Diode I-V Characteristics
Shockley: I = I_s(exp(V/V_T)−1) — knee at ~0.7 V Si; tiny reverse current.
Key Notes
A p-n junction diode has an asymmetric I-V curve: ALMOST NO current in reverse, EXPONENTIAL current in forward.
Forward characteristic: current is negligible until V ≈ V_threshold (knee voltage, ~0.7 V Si, ~0.3 V Ge), then rises steeply.
Reverse characteristic: tiny saturation current I₀ (nA-μA) until BREAKDOWN voltage, where current suddenly grows.
Static resistance R = V/I (depends on operating point). Dynamic (small-signal) resistance r_d = dV/dI = k_BT/(eI) at room temperature.
Ideal diode: zero forward resistance, infinite reverse resistance — used in idealized circuit analysis.
Real diode has a 'knee' due to V_threshold and finite forward slope (a few Ω).
Breakdown mechanisms: Zener (tunnelling, low V_BR, < 5V) and avalanche (impact ionisation, high V_BR).
Special diodes: LED (light emission), photodiode (photocurrent), Zener (voltage reference), Schottky (low V_F), tunnel diode.
Formulas
Shockley diode equation
Exponential growth in forward; saturation in reverse.
Threshold (knee) voltage
Below threshold, current is negligible.
Static resistance
Falls with V in forward bias (non-Ohmic).
Dynamic resistance
Useful for small-signal analysis. At I = 1 mA, r_d = 26 Ω.
Important Points
Diode is NON-LINEAR — Ohm's law does not apply.
V_threshold ~ 0.7 V for Si is rounded. Real-world diodes show forward conduction from ~0.5 V.
Reverse current is mostly thermally generated minority carriers — strongly T-dependent.
Dynamic resistance r_d falls with current — at high I, very low resistance.
Schottky diodes (metal-semiconductor) have V_F ~ 0.2 V and very fast switching — used in switching power supplies.
Reverse breakdown is NOT necessarily damaging if current is limited externally; Zener diodes exploit it.
Diode I-V Characteristics notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.