Course

Digital Electronics: from silicon to the logic gate

Name: Digital Electronics: from silicon to the logic gate
Price: 1999 BDT

Go inside the crystal to see where carriers come from, why a junction conducts in only one direction, and how a third terminal takes control of a current. Built on simulations, not slides.

42 lessons3h 45mEnglish

What you'll learn

Explain an electric current as a slow drift on top of fast random motion, and resistance as collisions
Say why pure silicon conducts poorly and what thermal energy changes
Predict which carrier a doped sample uses, and how many carriers doping creates
Identify the carrier type in a real sample from the sign of its Hall voltage
Describe the depletion region as fixed ions left behind, not as an empty gap
Predict how forward and reverse bias change a junction, and read the result on a diode curve
Explain how gate voltage forms a channel, and locate threshold voltage on a transfer curve
Contrast field control in a MOSFET with current control in a BJT, and justify which one logic uses
Explain why a resistor pull-up inverter wastes power and how the complementary pair removes it
Read a CMOS gate from its wiring, using series for AND and parallel for OR
Show that NAND alone can build every logic function, including XOR
Explain why a logic level depends on supply margin and what happens when the rail sags

Requirements

Comfortable with voltage, current and resistance at school level
Able to read a simple circuit diagram
No prior semiconductor physics is assumed

Course content

42 lessons · 3h 45m

Inside the crystal

Where a current actually comes from inside a solid, why pure silicon barely conducts, what doping changes, and how an experiment reveals which carrier is moving. Everything later in the course rests on these four ideas.

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What a current is inside a wire

Electrons are already moving fast. Voltage adds a slow drift, and that drift is the current.

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Why pure silicon barely conducts

Every outer electron is locked in a covalent bond, so almost nothing is free to drift.

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Doping: adding carriers on purpose

One replacement atom creates one carrier, and the count is set by chemistry rather than heat.

Which carrier is actually moving

Electrons and holes bend to the same edge but leave opposite charge, so the voltage sign tells you which.

Chapter check: carriers and conduction

Four questions on drift, bonding, doping and the Hall measurement.

The one-way valve

Join p-type silicon to n-type and the carriers themselves build a barrier. That barrier is the reason a diode passes current in one direction and blocks it in the other, and it is the first device in the course.

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The junction builds its own barrier

Carriers diffuse across, recombine, and leave fixed charged ions that oppose any further crossing.

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Bias makes the barrier move

Forward bias narrows the depletion region until carriers flood across. Reverse bias widens it.

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The curve a one-way valve produces

Why the forward knee sits near 0.7 V and why the reverse side is almost flat.

Putting the diode in a circuit

A diode is not a resistor. It drops a roughly fixed voltage, and the series resistor sets the current.

Chapter check: the junction and the diode

Three questions on the depletion region, bias direction and the diode drop.

The valve someone else controls

A diode decides for itself. A transistor lets a third terminal decide, and a small voltage there commands a large current. This is the single idea the whole of computing is built on.

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A third terminal, and a channel that appears

Gate voltage pulls a layer of electrons into the p-type body, bridging source and drain.

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Steering a real current

The gate draws almost no current, yet it controls the lamp. That gain is the point.

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Threshold, read off the curve

Below threshold nothing flows. Above it the current climbs steeply, and the curve shows where.

The mirror image: PMOS

Swap every carrier and the device turns on when the gate goes low instead of high.

Chapter check: the MOSFET

Three questions on the channel, the threshold and the PMOS mirror.

The current-controlled twin

The bipolar transistor reaches the same result by a different route. It is commanded by a small base current rather than by a field, and seeing why the base must be thin explains where its gain comes from.

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Control by current, not by field

Two junctions back to back, and carriers injected from the emitter that mostly survive the crossing.

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Why the base has to be thin

Only the carriers that recombine cost base current, so a thin base means a large gain.

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The PNP mirror

Swap every layer and every carrier, and the currents run the other way.

Choosing between the two transistors

A free input decides it. The MOSFET wins in logic because its gate costs almost nothing to hold.

Chapter check: the BJT

Three questions on injection, base width and the choice between device families.

From a transistor to a logic gate

Arrange transistors so the output is forced high or low and a physical device starts behaving as a Boolean one. The first attempt works and wastes power, and fixing that waste is what produced CMOS.

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Inversion from a single transistor

An NMOS pulls the output down, a resistor pulls it up, and the result is a NOT gate.

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The power this design wastes

Whenever the output is low, current runs straight from the supply to ground through the resistor.

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Two transistors, and no standing current

Replace the resistor with a PMOS, and exactly one of the pair conducts in either state.

Why the transition is sharp

The transfer curve is flat, then steep, then flat. That shape is what makes a value digital.

Chapter check: building an inverter

Three questions on pull-up, static power and the shape of the transfer curve.

The universal gates, and their limits

Two more arrangements of the same complementary pair give NAND and NOR, and either one alone can build every logic function there is. The chapter closes by showing what happens when the supply can no longer support a valid level.

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Four transistors become a NAND

Two NMOS in series, two PMOS in parallel, and the output falls only when both inputs are high.

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One gate is enough for everything

XOR built from four NANDs is the proof that a single gate type spans all of logic.

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Swap series for parallel and get NOR

The same four transistors in the opposite arrangement produce the other universal gate.

When a logic level stops being valid

Drop the rail toward the threshold and the output can no longer swing. A 1 is a margin, not a symbol.

Chapter check: universal gates

Three questions on the NAND network, universality and supply margin.

On the bench

The same logic on a real 74HC chip, where the supply has polarity, unused inputs cannot be left floating and the noise margin is a fraction of a rail that sags. Every failure here is one a simulation never shows.

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Power before logic

Find the notch, count from pin 1, and wire the supply. An unpowered output is not low, it is floating.

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Build AND from a chip that has no AND

A NAND with its inputs tied is an inverter, so two gates and one jumper turn a 7400 into an AND gate.

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An unused input is still an input

A floating CMOS input drifts to mid-rail and turns both transistors partly on, restoring the waste CMOS existed to remove.

What an output can actually drive

A logic output is a supply with a 4 mA ceiling, so the series resistor is chosen from the current you want, not left to the pin.

Chapter check: on the bench

Four bench faults: reversed supply, reading the wrong pin, floating inputs and a sagging rail.

About the creator

Classytic