Over the past five decades, microelectronics has revolutionized our lives. Cellphones, digital cameras, and laptop computers, once beyond the realm of possibility, are now integral parts of our daily affairs.
This lesson will provide an overview of microelectronics, exploring the fundamental devices, circuits, and systems that power the modern world.
Learning microelectronics can be fun. As we learn how each device operates, how devices form circuits that perform useful functions, and how circuits form sophisticated systems, we begin to see its beauty and appreciate the reasons for its explosive growth.
We'll look at key systems and identify the important circuit functions they employ.
The field of electronics began about a century ago. Early systems used "vacuum tubes" as amplifying devices. They operated by controlling the flow of electrons between plates in a vacuum chamber.
However, vacuum tubes had a finite lifetime, were large, and consumed a lot of power, motivating the search for a better alternative.
The first transistor was invented in the 1940s and rapidly displaced vacuum tubes. It was a true game-changer.
Compared to vacuum tubes, transistors offered a very long (in principle, infinite) lifetime and were much, much smaller. This invention marked the beginning of modern electronics.
It wasn't until the 1960s that the field of microelectronics began. This is the science of integrating many transistors onto a single chip.
Early "integrated circuits" (ICs) contained only a handful of devices, but technology advanced rapidly, allowing for a dramatic increase in the complexity of "microchips."
Today's microprocessors contain about 100 million transistors. What if we tried to build this with discrete, individual transistors instead of an integrated circuit?
If each discrete transistor occupies a volume of 3mm x 3mm x 3mm, the processor's minimum volume would be 27 cubic meters. That's a cube about 3 meters on each side!
A processor built from discrete components would be incredibly slow, as signals would need to travel over long wires. It would also be prohibitively expensive and heavy.
If each transistor cost 1 cent and weighed 1 gram, a 100-million-transistor processor would cost one million dollars and weigh 100 tons! Microelectronics solved these problems.
Let's explore how the concepts of microelectronics apply to a real-world system like a cellphone.
When you speak, a microphone converts your voice to an electrical signal. After processing in the "Transmitter" (TX), it's sent out via the antenna. A friend's phone picks up the signal, processes it in the "Receiver" (RX), and plays it through a speaker.
Why do we need the transmitter and receiver "black boxes"? For an antenna to work efficiently, its size must be a significant fraction of the signal's wavelength.
Human voice frequencies (20 Hz - 20 kHz) have enormous wavelengths (15 km - 15,000 km), requiring gigantic antennas. To use a small antenna (e.g., 5 cm), we need a much higher frequency, around 1.5 GHz.
To solve the antenna problem, we must "convert" the low-frequency voice signal to a high frequency. This is done through modulation.
One method is to multiply the voice signal with a high-frequency sine wave (a "carrier"). In the frequency domain, this action shifts the entire voice spectrum up to be centered around the carrier's frequency.
Based on our modulation solution, the transmitter's "black box" must contain several key components:
Now let's look at the receiver. If the signal from the antenna, which is centered at a gigahertz frequency, were sent directly to the speaker, it would produce no meaningful sound.
The receiver's job is to translate the spectrum from the high carrier frequency back down to the original voice band.
To restore the original voice signal, a process called demodulation is used. It's surprisingly similar to modulation.
The received signal is multiplied by the exact same high-frequency carrier wave. This action shifts the spectrum back down to the voice band. The unwanted high-frequency components are then removed with a low-pass filter.
The receiver's signal is incredibly weak (microvolts). It needs significant amplification before it can drive a speaker (millivolts).
A complete receiver path includes a Low-Noise Amplifier (LNA) to boost the weak signal without adding much noise, followed by the demodulation multiplier and the low-pass filter.
Digital cameras convert light into electricity using an array of light-sensitive elements called "pixels."
Each pixel contains a photodiode, which produces a current proportional to the intensity of the light it receives. This current charges a small capacitor, developing a voltage that represents the brightness at that point.
A modern camera has millions of pixels arranged in a grid. To read the voltage from each pixel without needing millions of circuits, the camera processes them in a shared, sequential manner.
Imagine a chessboard pattern of light falling on the array. Each pixel now holds a voltage corresponding to black or white.
To read the image, the camera activates one column at a time. Within that column, it turns on a switch for each pixel one-by-one, from top to bottom.
The voltage from the selected pixel travels down a common wire. This process generates an analog voltage waveform that represents the light pattern in that column.
The analog voltage from the pixel array must be converted into digital data (1s and 0s) for storage and processing. This is the job of an Analog-to-Digital Converter (ADC).
The ADC takes the continuous analog waveform and converts it into a stream of discrete binary numbers, which can then be processed by a Digital Signal Processor (DSP).
Why do we prefer to process signals digitally? A key reason is robustness against noise and distortion.
Analog signals are sensitive; any added noise directly corrupts the information. Digital signals, however, only have two valid levels (ONE and ZERO). As long as noise isn't large enough to flip a ONE to a ZERO, the information remains intact.
Even though digital is robust, as speeds increase, digital signals start to behave like analog ones. At low speeds (e.g., 100 Mb/s), a digital waveform has sharp, clean edges.
But at very high speeds (e.g., 1 Gb/s), the physical limitations of circuits cause the rise and fall times to become significant. The waveform looks more rounded, requiring careful analog design expertise to manage.
An analog signal is a waveform that carries information. It is continuous in both time and value, meaning it can assume any value within a given range at any moment.
Natural signals like voice, video, and music are analog. They are difficult to process and store perfectly due to their sensitivity to noise and distortion.
A digital signal assumes only a finite number of values at specific points in time. The most common form is a binary signal, which has only two levels: ONE and ZERO.
Digital signals are robust because logic circuits can correctly interpret the levels even in the presence of some noise. They are also much easier to store reliably in digital memory.
The most common analog function is amplification. An amplifier takes a small input signal and produces a larger output signal. The ratio of output to input voltage is its gain.
An amplifier's performance is also defined by its speed (bandwidth) and power dissipation. At high frequencies, capacitances in the circuit cause the gain to "roll off," limiting the usable bandwidth.
Digital microelectronics deals with the design of logic gates, flip-flops, and other components using transistors.
For example, a NOT gate inverts its input (A becomes NOT A), and a NOR gate produces a high output only if both of its inputs are low (Y = A+B).
We analyze these circuits to determine their speed, power consumption, and robustness to noise.
In this introduction, we've journeyed from the basics of electronics to the complex systems that define our modern world.
Key takeaways include: