BJTs, FETs, MOSFETs, SCR & TRIAC

NPN and PNP Transistor Operation

A BJT is a sandwich of three doped regions forming two P-N junctions: emitter (heavily doped carrier source), base (whisper-thin middle layer), and collector (the large region that gathers carriers). The magic is the thin base: carriers injected from the emitter mostly fly straight through it into the collector.

NPN in Action (the common case)

1. Raise the base ≈0.7 V above the emitter — the base-emitter junction forward biases exactly like a diode.
2. Electrons flood from the emitter into the paper-thin base. A tiny fraction exits the base lead as base current I_B.
3. The vast majority — typically 99% — are swept across the reverse-biased base-collector junction into the collector: I_C = β·I_B.

PNP is the exact mirror: the emitter sits at the higher potential, the base is pulled ≈0.7 V below the emitter to turn on, and conventional current flows emitter→collector. PNPs are the natural choice for switching loads on the positive rail (high-side switching).

The Three Operating Regions = The Three Jobs

The Three BJT Configurations

Any amplifier uses two terminals for input and two for output — so one of the transistor's three leads must be shared ("common"). Which lead you share completely changes the amplifier's personality.

How to Remember Them

• Common Emitter — "the amplifier": the only configuration with both voltage and current gain, so it's the default. Its 180° inversion matters when you trace signals: input rises, output falls.
• Common Collector — "the muscle": the output faithfully follows the input voltage (hence emitter follower) but with β-times the current behind it. Placed between a delicate source and a heavy load — like a speaker or a long cable.
• Common Base — "the specialist": rare in general work; shines at radio frequencies where its geometry avoids the internal feedback that destabilizes CE stages.

DC Load Line Analysis

The transistor's characteristic curves tell you what the device can do; the resistor and supply decide what the circuit allows. The DC load line is where those two constraints meet — a single straight line containing every operating point the circuit permits.

Drawing the Line (two points, 30 seconds)

1. Cutoff point: no collector current → no drop across R_C → V_CE = V_CC. Mark (V_CC, 0).
2. Saturation point: transistor fully on (≈0 V across it) → all of V_CC is across R_C → I_C = V_CC/R_C. Mark (0, V_CC/R_C).
3. Connect them. Done.

Finding the Q-Point

The circuit must satisfy the load line and the transistor's curve for the actual base current. Their intersection is the Q-point — the circuit's resting state.

Worked example: V_CC = 20 V, R_C = 2 kΩ, β = 100, I_B = 30 µA. Load line runs (20 V, 0) to (0, 10 mA). I_C = 100 × 30 µA = 3 mA, so V_CE = 20 − (3 mA)(2 kΩ) = 14 V. Q = (14 V, 3 mA) — comfortably in the active region.

• Amplifier bias: park Q near the middle of the line so the signal can swing both directions without hitting cutoff or saturation (clipping).
• Switch bias: deliberately overdrive the base so Q slams between the two ends of the line — fully off or fully on, never lingering in the lossy middle.

JFETs: The Voltage-Squeezed Channel

A JFET is conceptually a doped resistor with a valve around it. Current flows source→drain through a channel of N (or P) material. The gate forms a reverse-biased junction wrapped around that channel: increasing reverse gate voltage widens the junction's depletion region, squeezing the channel narrower until — at the pinch-off voltage V_P — it closes entirely.

The Two Headline Facts

• Normally ON: with V_GS = 0 the channel is wide open, conducting its maximum current I_DSS. Gate voltage only ever reduces current (depletion mode).
• Essentially zero gate current: the gate junction is always reverse biased, so input resistance is hundreds of megohms — the JFET loads its signal source almost not at all.

Where a Technician Meets JFETs

• Instrument and sensor inputs: oscilloscope front ends, pH meters, condenser microphone preamps — anywhere the source can't supply current.
• Low-noise audio stages and analog switches/choppers.
• Constant-current sources: a JFET with gate tied to source self-regulates at I_DSS.

MOSFETs: The Modern Default

The MOSFET insulates its gate from the channel with a nanometers-thin layer of silicon dioxide — the gate is one plate of a capacitor. Gate current is truly zero; the gate voltage's electric field creates or destroys the channel beneath the oxide.

Enhancement Mode (the one you'll actually service)

With no gate voltage there is no channel — the device is OFF. Raise V_GS past the threshold voltage V_TH (1–4 V) and the field pulls electrons to the surface, forming ("enhancing") a conductive channel. More gate voltage → wider channel → lower resistance:

Why MOSFETs Took Over Power Switching

• Drive is nearly free: no continuous gate current — a logic pin can hold a 50 A switch on (it only supplies brief pulses to charge the gate capacitance).
• Cool conduction: a saturated BJT always drops ≈0.2 V (0.2 V × 20 A = 4 W of heat); a 5 mΩ MOSFET at 20 A drops 0.1 V (2 W) — and modern parts do far better.
• Speed: no stored base charge to sweep out, so MOSFETs switch in nanoseconds — the enabler of switching power supplies and PWM motor drives.

JFET vs. MOSFET at a Glance

SCRs: The Latching Power Switch

The Silicon Controlled Rectifier is a four-layer PNPN device — think of it as a diode with a trigger. Off, it blocks in both directions. Pulse the gate while the anode is positive and internal regenerative feedback snaps it fully on in microseconds. Then comes the defining behavior:

The Three Numbers on Every SCR Datasheet

• Gate trigger current (I_GT): the pulse needed to fire it — often just 5–50 mA controlling hundreds of amps.
• Latching current: the anode current that must be reached during the gate pulse or the SCR falls back off.
• Holding current (I_H): the minimum anode current that keeps it on afterward.

Where Technicians Meet SCRs

• Controlled rectifiers: replacing diodes in a bridge so the DC output is adjustable — battery chargers, DC motor drives, welders, electroplating supplies.
• Crowbar overvoltage protection: a Zener senses overvoltage and fires an SCR straight across the supply, blowing the fuse and saving the load — the latch is the feature.
• Soft-start and phase-controlled heaters at industrial power levels (SCRs scale to thousands of volts and amps).

TRIACs and Phase Control

An SCR wastes half of every AC cycle — it's a rectifier at heart. The TRIAC fuses two SCRs in antiparallel with a single gate, conducting in both directions between MT1 and MT2. One small gate pulse per half-cycle gives complete AC power control.

Phase (Firing-Angle) Control

On AC, every zero crossing turns the TRIAC off for free. Power control becomes a question of when in each half-cycle you fire it:

• Fire at 10°: the load receives almost the entire half-cycle — near full power.
• Fire at 90°: the load gets the second half of each hump — about 50% power.
• Fire at 160°: only a sliver conducts — a dim glow.

A simple RC network delays a sample of the line voltage; when it reaches the DIAC's ±32 V breakover, the DIAC dumps a crisp pulse into the TRIAC gate. Turning the potentiometer changes the RC delay — that potentiometer is the dimmer knob on your wall.

Applications and Limits

• Lamp dimmers, heater controls, small universal-motor speed controls (drills, fans, mixers) up to roughly 40 A.
• Solid-state relays (SSRs): an optocoupler (Lesson 2!) firing a TRIAC — logic-controlled, isolated AC switching with no moving parts.
• Snubbers: an RC network across the TRIAC prevents false triggering from fast voltage spikes (dV/dt) — a missing or failed snubber is a classic cause of a dimmer that flickers or self-fires.
• For very large loads, designers return to back-to-back SCR pairs, which outrun TRIACs in ratings.

Choosing the Right Device: A Technician's Decision Guide

Every device in this lesson switches or amplifies. The differences that matter on the bench are what controls it, whether it latches, and how much power it moves.

Fast Selection Rules

• Amplifying a signal? BJT (or small-signal FET for high-impedance sources). Thyristors need not apply — they only know ON and OFF.
• Switching DC efficiently or quickly (PWM)? MOSFET, almost without exception in modern designs.
• Switching AC power proportionally? TRIAC (with DIAC trigger) up to ~40 A; back-to-back SCRs beyond.
• Need the switch to stay on after a momentary trigger — alarms, crowbars, controlled rectifiers? SCR: the latch is the point.
• Driving a relay or LED from a logic pin? Small NPN (2N3904-class) or logic-level MOSFET — either works; the MOSFET wastes no drive current.