Semiconductor Basics
PN junctions, diode characteristics, BJT operating regions, and MOSFET fundamentals — with diagrams. Sections 5 to 11 cover the rest of the semiconductor part of Competency 3: silicon and germanium diode analysis, zener ratings, varactors, bias polarities with alpha and beta, Darlington pairs, IGBTs and CMOS, IC types, packages and pin-outs, and relays. The passive parts of Competency 3 are taught on other guides: resistors (3.1), capacitors (3.2) and coils (3.3, with the core laminations of 3.4.2) in sections 8, 9 and 10 of the Electrical Theory guide; transformers (3.4) in sections 2, 7 and 9 of the Power Supplies guide; and diacs, triacs and SCRs (3.6) in section 12 of the Circuits guide.
On the exam from this page
- a conducting diode drops about 0.7 V (silicon) or 0.3 V (germanium); subtract that drop from the source before using Ohm's law, and its power is the drop times the current
- a zener regulates only when it is reverse-biased in breakdown; it is rated by its zener voltage at a test current, its tolerance and its power rating, and its maximum current is the power rating divided by the zener voltage
- a varactor is operated reverse-biased; raising the reverse voltage widens its depletion region and lowers its capacitance, so a voltage can tune a resonant circuit
- an enhancement MOSFET is normally off and needs a gate voltage beyond its threshold; a JFET and a depletion MOSFET conduct with zero gate voltage, and a reverse gate voltage pinches them off
- a Darlington pair multiplies the two transistors' current gains but drops two base-emitter voltages, about 1.4 V; an IGBT is driven like a MOSFET, with practically no gate current, and conducts like a bipolar transistor
- pin 1 is marked by a dot, notch or bevel, and the pins are numbered counter-clockwise from it as seen from the top of the package
- a relay's normal state is with its coil de-energized: normally-open contacts are then open and normally-closed contacts are closed, and a schematic draws them that way
- a solid-state relay has no moving contacts, so it switches fast without arcing or contact wear, but it leaks a little current when off, drops voltage and makes heat when on, and usually fails shorted
PN Junction
What is a PN Junction?
A PN junction forms when p-type silicon (doped with boron — extra holes) is joined to n-type silicon (doped with phosphorus — extra electrons).
At the boundary, electrons from the n-side diffuse into the p-side and recombine with holes, creating a depletion region that acts as a barrier to further flow.
Two different things are often both called "the barrier". The built-in potential is the voltage across the depletion region at rest; it is set by doping (roughly 0.6–0.9 V in silicon) and a meter cannot read it. The forward drop is what you measure across a conducting diode, about 0.6–0.7 V for silicon at ordinary currents. Exam questions mean the forward drop.
Forward vs Reverse Bias
Forward bias: Positive voltage on the p-side shrinks the depletion region. At ≈0.7 V (silicon) current flows freely.
Reverse bias: Positive voltage on the n-side widens the depletion region. Only tiny leakage current flows until breakdown voltage is reached.
BJT Operating Regions
NPN BJT — Four Operating Regions
Three terminals: Base (B), Collector (C), Emitter (E). For NPN: IC = β × IB in the active region.
| Region | B-E Junction | B-C Junction | Condition | Use |
|---|---|---|---|---|
| Cutoff | Reverse | Reverse | IB = 0, IC ≈ 0 | Switch OFF |
| Active (Linear) | Forward | Reverse | IC = β×IB | Amplifier |
| Saturation | Forward | Forward | VCE ≈ 0.2 V | Switch ON |
| Reverse Active | Reverse | Forward | IC = βR×IB | Rarely used |
BJT as Switch
To saturate an NPN (switch fully ON): drive enough base current so IC_desired / β < IB_actual. Exactly IC/β is the bare minimum and is not enough in practice, because β varies widely between parts and falls in saturation. Designers overdrive the base with a forced β of 10–20: IB ≈ IC/10 to IC/20.
IC = Vcc / RL (saturation)
VBE ≈ 0.7 V, VCE_sat ≈ 0.2 V
BJT as Amplifier
Common-emitter amplifier inverts the signal and provides voltage gain. Gain depends on collector resistor and transconductance. The formula assumes the emitter resistor is fully bypassed and there is no load; with a load, use RC in parallel with RL.
gm = IC / VT (VT ≈ 26 mV near room temperature; 25.7 mV at 25°C)
Av ≈ −RC × IC / 0.026
MOSFET Fundamentals
Enhancement N-Channel MOSFET
Gate voltage creates an electric field that induces a channel between Drain and Source. No DC gate current flows (oxide insulation). Key parameter: threshold voltage Vth.
Gate current ≈ 0 A (DC)
Vth typically 1–4 V (power MOSFETs)
MOSFET Operating Regions
| Region | Condition | Use |
|---|---|---|
| Cutoff | VGS < Vth | Switch OFF |
| Linear (Ohmic) | VGS > Vth and VDS < VGS−Vth | Switch ON |
| Saturation | VGS > Vth and VDS ≥ VGS−Vth | Amplifier |
P_sw ≈ ½ × VDS × ID × (tr + tf) × fsw (switching loss)
P_total ≈ P_cond + P_sw
Semiconductor Materials
| Material | Bandgap (eV) | Applications | Notes |
|---|---|---|---|
| Silicon (Si) | 1.12 | Most ICs, transistors, diodes, solar cells | Most widely used; excellent native oxide (SiO₂) |
| Germanium (Ge) | 0.66 | Early transistors, low-Vf diodes | Lower threshold; poor high-temp performance |
| GaAs | 1.42 | RF/microwave, LEDs, solar cells | High electron mobility; direct bandgap |
| SiC | 2.86–3.26 | High-power/high-temp power devices | Very high breakdown voltage |
| GaN | 3.4 | Power electronics, blue LEDs, 5G | Very high electron velocity |
Silicon and germanium diodes: bias, current and power
Competency 3.7.2 asks you to analyze the voltage bias, current and power of silicon and germanium diodes. Kuphaldt gives the nominal forward voltage of a conducting silicon diode as 0.7 V and of a germanium diode as only 0.3 V; the chemistry of the junction sets the figure, and it stays roughly constant over a wide range of currents. That makes a forward-biased diode easy to analyze: take its drop away from the source, and the rest of the voltage drives the current through whatever resistance is in series. Reverse-biased, it passes only leakage, so the whole source voltage appears across it.
| Silicon | Germanium | |
|---|---|---|
| Forward drop | About 0.7 V (Kuphaldt) | About 0.3 V |
| Current, 5 V and 1 kΩ | (5 − 0.7) / 1000 = 4.3 mA | (5 − 0.3) / 1000 = 4.7 mA |
| Diode power | P = VF × IF = 0.7 × 4.3 mA ≈ 3.01 mW | P = VF × IF = 0.3 × 4.7 mA ≈ 1.41 mW |
| Heat | The material most devices use | Poor high-temperature performance (section 4) |
Worked example
One loop, two diodes: a 5 V source drives a diode and a 1 kΩ resistor in series. With a silicon diode, I = (5 − 0.7) / 1000 = 4.3 mA, and the diode dissipates 0.7 × 0.0043 ≈ 3.01 mW. Swap in germanium and I = (5 − 0.3) / 1000 = 4.7 mA, with 0.3 × 0.0047 ≈ 1.41 mW in the diode. Turn either diode around and only its leakage flows: almost no current, and the full 5 V across the diode. A rectifier carrying 2 A at 0.7 V must shed 0.7 × 2 = 1.4 W as heat.Kuphaldt warns that a meter's diode-check reading is usually lower than 0.7 V or 0.3 V, because the meter's test current is tiny; section 15 of the Test Equipment guide covers testing diodes in and out of circuit.
On the exam
Exam point: a conducting diode drops about 0.7 V (silicon) or 0.3 V (germanium); subtract that drop from the source before using Ohm's law, and its power is the drop times the current. A reverse-biased diode passes only leakage, so it takes the whole source voltage.Zener diode ratings
Competency 3.7.3 asks you to explain zener diode ratings and their use in regulator circuits. Kuphaldt: a zener is built to break down in reverse bias without damage. Forward-biased it drops about 0.7 V like any diode; reverse-biased it conducts little until the applied voltage reaches its zener voltage, and then it conducts whatever current it must to hold that voltage, as long as its power rating is not exceeded. That is why it is always connected reverse-biased in a regulator. A datasheet rates it in these terms (values from Vishay's 1N4728A–1N4761A family):
| Rating | What it means | Example |
|---|---|---|
| Zener voltage | VZ, the nominal voltage, measured at a stated test current IZT, because VZ shifts a little with current and temperature | 1N4733A: 5.1 V at 49 mA |
| Tolerance | How far a part may sit from nominal. Vishay's standard is ±5%; Kuphaldt gives 5 to 10% as typical | 5.1 V ±5% is 4.845 to 5.355 V |
| Power rating | PZ, the heat the die and package can shed. Kuphaldt lists common 0.3 W and 1.3 W parts; Vishay's rating holds only with the leads 4 mm from the case kept at ambient, and its Fig. 1 plots the power allowed against ambient temperature, so check it for a hot enclosure | Vishay family: 1300 mW at 25 °C ambient |
| Maximum current | IZM, the most current it can carry: Vishay writes the limit as the power divided by VZ. Use the datasheet's own IZM when it gives one | 1N4733A: 178 mA |
| Dynamic resistance | ZZT: how much VZ moves as the current changes; lower regulates better | 1N4733A: 7 Ω at IZT |
In a regulator. A series resistor drops the difference between the supply and VZ, and the zener draws more or less of the resistor's current so the load voltage stays put; Kuphaldt likens it to controlling a car's speed with the brake. Two limits follow. The zener's current is the resistor's current minus the load's, and if the load draws more than the resistor can supply, the zener stops conducting and stops regulating. Kuphaldt adds that a zener destroyed by too much power usually fails shorted. Section 6 of the Power Supplies guide covers zener and IC regulators in supplies.
Worked example
Kuphaldt's regulator: a 12.6 V zener fed from 45 V through 1 kΩ. The resistor drops 45 − 12.6 = 32.4 V, so it carries 32.4 mA. With no load all of it flows in the zener: 12.6 × 0.0324 ≈ 0.408 W, so a 0.5 W zener will do, and the resistor dissipates 32.4 × 0.0324 ≈ 1.05 W, so it needs a 1.5 or 2 W rating. Add a 500 Ω load: it draws 12.6 / 500 = 25.2 mA, leaving 32.4 − 25.2 = 7.2 mA in the zener. A 12 V zener rated 0.5 W can carry at most 0.5 / 12 ≈ 41.7 mA.Trap
The zener current is not the resistor current: subtract the load current from the series-resistor current. In the example the resistor carries 32.4 mA, but with the load connected the zener carries only 7.2 mA.On the exam
Exam point: a zener regulates only when it is reverse-biased in breakdown; it is rated by its zener voltage at a test current, its tolerance and its power rating, and its maximum current is the power rating divided by the zener voltage.Varactor diodes: capacitance set by voltage
Competency 3.2.1 asks you to identify methods of varying capacitance. Section 9 of the Electrical Theory guide covers the mechanical ones; the varactor adds an electronic one.
| Method | What changes |
|---|---|
| Variable capacitor | Turning the rotor changes the overlap of the plates, so their area (NEETS: the rotor-stator type tunes most radio receivers) |
| Trimmer | Its screw sets the spacing between its two plates (NEETS) |
| Dielectric | A different material changes the permittivity between the plates, fixed when the part is made |
| Varactor | Its reverse bias sets the width of its depletion region, which acts as the dielectric |
Kuphaldt: a reverse-biased diode has an insulating depletion region between its two semiconductor layers, so it is a small capacitor, and changing the reverse bias changes the region's width and so the capacitance. A varactor, or varicap, is a diode built to make the most of this. More reverse voltage widens the depletion region, which is like moving the plates apart, so the capacitance falls. Kuphaldt gives the range over the rated bias: about 4:1 for an abrupt varactor, 10:1 for a hyperabrupt one and 20:1 for a super-hyperabrupt one. In a tuned circuit a control voltage then sets the frequency, with a large series capacitor to keep the control voltage from being shorted by the coil. It is the tuning element in the voltage-controlled oscillator of a phase-locked loop (section 11 of the Circuits guide).
Worked example
How far a varactor tunes: resonance is f = 1 / (2π√(LC)), so frequency goes as one over the square root of C. If more reverse bias takes a varactor from 100 pF to 25 pF (an abrupt part's 4:1), the tank's frequency rises by √(100 / 25) = 2 times. A hyperabrupt part's 10:1 gives √10 ≈ 3.16 times.On the exam
Exam point: a varactor is operated reverse-biased; raising the reverse voltage widens its depletion region and lowers its capacitance, so a voltage can tune a resonant circuit.Bias polarities, beta and alpha, enhancement and depletion
Competency 3.5.2 asks you to analyze the biasing voltages of NPN and PNP BJTs, JFETs and MOSFETs, and 3.5.3 to explain beta and alpha and enhancement and depletion mode. Kuphaldt: the difference between a PNP and an NPN transistor is the polarity of their bias, and for any state of operation every current direction and voltage polarity of one is the reverse of the other. Bipolar transistors are normally off: with no base current there is no collector current. A JFET is normally on, and a depletion-type MOSFET partly on. Read the N-type rows first; every P-type part takes the opposite polarities.
| Device | With no input | Bias that makes it conduct |
|---|---|---|
| NPN BJT | Off | Base-emitter forward-biased: base about 0.7 V above the emitter. Base-collector reverse-biased: the collector is the most positive terminal (section 2) |
| PNP BJT | Off | Every polarity reversed: base about 0.7 V below the emitter; the emitter is the most positive terminal and the collector the most negative |
| N-channel JFET | On: maximum drain current at zero gate voltage | The gate is made negative to the source, reverse-biasing the gate junction; more negative pinches the channel off. Kuphaldt: never forward-bias the gate in practice |
| N depletion MOSFET | Partly on | A negative gate depletes (narrows) the channel and a positive gate enhances it; the insulated gate tolerates either polarity |
| N enhancement MOSFET | Off: no channel at zero gate bias | The gate is made positive to the source, beyond the threshold voltage Vth (section 3) |
Beta and alpha. Beta (β, hFE) is the collector current divided by the base current (section 2). Alpha (α) is the collector current divided by the emitter current. The emitter current is the collector current plus the base current, so α is always just under 1, and Kuphaldt derives it from β: α = β / (β + 1). Alpha is the current gain of the common-base amplifier, which is why that circuit attenuates current rather than amplifying it.
Enhancement and depletion. Kuphaldt: an enhancement-mode MOSFET has no channel at zero gate bias, so it is normally off, and its schematic symbol draws the channel as a broken line; a gate voltage of the right polarity enhances a channel into being. It is the most common kind. A depletion-mode MOSFET is built with a channel, so it conducts with zero gate voltage, and a gate voltage of either polarity can deplete or enhance it.
Worked example
Alpha from beta, and a PNP reading: an NPN with β = 120 and 50 µA of base current carries IC = 120 × 50 µA = 6 mA, so IE = 6 + 0.05 = 6.05 mA and α = 6 / 6.05 ≈ 0.992, the same as 120 / 121. A transistor with β = 99 has α = 99 / 100 = 0.99. A PNP biased into conduction with its emitter at +12 V has its base at about 12 − 0.7 = 11.3 V, and its collector lower still.On the exam
Exam point: an enhancement MOSFET is normally off and needs a gate voltage beyond its threshold; a JFET and a depletion MOSFET conduct with zero gate voltage, and a reverse gate voltage pinches them off. Alpha, the collector current over the emitter current, is always just under 1.Darlington pairs, IGBTs, CMOS and the FET family
Competency 3.5.1 asks you to describe the operation and applications of MOS, CMOS, FET, IGBT and Darlington-pair devices.
| Device | How it works (Kuphaldt) | Strengths and limits |
|---|---|---|
| Darlington pair | The emitter of the first transistor feeds the base of the second, so the second amplifies the first's already amplified current | Overall current gain is the product of the two gains (each β + 1); sold as one package or built from two parts. It loses two base-emitter drops, about 1.4 V |
| JFET | A reverse-biased gate junction widens its depletion region and squeezes the channel | Voltage-controlled, with almost no gate current; normally on |
| MOSFET (IGFET) | The gate is a metal plate insulated from the channel by a thin oxide, so it acts on the channel like a capacitor | No continuous gate current, only a surge to charge the gate capacitance; Kuphaldt notes a larger saturated voltage drop than a BJT at high current |
| CMOS | Complementary P- and N-channel MOSFETs in series, one on while the other is off | Ideally draws no supply current while static, more as its switching rate rises; its inputs are damaged by static and must not be left floating |
| IGBT | A MOSFET drives the base of a bipolar transistor, which carries the load current: in effect a Darlington pair of a MOSFET and a BJT | Practically no gate current, with a conducting drop as low as a BJT's; turns off more slowly than a BJT. Used for high-power control |
Why these devices exist. Kuphaldt's example: a power BJT with β = 20 needs at least 100 / 20 = 5 A of base current to conduct 100 A, a lot for small control circuitry to supply. A Darlington pair cuts that drive current by multiplying the gains; an IGBT, with its insulated gate, cuts it to practically nothing while keeping the bipolar transistor's low conducting drop.
Worked example
A Darlington's gain: transistors with β = 100 and β = 50 make a pair whose gain is (100 + 1) × (50 + 1) = 5151, or roughly 100 × 50 = 5000. A base current of 0.1 mA can then control about 0.0001 × 5151 ≈ 0.52 A, and the input must rise about 2 × 0.7 = 1.4 V above the emitter before the pair conducts.Safety
Handle MOS parts as static-sensitive: a MOSFET's gate oxide is very thin, and Kuphaldt warns that CMOS inputs are damaged by static voltages. Use a wrist strap and grounded mat, and keep parts in their protective packaging until they are installed (section 5 of the Safety guide).On the exam
Exam point: a Darlington pair multiplies the two transistors' current gains but drops two base-emitter voltages, about 1.4 V; an IGBT is driven like a MOSFET, with practically no gate current, and conducts like a bipolar transistor.Integrated circuits: types, packages and pin-outs
Competency 3.8 asks you to describe types of ICs, such as microprocessors, with their basic components and pin-outs. The types you will meet on the exam are each taught on their own guide:
| Type | What it is | Taught in |
|---|---|---|
| Op-amp | A high-gain differential amplifier, used with feedback | Amplifiers guide, section 8 |
| Voltage regulator | Holds a supply rail steady; a 7805 gives a fixed +5 V | Power Supplies guide, section 6 |
| 555 timer | The usual IC multivibrator, astable or monostable (one-shot) | Circuits guide, section 8 |
| Logic | TTL (74xx) and CMOS gates, flip-flops, counters and registers built from transistors | Digital Electronics guide |
| Microprocessor | A CPU with only minimal memory of its own (its registers), so it needs external memory | Digital Electronics guide, section 10 |
| Microcontroller | A CPU, memory and input/output on one chip | Computer Electronics guide, section 6 |
Packages. The same die may come in several packages (Siemens): the DIP (dual in-line package) is through-hole, with two parallel rows of pins; the SOIC has gull-wing leads and a smaller profile than the DIP; the QFP has leads on all four sides for higher pin counts; and the BGA connects through solder balls (the Soldering guide, section 11, covers handling them).
Finding pin 1. Pin 1 is marked by a dot, a notch or half-moon at one end, a bevel along the pin-1 side, or a small triangle or tab (DigiKey; MSOE). From pin 1, the pins are numbered counter-clockwise around the package as you look down on its top. On a DIP held notch-up, pin 1 is the top-left pin, the numbers run down the left side and back up the right, so on a 14-pin DIP pin 14 sits directly across from pin 1 and pin 8 across from pin 7. The IC Pinouts tool shows common parts.
Trap
The pin-out belongs to the package: TI's SN7400 has VCC on pin 14 and ground on pin 7 in its 14-pin DIP and SOIC, but the military SN5400 version of the same gate in TI's 14-pin flat pack (CFP) has VCC on pin 4 and ground on pin 11. Read the datasheet's pin-out for the exact package before you probe or replace.On the exam
Exam point: pin 1 is marked by a dot, notch or bevel, and the pins are numbered counter-clockwise from it as seen from the top of the package.Relays: types, contacts and solid-state relays
Competency 3.9 asks you to describe relay types, 3.9.1 to identify normally-open and normally-closed contacts, 3.9.2 to identify the coil (section 4 of the Electrical Theory guide) and 3.9.3 to compare a solid-state relay with a mechanical one. Kuphaldt: a relay is a solenoid set up to operate switch contacts. Its coil may use only a fraction of a watt while its contacts switch hundreds of times that power, and the coil circuit is electrically isolated from the contact circuit, so one can be DC and the other AC. The coil needs a minimum pull-in current to operate, then holds at less, and releases only below a lower drop-out current.
| Relay type (Kuphaldt) | How it differs | Typical use |
|---|---|---|
| Electromechanical | A coil pulls an iron armature that moves open-air metal contacts: the best contacts for high current, but they can spark and corrode | Switching loads from a low-power control circuit |
| Reed relay | Two thin magnetic reeds sealed in a tube, pulled together by the coil's field: sparkless and corrosion-free, but limited in current | Low-current switching |
| Contactor | A large relay with several heavy contacts, usually normally open, so the load loses power when the coil drops out | Motor and other high-power loads |
| Time-delay relay | Its contacts move only after a set time, on energizing (on-delay) or de-energizing (off-delay); modern ones time with an RC circuit | Sequencing start-ups, purge timers, flashers |
| Protective relay | Watches current, voltage or frequency and trips a large circuit breaker | Power generation and distribution |
| Solid-state relay (SSR) | An LED inside drives an optically coupled SCR, triac or transistor output; no moving contacts | Fast or frequent switching, such as heater control (Omron) |
Normally open and normally closed. Kuphaldt: a relay's "normal" state is its state with the coil de-energized, as it sits on a shelf. A normally-open (NO) contact is open then and closes when the coil is energized; a normally-closed (NC) contact is closed then and opens. Schematics draw contacts in that normal state, so an NO contact is drawn open even in a circuit where it spends most of its time closed. The contact forms (National Instruments): Form A is a single normally-open contact, Form B a single normally-closed one, and Form C a single-pole double-throw changeover, a common terminal with one NO and one NC contact, which breaks one connection before it makes the other. When the coil energizes, the common moves from the NC contact to the NO contact (DigiKey).
| Mechanical relay | Solid-state relay | |
|---|---|---|
| Switching element | Metal contacts moved by an armature | An SCR, triac or transistor (Kuphaldt) |
| Isolation | Coil insulated from the contacts | Optical: Kuphaldt says it rivals the best mechanical relays |
| Life and speed | Limited contact life; slow, as the armature must move | No moving parts to wear out; switches far faster |
| Arcing and noise | Contacts can spark and corrode | No sparking and no contact corrosion |
| When off | An air gap: open | A small leakage current still flows: 10 mA at 200 V AC on an Omron heater SSR |
| When on | Metal-to-metal contacts; Kuphaldt: open-air contacts are the best for high current | A voltage drop (up to 1.8 V on the same Omron part) that makes heat: that SSR is rated 90 A on a heat sink but 7 A without one |
| AC turn-off | Can break the current mid-cycle, which makes a voltage spike in an inductive load | An SCR or triac output opens only as the current falls through zero, so no spike (Kuphaldt) |
| Usual failure | Fails open | Fails shorted, which is why mechanical relays are still preferred where failing open is safer |
Worked example
Why an SSR needs a heat sink: with a 1.8 V drop, an SSR switching a 25 A heater turns 1.8 × 25 = 45 W into heat; at 20 A it is 1.8 × 20 = 36 W.Trap
An SSR that is off is not an open circuit: its leakage current still flows into the load. Never use an SSR, or any relay, as the disconnect when you lock out a circuit for work: OSHA 1910.147 accepts only mechanical energy-isolating devices, such as a manually operated breaker or disconnect switch, and says control-circuit devices are not energy-isolating devices (section 3 of the Safety guide).On the exam
Exam point: a relay's normal state is with its coil de-energized: normally-open contacts are then open and normally-closed contacts are closed, and a schematic draws them that way.On the exam
Exam point: a solid-state relay has no moving contacts, so it switches fast without arcing or contact wear, but it leaks a little current when off, drops voltage and makes heat when on, and usually fails shorted.Check yourself
Answer in your head first, then reveal.
What is the forward voltage of a conducting silicon diode, and of a germanium one?
Forward voltage when conducting ≈ 0.7 V (silicon) | ≈ 0.3 V (germanium)
When an exam question asks about the diode "barrier", which voltage does it mean?
Exam questions mean the forward drop.
What does forward bias do to the depletion region?
Positive voltage on the p-side shrinks the depletion region.
In which region does IC = β × IB hold for an NPN BJT?
For NPN: IC = β × IB in the active region.
How much DC gate current does an enhancement MOSFET draw?
No DC gate current flows (oxide insulation).
How do you find the current through a conducting silicon or germanium diode, and its power?
a conducting diode drops about 0.7 V (silicon) or 0.3 V (germanium); subtract that drop from the source before using Ohm's law, and its power is the drop times the current
How is a zener diode rated, and what limits its current?
a zener regulates only when it is reverse-biased in breakdown; it is rated by its zener voltage at a test current, its tolerance and its power rating, and its maximum current is the power rating divided by the zener voltage
What happens to a varactor's capacitance as its reverse bias rises?
a varactor is operated reverse-biased; raising the reverse voltage widens its depletion region and lowers its capacitance, so a voltage can tune a resonant circuit
Which FETs conduct with zero gate voltage, and which need a gate voltage to turn on?
an enhancement MOSFET is normally off and needs a gate voltage beyond its threshold; a JFET and a depletion MOSFET conduct with zero gate voltage, and a reverse gate voltage pinches them off
What does a Darlington pair gain, and what does it cost?
a Darlington pair multiplies the two transistors' current gains but drops two base-emitter voltages, about 1.4 V
How are an IC's pins numbered?
pin 1 is marked by a dot, notch or bevel, and the pins are numbered counter-clockwise from it as seen from the top of the package
In what state does a schematic draw a relay's contacts?
a relay's normal state is with its coil de-energized: normally-open contacts are then open and normally-closed contacts are closed, and a schematic draws them that way
How does a solid-state relay compare with a mechanical relay?
a solid-state relay has no moving contacts, so it switches fast without arcing or contact wear, but it leaks a little current when off, drops voltage and makes heat when on, and usually fails shorted