needed to calculate the current in a semiconductor. First, we need to know how many fixed and mobile charges are present in the material. Second, we need to understand the transport of the mobile carriers through the semiconductor.

In this chapter, we explain the concepts of energy bands, energy band gaps and the density of states in an energy band. We also show how the current in an almost filled band can more easily be analyzed using the concept of holes.

Two carrier transport mechanisms will be considered. The drift of carriers in an electric field and the diffusion of carriers due to a carrier density gradient will be discussed. Recombination mechanisms and the continuity equations are then combined into the diffusion equation. Finally, we present the drift-diffusion model, which combines all the essential elements discussed in this chapter.

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Chapter

6

SEQUENTIAL LOGIC CIRCUIT

DESIGN

6.1 INTRODUCTION

If we neglect the propagation delay time, the output of the combinational logic circuit, at any given time point are directly determined as Boolean function of the input variables applied at that time. Thus, the combinational circuits lack of the capability of storing any previous events, or displaying an output behaviour which is dependent upon the previously applied inputs.

The sequential circuit gives the output, which is determined by the current inputs as well as the previously applied input variables.

Bistable circuits have, as their name implies, two stable stages or operation modes, each of which can be attained under certain input and output condition.

Monostable circuits, have only one stable operating point (state).

All basic Latch and flip-flop circuits, registers and memory elements used in digital systems fall into this category.

6.2 BEHAVIOUR OF BI-STABLE ELEMENTS

The basics of a bi-stable elements is that, it has two identical cross-coupled inverter circuits, as shown in figure given below:

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