9.2 FETs, LEDs & Specialty Devices

9.2 FETs, LEDs & Specialty Devices

Quick Answer: FETs are voltage-controlled with high input impedance (BJTs are current-controlled with low Z_in). JFETs = N-channel or P-channel. MOSFET/CMOS gates fear static; many MOSFETs have a gate zener. LEDs need forward bias, color from materials, ≈ 20 mA, protected by a series resistor. LED + photodiode in a sealed package = optoisolator. CMOS = complementary metal-oxide semiconductor. Piezoelectric crystals vibrate when voltage is applied; a relay is a remotely controlled switch; Pierce is the classic crystal oscillator.

If §9.1 was all about current-controlled BJTs, this section adds voltage-controlled FETs, light-emitting and optical devices, and the mixed “devices” bag of key topic 028—exactly the parts you meet on modern marine and aviation radio boards.

Field-effect transistors — voltage control

A field-effect transistor (FET) controls drain-to-source current with an electric field at the gate. Gate current is ideally tiny, so the input looks almost open-circuit to DC—very different from a BJT base that must draw continuous current.

How does the input impedance of a field-effect transistor compare with that of a bipolar transistor? An FET has high input impedance; a bipolar transistor has low input impedance.

That single contrast explains why FETs star as RF front-end amplifiers, high-impedance buffers, and CMOS logic inputs, while BJTs remain common where current drive and predictable β matter.

JFET types

The two basic types of junction field-effect transistors: N-channel and P-channel.

JFETs are normally on at zero gate-source voltage and are pinched off by reverse gate bias (depletion-mode behavior). They appear in low-noise RF preamps and analog switches in legacy and some current radio designs.

MOSFET, depletion vs enhancement (technician view)

MOSFET = metal-oxide-semiconductor FET. A thin oxide insulator separates the gate electrode from the channel, giving extremely high DC gate resistance—and extreme static sensitivity.

Most power switching MOSFETs in supplies and PA drivers are enhancement-mode. RF power LDMOS and small-signal MOSFET RF amps follow the same high-Z_in story with careful matching networks.

MOSFET gate protection and CMOS

MOSFETs are manufactured with this protective device built into their gate to protect the device from static charges and excessive voltages: a zener diode (often a dual-zener clamp structure to ground/source).

Why are special precautions necessary in handling FET and CMOS devices? They are susceptible to damage from static charges. Body voltage, carpet shuffle, or an ungrounded iron tip can punch through the gate oxide forever—even if the part still “sort of works” with shifted thresholds.

What do the initials CMOS stand for? Complementary metal-oxide semiconductor. CMOS logic pairs complementary N- and P-channel MOSFETs for low static power—the backbone of modern synthesizer, microcontroller, and DSP sections in radios.

LEDs — light from a forward-biased junction

What type of bias is required for an LED to produce luminescence? Forward bias.

Reverse-biasing an LED does not light it (and may damage low-PIV optical diodes). Electrons and holes recombine in the junction and release photons—the solid-state version of “glow.”

What determines the visible color radiated by an LED junction? The materials used to construct the device (bandgap of the semiconductor alloy), not the color of a plastic lens alone, and not simply “more voltage means redder.” Voltage and current affect brightness and can shift spectra slightly, but Element 3’s correct selector is materials.

What is the approximate operating current of a light-emitting diode? 20 mA.

What would be the maximum current to safely illuminate an LED? 20 mA (same pool figure—treat ~20 mA as the standard small indicator LED design point unless a datasheet says otherwise).

Series resistor — always

What circuit component must be connected in series to protect an LED? A series resistor.

An LED’s forward voltage is roughly fixed (often ~1.2 V IR, ~1.8–2.2 V red/yellow, ~3+ V blue/white). Without a resistor, supply voltage minus V_F across a nearly vertical I–V curve produces runaway current and a dead LED.

Worked example — series resistor sizing

Panel LED on a 12 V ship’s DC bus; red LED V_F ≈ 2.0 V; design current 20 mA.

[ R = \frac{V_{\mathrm{supply}} - V_F}{I} = \frac{12 - 2.0}{0.020} = 500,\Omega ]

Use 470 Ω or 510 Ω standard value; check resistor power: (P = I^2 R \approx 0.2,\mathrm{W}) → ½ W part is comfortable. Same math applies on 5 V logic rails with smaller R.

Photodiodes, optoisolators, and optical isolation

An LED facing a photodiode in a light-tight enclosure is commonly known as an optoisolator (opto-coupler). Light carries the signal across an electrical isolation barrier—no shared copper path for ground loops or dangerous voltages.

GROL maintainers use optoisolators in interface boards between ship systems, computer controllers, and RF decks so a fault on one side does not inject destructive voltage on the other.

Forward-biased diode junction (device context)

What describes a diode junction that is forward biased? It is a low impedance.

Forward bias collapses the barrier; current flows easily. Reverse bias → high impedance (until breakdown). That rule applies to ordinary silicon diodes, the BE junction of a BJT, and the LED when lit. When troubleshooting, a forward diode drop of ~0.7 V silicon (or LED V_F) across a junction that should conduct is a healthy sign; open (infinite) or short (0 V both ways) is not.

Varactors — voltage-variable capacitance (RF tuning)

A varactor (varicap) diode is used for voltage-controlled variable capacitance. Reverse bias width-modulates the depletion region; capacitance falls as reverse voltage rises. Synthesizers, VCOs, and automatic tuning networks in marine/aviation radios use varactors so a DC control voltage steers frequency without a mechanical capacitor. Remember: varactors are reverse-biased in normal RF use—not forward-lit like LEDs.

Specialty devices from key topic 028

Piezoelectric effect

What is the piezoelectric effect? Mechanical vibration of a crystal by the application of a voltage (and the inverse: mechanical stress producing voltage). Quartz crystals in oscillators rely on this electromechanical resonance for stable frequency references in transmitters, receivers, and GPS-disciplined gear.

Relays

An electrical relay is a remotely controlled switching device. Coil current creates a magnetic field that opens or closes contacts—isolation between control circuit and switched RF/power paths. Antenna TR relays, band-switch relays, and fault contactor relays are everyday GROL territory. A relay is not primarily a current-limiting resistor or multi-voltage regulator.

Crystal oscillators — Pierce

In which oscillator circuit would you find a quartz crystal? The Pierce oscillator is the classic textbook crystal configuration (crystal between gate/drain or collector/base style nodes with capacitors to ground). Hartley and Colpitts are LC oscillator families that can be crystal-controlled in variants, but Element 3 points you at Pierce as the crystal-associated answer when that name appears among the options.

Putting §9.2 together on a radio chassis

Walk a modern VHF marine transceiver mentally:

1. Front-end / RF switch FETs — high Z, voltage control, ESD discipline when you probe.
2. CMOS synthesizer / MCU — complementary MOSFET logic; static handling rules.
3. Panel LEDs — forward bias, 20 mA class, series resistors on the 5 V or 12 V rail.
4. Optoisolators on accessory ports — LED/photodiode isolation from ship grounds.
5. Varactors in the VCO — reverse bias tuning voltage from the PLL.
6. Quartz reference — piezoelectric resonator, often in a Pierce-style gate oscillator.
7. TR or band relay — remote coil, switched RF contacts.

When Element 3 asks about luminescence bias, 20 mA, series resistors, CMOS expansion, static on FETs, low impedance of a forward diode, or remote switching, this section is the map. The next section turns device knowledge into selection, ratings, thermal design, and substitution—the maintainer’s daily decisions.