Every processor, memory chip and power supply is built from the devices in this subject. Digital logic hides them behind gates, but underneath, every gate is a handful of transistors. This article revisits the five units of the JNTUK course: how semiconductors work, how diodes and transistors behave, and how they are used for rectifying, switching and amplifying.
Unit 1: Semiconductors and the PN junction diode
Semiconductor basics
A semiconductor such as silicon conducts better than an insulator but worse than a metal, and its conductivity can be controlled.
| Type | How it is made | Majority carriers |
|---|---|---|
| Intrinsic | Pure silicon | Equal electrons and holes |
| N-type | Doped with a donor (phosphorus, arsenic) | Electrons |
| P-type | Doped with an acceptor (boron) | Holes |
A hole is a missing electron that behaves like a positive carrier. Charge moves by drift (under an electric field) and diffusion (from high to low concentration). Conductivity rises with temperature, unlike a metal.
The PN junction
Joining P and N material makes a depletion region free of carriers, with a built-in barrier potential of about 0.7 V for silicon (0.3 V for germanium).
| Bias | Effect |
|---|---|
| Forward (P positive) | Barrier shrinks; current rises sharply once voltage exceeds about 0.7 V |
| Reverse | Barrier grows; only a tiny leakage current flows |
The diode equation is I = Is (eV/(ηVT) − 1), with thermal voltage VT about 26 mV at room temperature. In reverse bias, too much voltage causes breakdown: Zener breakdown (low voltage, field-driven) or avalanche breakdown (higher voltage, collision-driven).
Diode models and capacitance
The ideal diode is a perfect one-way switch. The practical model adds a 0.7 V drop and a small resistance. A diode also has junction capacitance: depletion (transition) capacitance in reverse bias, diffusion capacitance in forward bias.
Today: the LED is a diode that emits light, a photodiode senses it (cameras, optical drives), and a solar cell is a large PN junction. The 0.7 V drop is why a diode in a power path wastes energy as heat.
Unit 2: Diode applications: rectifiers, filters and special diodes
Rectifiers
A rectifier converts AC to pulsating DC.
| Type | Diodes | Output | Ripple factor |
|---|---|---|---|
| Half-wave | 1 | One half of each cycle | 1.21 |
| Full-wave (centre-tap) | 2 | Both halves | 0.48 |
| Bridge | 4 | Both halves, no centre tap | 0.48 |
Key values for a full-wave rectifier: Vdc = 2Vm / π, rectification efficiency about 81.2% (half-wave: 40.6%). The peak inverse voltage (PIV) is the largest reverse voltage a diode must withstand.
Filters
A filter smooths the pulsating DC. A capacitor filter charges to the peak and discharges slowly through the load; larger C and larger RL mean less ripple. Inductor, LC and π (CLC) filters give better smoothing. Ripple factor measures the remaining AC in the DC output.
Voltage regulation and special diodes
- Zener diode regulator: a Zener in reverse breakdown holds a nearly constant voltage across the load.
- Clippers cut off part of a waveform above or below a level; clampers shift the DC level of a waveform.
- Special diodes: the Schottky (metal-semiconductor, fast with low drop), varactor (voltage-controlled capacitance), tunnel diode (negative resistance), photodiode and LED.
Today: the adapter that powers your laptop is a bridge rectifier, a large capacitor and a regulator, usually in a switching form. Schottky diodes appear in power supplies, and Zener diodes protect inputs from voltage spikes.
Unit 3: Bipolar junction transistor (BJT)
A BJT has three layers and three terminals: emitter, base, collector. It comes as NPN or PNP. A small base current controls a much larger collector current, so it can switch or amplify.
Operation and key relations
For normal (active) operation, the emitter-base junction is forward biased and the collector-base junction is reverse biased.
| Relation | Value |
|---|---|
| Emitter current | IE = IB + IC |
| Current gain α | IC / IE, slightly less than 1 |
| Current gain β | IC / IB, typically 50 to 300 |
| Link | β = α / (1 − α) |
Configurations
| Configuration | Input / output | Gain | Typical use |
|---|---|---|---|
| Common base (CB) | Emitter / collector | Voltage gain, no current gain | High-frequency circuits |
| Common emitter (CE) | Base / collector | High voltage and current gain, 180° phase shift | General amplifier |
| Common collector (CC), emitter follower | Base / emitter | Voltage gain about 1, high current gain | Buffer, impedance matching |
Regions of operation
| Region | Junctions | Acts as |
|---|---|---|
| Cut-off | Both reverse | Open switch (OFF) |
| Saturation | Both forward | Closed switch (ON) |
| Active | EB forward, CB reverse | Amplifier |
Today: a transistor as a switch is the basis of digital logic. Older logic families (TTL) used BJTs directly; today BJTs live mainly in analog, power and RF circuits.
Unit 4: Biasing and small-signal amplifiers
Why biasing
Biasing sets the DC operating point (the Q-point) so the transistor stays in the active region for the whole input swing. A badly placed Q-point causes clipping and distortion. The Q-point can drift with temperature (thermal runaway: more heat, more current, even more heat), so a bias circuit also needs stability.
| Bias method | Idea | Stability |
|---|---|---|
| Fixed bias | One base resistor to VCC | Poor |
| Collector-to-base | Feedback from collector | Better |
| Voltage-divider (self) bias | Two resistors set the base voltage, emitter resistor adds feedback | Best; most widely used |
The stability factor S = ΔIC / ΔICO measures how much the collector current changes with the leakage current; smaller is better.
Small-signal analysis
For small AC signals the transistor is replaced by a simple equivalent circuit:
- The hybrid (h) parameters hie, hre, hfe, hoe describe a transistor as a linear two-port.
- The re model uses re = 26 mV / IE.
From the model you get voltage gain Av, current gain Ai, input resistance Ri and output resistance Ro. A typical CE amplifier has a large voltage gain and a moderate input resistance; the emitter follower has a very high input and low output resistance, which is why it is used to buffer a source.
Amplifier classes and frequency response
| Class | Conduction | Efficiency | Use |
|---|---|---|---|
| A | Full cycle | Low (25 to 50%) | Low distortion small signals |
| B | Half cycle | Up to 78.5% | Push-pull power amplifiers |
| AB | Slightly over half | Between the two | Audio output stages |
| C | Less than half | High | Radio-frequency tuned circuits |
The frequency response shows gain across frequencies: gain falls at low frequency (coupling and bypass capacitors) and at high frequency (junction and stray capacitance). The bandwidth lies between the half-power (−3 dB) points.
Today: the “amplifier” in a Wi-Fi radio, the microphone preamp in a laptop and the sensor interface on an IoT board are all small-signal stages. The biasing and thermal-stability ideas also explain why electronics are derated at high temperatures.
Unit 5: Field-effect transistors (FET, MOSFET) and special devices
A FET controls current with an electric field instead of a base current. It has very high input resistance because the gate draws almost no current, and only one type of carrier is involved (it is unipolar).
| Terminal | Role |
|---|---|
| Source | Where carriers enter |
| Drain | Where carriers leave |
| Gate | Controls the channel |
JFET
The JFET has a channel of N or P material with a reverse-biased gate junction. As the gate voltage becomes more negative (N-channel), the channel narrows. Key characteristics are the pinch-off voltage VP, the drain saturation current IDSS, and the transconductance gm = ΔID / ΔVGS. In saturation, ID = IDSS (1 − VGS/VP)².
MOSFET
The MOSFET insulates the gate with a thin oxide layer, so input resistance is extremely high.
| Type | Behaviour |
|---|---|
| Depletion mode | Channel exists at zero gate voltage; voltage can enhance or deplete it |
| Enhancement mode | No channel at zero gate voltage; a gate voltage above the threshold VT creates one |
The enhancement-mode MOSFET dominates modern electronics because it is simple to make and easy to turn fully on or off.
BJT versus FET and special devices
| Point | BJT | FET |
|---|---|---|
| Control | Current-controlled | Voltage-controlled |
| Input resistance | Low to moderate | Very high |
| Noise and size | More noise, larger | Less noise, smaller |
| Carriers | Two types (bipolar) | One type (unipolar) |
Other devices in the syllabus: the UJT (a relaxation oscillator and trigger device), the SCR (a latching switch for controlling AC power, used in dimmers and motor controls), and the photo-devices (photodiode, phototransistor, LDR, optocoupler).
Today: the CMOS logic in every CPU, GPU and memory chip uses pairs of N-type and P-type MOSFETs, which draw power mainly when switching. Billions of MOSFETs, made smaller each generation, are what “7 nm” and “3 nm” refer to. Power MOSFETs and their successors, such as GaN devices, are in the fast chargers and server power supplies.
One-minute recap
| Unit | Key idea | You meet it in |
|---|---|---|
| 1. PN junction | Doping, depletion region, 0.7 V forward drop | LEDs, solar cells, photodiodes |
| 2. Rectifiers and filters | Bridge rectifier, capacitor filter, Zener regulator | Laptop adapters, power supplies |
| 3. BJT | β = IC / IB; CB, CE, CC; cut-off, active, saturation | Amplifiers, drivers, analog circuits |
| 4. Biasing and amplifiers | Q-point, voltage-divider bias, h-parameters, classes A to C | Audio, radio and sensor interfaces |
| 5. FET and MOSFET | Voltage-controlled, very high input resistance | CMOS chips, power MOSFETs |
Next in Engineering Revisited: Mathematical Methods.
Written by Claude, Anthropic’s AI assistant, as part of Engineering Revisited, a quick refresher of the B.Tech subjects one at a time.