Almost a century ago, computers were built with large vacuum tubes and were very big, and by big,
I mean HUGE. Some computers, in fact, were large enough to occupy an entire room. Fast-forward
to 2018 and computers are smaller than a matchbox and can be strapped to our wrists.
This incredible evolution was made possible by the invention of a device no larger than
a hairclip… the transistor. The transistor is probably
the greatest invention of the last century or, perhaps, one of the greatest all time.
A Transistor is just three semiconductors glued to one another A semiconductor, as the name suggests,
is a material that conducts, but only partly. The conductivity of a semiconductor like silicon
or germanium lies between the conductivity of a metal, like copper, and an insulator, like rubber.
A semiconductor normally conducts at a high temperature, but we can also make it conduct
by injecting it with impurities. This is called doping and is done, just like when
it’s done in sports, to improve performance. A semiconductor is either doped with an atom
that carries an extra electron or with an atom that is deprived of an electron. The vacancies
left by migrated electrons are called holes and are considered to be positive charges.
The former then becomes an n-type semiconductor, while the latter becomes a p-type semiconductor.
It is critical to remember that the resulting semiconductors are not charged; they are,
in fact, electrically neutral. The donors, having donated electrons to the n-type semiconductor,
are now positive. Similarly, the acceptors having accepted electrons from the p-type semiconductor,
are now negative. Therefore, the semiconductors contain the same number of charges. However, they
are named so because of the type of charge that is free to move and carry the current in them.
When a p-type and an n-type semiconductor are sandwiched together, what is formed is a
diode. However, when an n-type semiconductor is sandwiched between two p-type semiconductors or
vice versa, a transistor is formed. A diode is a pipe through which a
constant current of water can flow in only a single direction. A transistor, however,
is like a valve; it allows us to control the current flowing through this pipe by rotating it.
In one of the most commonly used transistors, the source of water is formed by a highly doped
n-type semiconductor called the source, and the end of the pipe is formed by a lightly
doped n-type semiconductor called the drain. The valve is formed by a moderately doped
p-type semiconductor that is sandwiched between them. This is called the gate.
Now, because the semiconductors are electrically neutral, no current flows through them. We need
to create an excuse for their charges to move and therefore constitute a current.
When we apply a positive voltage to the gate, the electrons of the highly doped source are
lured towards it and the holes of the gate are repelled away from it. This gradually opens a
channel from the source to the drain, which the electrons cross, thereby constituting a current.
Applying a voltage to the gate is analogous to rotating the valve to remove the barrier
that prevents the water from flowing from the source and out from the pipe. By varying the
degree of its rotation, we can control the amount of water that flows in the pipe. This
is the working principle of a transistor. Now, we can design the valve to allow the
water to flow through it only when it’s rotated to a certain degree. When rotated
less than this threshold, it stifles the water flowing through the pipe. In this way,
a transistor acts as a switch. This is how binary numbers are generated. Each bit, “1” or “0”,
is either an open valve whose magnitude of current is standardized as “1”, or a closed valve,
which translates to “0”. The sequence of bits is then manipulated by microprocessors
to implement a myriad of operations. These switches are extremely fast,
but most importantly, extremely tiny. The computer on which you are watching this right now is
powered by a bed of silicon covered with billions of transistors thinner than a strand of hair.
The ridiculous scalability of transistors is dictated by Moore’s law, according to which the
number of transistors populating a microprocessor must double every year. Naturally, an increase in
transistors causes an increase in processing speed. Intel’s best chip in 1985 employed
less than a quarter of a million transistors, compared to the almost 3 billion transistors
that Intel’s best chip now employs. The increase in speed? Intel’s best chip in 1985 would take 4
and a half hours to process what Intel’s best chip now processes in a second. That’s speed.
If it wasn’t for this scalability, we couldn’t have developed billions of smaller,
faster and cheaper devices, including computers, telephones, GPS devices, flashlights, radios,
gaming consoles, televisions, routers and a device -- no larger than a human
hand -- that integrates all these indispensable technological marvels under the same screen.