Diodes#
A diode is the simplest semiconductor device β one junction, two terminals, and behavior that depends on which way current wants to flow. The ideal model (conducts forward, blocks reverse) is useful for first-pass understanding, but real diodes have forward voltage drops, reverse leakage, breakdown behavior, and dynamic characteristics that matter in practice.
The Ideal vs. Real Gap#
The ideal diode is a perfect one-way valve: zero resistance forward, infinite resistance reverse, instant switching. No real diode does any of this.
Forward bias reality:
- Silicon diodes drop about 0.6-0.7 V forward (not zero). Schottky diodes drop 0.2-0.4 V. LEDs drop 1.8-3.3 V depending on color
- The forward voltage varies with current β higher current means higher V_f. The datasheet V_f is specified at a particular test current
- Forward voltage has a negative temperature coefficient (about -2 mV/Β°C for silicon). Hotter diodes drop less voltage. This matters in precision circuits and can cause thermal runaway in parallel diodes
Reverse bias reality:
- Real diodes have reverse leakage current β nanoamps to microamps for silicon, more for Schottky. Leakage increases with temperature, roughly doubling every 10Β°C
- At high enough reverse voltage, the diode breaks down. Avalanche breakdown is usually non-destructive (if current is limited); exceeding rated reverse voltage without current limiting is destructive
Rectification#
Converting AC to DC. The most basic diode application.
Half-wave rectifier: One diode passes one polarity and blocks the other. Simple but wastes half the input cycle. Output has large ripple at the input frequency.
Full-wave rectifier (bridge): Four diodes in a bridge configuration pass both polarities. Two diode drops in the current path (1.2-1.4 V for silicon). Output ripple is at twice the input frequency, which is easier to filter.
Rectifier design considerations:
- Forward voltage drop means lost power: P_loss = V_f Γ I_load. At 10 A with a silicon bridge, that’s 14 W of heat in the diodes alone. Schottky bridges cut this significantly
- Reverse recovery time matters at higher frequencies. Standard rectifiers are fine at 50/60 Hz. Fast-recovery or Schottky diodes are needed in switching power supplies
- Surge current rating β inrush into a discharged capacitor can momentarily exceed the steady-state current by 10Γ or more
Clamping and Clipping#
Clamping (DC restoration): A diode plus a capacitor shifts the DC level of an AC signal without changing its shape. Used in video circuits, power supplies, and level-shifting applications.
Clipping: Diodes limit signal excursion by conducting when the signal exceeds a threshold. Used to protect inputs from overvoltage. Back-to-back diodes clip both polarities.
The clipping threshold is the diode’s forward voltage β soft for silicon (~0.6 V), sharper for Schottky (~0.3 V). This is not a hard wall; the transition from non-conducting to conducting is exponential, so clipping introduces distortion before the signal reaches the full V_f.
Protection#
Diodes are the first line of defense for many protection schemes:
- Reverse polarity protection β A series diode blocks current if the supply is connected backwards. Costs one V_f drop. A parallel reverse-biased diode (with a fuse) shorts a reversed supply and blows the fuse
- ESD protection β Clamping diodes to VCC and ground steer ESD current away from sensitive inputs. Most IC inputs have these built in, but external diodes handle higher energy
- Flyback protection β A diode across an inductive load (relay, motor, solenoid) provides a current path when the drive transistor turns off. Without it, the inductive voltage spike can destroy the transistor. The diode clamps the spike to one V_f above the supply
- TVS (Transient Voltage Suppressor) β A specialized Zener-like device designed to absorb large transient energy pulses. Faster than MOVs, with well-defined clamping voltages
Zener Behavior#
Zener diodes are designed to operate in reverse breakdown at a specific voltage. This makes them useful as voltage references and simple regulators.
- Below about 5 V, the mechanism is true Zener effect (quantum tunneling). Above about 7 V, it’s avalanche breakdown. Between 5-7 V, both contribute β and this crossover region has the best temperature stability
- A Zener “regulator” is just a resistor feeding a Zener. Load regulation is poor (output impedance is the Zener’s dynamic impedance, typically 5-50 Ξ©). For anything beyond a rough reference, use a proper regulator
- Zener diodes are noisy. The breakdown process generates broadband noise. This is actually exploited in noise generators, but it’s a problem in precision reference circuits
Tips#
- Use Schottky diodes in low-voltage, high-current applications to minimize forward voltage drop and power loss
- For flyback protection, place the diode directly across the inductive load, not at the driver
- Choose Zener diodes in the 5-7 V range for best temperature stability
Caveats#
- Forward voltage is not constant β It varies with current, temperature, and device type. Don’t assume 0.7 V for everything
- Reverse recovery β When a diode switches from forward to reverse, it conducts in reverse briefly while stored charge is swept out. This causes current spikes in switching circuits. Fast-recovery and Schottky diodes minimize this
- Schottky leakage β Schottky diodes have much higher reverse leakage than silicon junction diodes. At elevated temperatures, leakage can become significant, especially in battery-powered circuits where quiescent current matters
- Capacitance β Diodes have junction capacitance that varies with reverse voltage. This matters in high-frequency and tuning circuits (varactors exploit this deliberately)
- Thermal runaway in parallel diodes β The hotter diode drops less voltage, draws more current, gets hotter. Parallel diodes need current-sharing resistors or matched thermal coupling
In Practice#
- A diode that measures low resistance in both directions is shorted β replace it
- A diode that measures high resistance in both directions is open β replace it
- Forward voltage significantly higher than expected under load suggests the diode is undersized and overheating
- Voltage spikes on the collector of a transistor driving an inductive load indicate missing or failed flyback diode