Difference between VHDL and Verilog

Why Silicon is preferred over Germanium for Semiconductor Devices?

 As we all know, both Silicon and Germanium are semiconductor devices. But the present trend is to use Silicon instead of Germanium. What may be the reasons?

1. At room temperature, Silicon crystal has fewer free electrons than Germanium crystal. This implies that silicon will have much smaller Collector cut off current than Germanium.
2. The variation of Collector cut off current with temperature is less in Silicon compared to Germanium. 
3. The structure of Germanium crystals will be destroyed at higher temperature. However, Silicon crystals are not easily damaged by excess heat.
4. Peak Inverse Voltage ratings of Silicon diodes are greater than Germanium diodes.  
5. Si is less expensive due to the greater abundance of element. The major raw material for Si wafer fabrication is sand and there is lots of sand available in nature. 

But there is a disadvantage for Silicon over Germanium. 

The potential Barrier of Silicon is more compared to Germanium. 

But if we consider the advantages listed above, we can conclude that Silicon is the best element for the Semiconductor Devices and Applications. 

However,  the  first transistor was made of germanium (Ge). :)

Static RAM Vs Dynamic RAM

An Introduction to Digital Logic Families

What is a Logic Family?

In Digital Designs, our primary aim is to create an Integrated Circuit (IC).

A Circuit configuration or arrangement of the circuit elements in a special manner will result in a particular Logic Family.

What are the advantages of creating different Logic Families?

Electrical Characteristics of the  IC will be identical. In other words, the different parameters like Noise Margin, Fan In, Fan Out etc will be identical.

Different ICs belonging to the same logic families will be compatible with each other.

Some Characteristics we consider for the selection of a particular Logic Family are:

• Supply voltage range 
• Speed of response 
• Power dissipation 
• Input and output logic levels 
• Current sourcing and sinking capability
• Fan-out 
• Noise margin

The basic Classification of the Logic Families are as follows:

• Bipolar Devices
• MOS Devices
• Hybrid Devices

 Bipolar Families:

1. Diode Logic (DL)
2. Resistor Transistor Logic (RTL)
3. Diode Transistor Logic (DTL)
4. Transistor- Transistor Logic (TTL)
5. Emitter Coupled Logic (ECL) or Current Mode Logic (CML)
6. Integrated Injection Logic (IIL)

MOS Families:

1. P-MOS Family
2. N-MOS Family
3. Complementary-MOS Family 
1.  Standard C-MOS
2. Clocked C-MOS 
3. Bi-CMOS
4. Pseudo N-MOS
5. C-MOS Domino Logic
6. Pass Transistor Logic

 Hybrid Family:

1. Bi-CMOS Family

Diode Logic 

In DL (diode logic), only Diode and Resistors are used for implementing a particular Logic.Remember that the Diode conducts only when it is Forward Biased.

Disadvantages of Diode Logic

• Diode Logic suffers from voltage degradation from one stage to the next.
• Diode Logic only permits OR and AND functions.

Resistor Transistor Logic
In RTL (resistor transistor logic), all the logic are implemented using resistors and transistors. One basic thing about the transistor (NPN), is that HIGH at input causes output to be LOW (i.e. like a inverter). In the case of PNP transistor, the LOW at input causes output to be HIGH.

RTL Circuit

Advantage:

• Less number of Transistors

Disadvantage:

• High Power Dissipation
• Low Fan In

Diode Transistor Logic
In DTL (Diode transistor logic), all the logic is implemented using diodes and transistors.

DTL Logic

Disadvantage:

• Propagation Delay is Larger

Transistor Tansistor Logic
In Transistor Transistor logic or just TTL, logic gates are built only around transistors.
TTL Logic has the following sub-families:

• Standard TTL.
• High Speed TTL
• Low Power TTL.
• Schhottky TTL.
• Low Power Schottky TTL
• Advanced Schottky TTL
• Advanced Low Power Schottky TTL
• Fast Schottky

Emitter Coupled Logic
The main specialty of ECL is that it is operating in Active Region than the Saturation Region. That is the reason for its high speed operation. As you can see in the figure, the Emitters of the Transistors Q1 and Q2 are coupled together.

Emitter Coupled Pair

Disadvantage:

• Large Silicon Area
• Large Power Consumption

What are MEMS?

Think if we can integrate a GPS System on every Parcel we are sending via Courier or Post. It will be very easy for us to track the current location of the Parcel and the Handling of the Parcel. But, the system should be small like a Chip, and it should be of least cost. Don't forget that the GPS System has a Processor and an Antenna to receive the signals from the Satellite. How can we integrate the entire system inside a Small Chip? Here comes the application of MEMS.

MEMS is an emerging technology in which Microscopic Machines are developed by the tools and techniques that were developed for the Integrated Circuit (IC)ndustry.

MEMS structure

Micro-Electro Mechanical systems (MEMS) is a technology that combines computers with tiny mechanical devices such as sensors, valves, gears, mirrors, and actuators embedded in semiconductor chips. 

These machines are built on standard silicon wafers.

MEMS are made up of components between 1 to 100 micrometres in size (i.e. 0.001 to 0.1 mm), and MEMS devices generally range in size from 20 micrometres (20 millionths of a metre) to a millimetre (i.e. 0.02 to 1.0 mm).

They usually consist of a Central Unit that processes data (the microprocessor) and several components that interact with the outside such as Microsensors.

Components of MEMS

What are the Advantages of MEMS?

By utilizing this technology, it is possible to integrate both Microelectronic circuits and Mechanical structures on the same chip, enabling monolithic integration while reducing the microsystem size and cost considerably.

What are the Applications of MEMS? 

SMART BIOMEM (LAB ON A CHIP)

This technology has an enormous number of application areas, including 

• Automotive Eg. Accelerometers for airbag systems, Roll-over detection systems, etc.
• Biomedical Eg.Neural prosthesis devices like hearing and visual aids, Smart drug delivery systems, On Chip body fluid analysis systems, Microsurgery tools, Pacemakers
• Telecommunication Eg. Micromirrors for fiber optic switching for fast internet,Smart Antennas
• Household appliances pressure sensors for water level detection, frost sensors for refrigerators
• Consumer Applications DLP projectors, i-phone,
• Defense applications Eg. Low cost night vision, Smart munitions

Image Courtesy

www.engineersgarage.com 
www.medgadget.com

SMART CARDS (SIM)

A smart card, typically a type of chip card, is a plastic card that contains an embedded computer chip–either a memory or microprocessor type–that stores and transacts data. This data is usually associated with either value, information, or both and is stored and processed within the card's chip. The card data is transacted via a reader that is part of a computing system. Systems that are enhanced with smart cards are in use today throughout several key applications, including healthcare, banking, entertainment, and transportation. All applications can benefit from the added features and security that smart cards provide.

Applications

• SIM Cards and Telecommunication 
• Loyalty and Stored Value 
• Securing Digital Content and Physical Assets 
• E-Commerce 
• Bank Issued Smart Cards and Healthcare Informatics

Types of Smart Card

1. Contact Cards

These are the most common type of smart card. Electrical contacts located on the outside of the card connect to a card reader when the card is inserted. This connector is bonded to the encapsulated chip in the card.

2. Contactless Cards

A second card type is the contactless smart card, in which the card communicates with and is powered by the reader through RF induction technology (at data rates of 106–848 kbit/s). These cards require only proximity to an antenna to communicate. Like smart cards with contacts, contactless cards do not have an internal power source. Instead, they use an inductor to capture some of the incident radio-frequency interrogation signal, rectify it, and use it to power the card's electronics.

3. Multifunction Cards


These cards have on-card dynamic data processing capabilities. Multifunction smart cards allocate card memory into independent sections or files assigned to a specific function or application. Within the card is a microprocessor or micro controller chip that manages this memory allocation and file access. This type of chip is similar to those found inside all personal computers and when implanted in a smart card, manages data in organized file structures, via a card operating system (COS). Unlike other operating systems, this software controls access to the on-card user memory. This capability permits different and multiple functions and/or different applications to reside on the card, allowing businesses to issue and maintain a diversity of ‘products’ through the card. One example of this is a debit card that also enables building access on a college campus. Multifunction cards benefit issuers by enabling them to market their products and services via state-of-the-art transaction and encryption technology. Specifically, the technology enables secure identification of users and permits information updates without replacement of the installed base of cards, simplifying program changes and reducing costs. For the card user, multifunction means greater convenience and security, and ultimately, consolidation of multiple cards down to a select few that serve many purposes.

CMOS FABRICATION PROCESS

CMOS fabrication process

Semiconductor Fabrication Processes

·        Front-End Processing (Wafer fabrication)

·        Back-End Processing (Assembly and Testing)

Semiconductor Applications

• Personal Computer--- Desktop Computer (DT)  /
                    Notebook (NB)
• Communication---
                 ADSL  / Cable Modem  /
                   IEEE802.11X / Bluetooth  / VoIP
• Consumables---
                 Game  / DVD /  Digital Camera 
• 3C merge--- Digital Home

Types of Chips

•       Dynamic Random Access Memory chips (DRAMs) - serve as the primary memory for computers

•       Microprocessors (MPUs) - act as the brains of computers.

•       Application Specific Integrated Circuits (ASICs) - are custom semiconductors designed for very specific functions

•       Digital Signal Processors (DSPs) - process signals, such as image and sound signals or radar pulses.

•       Programmable memory chips (EPROMs, EEPROMs, and Flash) - are used to perform functions that require programming on the chip.

VLSI Circuits

DEALING WITH VLSI CIRCUITS

Digital VLSI circuits are predominantly CMOS based. The way normal blocks like latches and gates are implemented is different from what students have seen so far, but the behaviour remains the same. All the miniaturisation involves new things to consider. A lot of thought has to go into actual implementations as well as design. Let us look at some of the factors involved ...

1. Circuit Delays. Large complicated circuits running at very high frequencies have one big problem to tackle - the problem of delays in propagation of signals through gates and wires ... even for areas a few micrometers across! The operation speed is so large that as the delays add up, they can actually become comparable to the clock speeds.

2. Power. Another effect of high operation frequencies is increased consumption of power. This has two-fold effect - devices consume batteries faster, and heat dissipation increases. Coupled with the fact that surface areas have decreased, heat poses a major threat to the stability of the circuit itself.

3. Layout. Laying out the circuit components is task common to all branches of electronics. Whats so special in our case is that there are many possible ways to do this; there can be multiple layers of different materials on the same silicon, there can be different arrangements of the smaller parts for the same component and so on.

The power dissipation and speed in a circuit present a trade-off; if we try to optimise on one, the other is affected. The choice between the two is determined by the way we chose the layout the circuit components. Layout can also affect the fabrication of VLSI chips, making it either easy or difficult to implement the components on the silicon.

VLSI basics

What is VLSI?


VLSI stands for "Very Large Scale Integration". This is the field which involves packing more and more logic devices into smaller and smaller areas.Thanks to VLSI, circuits that would have taken boardfuls of space can now be put into a small space few millimeters across! 

This has opened up a big opportunity to do things that were not possible before. VLSI circuits are everywhere ... your computer, your car, your brand new state-of-the-art digital camera, the cell-phones, and what have you. All this involves a lot of expertise on many fronts within the same field, which we will look at in later sections.

VLSI has been around for a long time, there is nothing new about it ... but as a side effect of advances in the world of computers, there has been a dramatic proliferation of tools that can be used to design VLSI circuits. 

Alongside, obeying Moore's law, the capability of an IC has increased exponentially over the years, in terms of computation power, utilisation of available area, yield. The combined effect of these two advances is that people can now put diverse functionality into the IC's, opening up new frontiers. 

Examples are embedded systems, where intelligent devices are put inside everyday objects, and ubiquitous computing where small computing devices proliferate to such an extent that even the shoes you wear may actually do something useful like monitoring your heartbeats! 

Breakthroughs in ICs over a decade

We know that the advancement in the IC technology changed the world and it is the main reason for the portable devices available now-a-days.

First Integrated Chip

In the below animations, you can see what happened in the past years after the first IC has been manufactured.


Animation 1
Animation 2

ASICs vs FPGA

FPGA	ASIC
Definition A semiconductor device containing programmable logic components called "logic blocks", and programmable interconnects. Logic blocks can be programmed to perform the function of basic logic gates such as AND, and XOR, or more complex combinational functions such as decoders or mathematical functions.	Definition An integrated circuit designed for a particular use, rather than intended for general-purpose use. Processors, RAM, ROM, etc are examples of ASICs.
Speed Less speed.	Speed ASIC rules out FPGA in terms of speed. As ASIC are designed for a specific application they can be optimized to maximum, hence we can have high speed in ASIC designs. ASIC can have hight speed clocks.
Cost FPGAs are cost effective for small applications.	Cost For complex and large volume designs (like 32-bit processors) ASIC products are cheaper.
Size/Area FPGA contains lots of LUTs, and routing channels which are connected via bit streams(program). As they are made for general purpose and because of re-usability. They are in-general larger designs than corresponding ASIC design.	Size/Area  Optimised , so smaller
Power FPGA designs consume more power than ASIC designs. The unwanted circuitry results in wastage of power. FPGA wont allow us to have better power optimization.	Power When it comes to ASIC designs we can optimize them to the fullest.
Time to Market FPGA designs will take less time, as the design cycle is small when compared to that of ASIC designs.	Time to Market ASIC designs will take more time
Type of Design It is not possible to design mixed signal designs or only analog designs using FPGA chips.	Type of Design ASIC can have mixed-signal designs, or only analog designs
Customization FPGA has less customization.	Customization ASIC has the upper hand when comes to the customization. The device can be fully customized as ASICs will be designed according to a given specification.
Prototyping Because of re-usability of FPGAs, they are used as ASIC prototypes. ASIC design HDL code is first dumped onto a FPGA and tested for accurate results. Once the design is error free then it is taken for further steps. Its clear that FPGA may be needed for designing an ASIC.
Non Recurring Engineering/Expenses NRE refers to the one-time cost of researching, designing, and testing a new product. No such thing is associated with FPGA. Hence FPGA designs are cost effective.	Non Recurring Engineering/Expenses NRE  is generally associated with ASICs.
Design Cycle  Due to software that handles much of the routing, placement, and timing, FPGA designs have smaller design cycle than ASICs.	Design Cycle  ASICs have larger design cycle.
More Predictable Project Cycle Due to elimination of potential re-spins, wafer capacities, etc. FPGA designs have better project cycle.
Tools Tools which are used for FPGA designs are relatively cheaper than ASIC designs.
Re-Usability A single FPGA can be used for various applications, by simply reprogramming it (dumping new HDL code). By definition ASIC are application specific cannot be reused.

VLSI Design Animation

Please CLICK HERE to watch a fantastic animation of the Complete VLSI Design Process.

All the steps from Wafer preparation to Packaging is explained here.

Courtesy: http://www.fsi-intl.com/