Statement A — TRUE ✓
When forward bias voltage increases above the threshold (knee) voltage (~0.7V for Si, ~0.3V for Ge), the potential barrier is significantly reduced. Majority carriers can now easily cross the junction, and the diode current increases steeply (exponentially). This is the normal forward conduction behaviour.
Statement B — FALSE ✗
The large current flowing in forward bias is called forward current, NOT reverse saturation current. Reverse saturation current (I₀) is the tiny current (microamperes to nanoamperes) that flows when the diode is reverse biased — it is due to minority carriers and is essentially independent of reverse voltage. Statement B incorrectly names the forward current.
Answer: Option 2 — A true, B false
Semiconductors are materials with electrical conductivity between conductors and insulators. Silicon (Si) and Germanium (Ge) are the most important semiconductors. At absolute zero, they behave as insulators. As temperature increases, more electrons get enough energy to jump the bandgap from valence band to conduction band, increasing conductivity.
The bandgap energy determines whether a material is a semiconductor. For Silicon: E_g = 1.1 eV. For Germanium: E_g = 0.7 eV. Materials with E_g < 3 eV are semiconductors; those with E_g > 3 eV are insulators; metals have zero or overlapping bandgaps.
n-type: Formed by doping pure semiconductor with pentavalent atoms (phosphorus, arsenic, antimony). The extra electron from each dopant atom becomes a free electron (majority carrier). Minority carriers are holes. The material remains electrically neutral overall.
p-type: Formed by doping with trivalent atoms (boron, aluminium, gallium). Each dopant creates a hole (missing electron) that can accept electrons from neighbouring atoms. Holes are majority carriers; electrons are minority carriers. Again, electrically neutral overall.
When p-type and n-type materials are joined, electrons from n-side diffuse to p-side (down the concentration gradient) and holes from p-side diffuse to n-side. This creates an electric field in the depletion region (pointing n→p) that builds up until it opposes further diffusion. The depletion region has no free carriers and acts as an insulating layer.
📌 Built-in potential barrier: ~0.3V for Ge, ~0.7V for Si
📌 Depletion width: ~0.1–1 μm (wider for lightly doped)
📌 Electric field points from n-side to p-side across depletion region
📌 At equilibrium: drift current = diffusion current (no net flow)
In forward bias: p-side connected to positive terminal (+), n-side to negative terminal (−). The external voltage opposes the built-in potential, reducing the potential barrier and narrowing the depletion region. When forward voltage exceeds the threshold (knee voltage), the barrier is nearly eliminated and current flows freely:
📌 Si diode knee voltage: ~0.7 V
📌 Ge diode knee voltage: ~0.3 V
📌 Below knee voltage: very small current (μA range)
📌 Above knee voltage: large current (mA to A range)
📌 This large current = FORWARD current (not reverse saturation current!)
📌 Depletion layer narrows → more majority carriers cross junction
In reverse bias: p-side to negative terminal (−), n-side to positive terminal (+). This increases the potential barrier and widens the depletion region. Majority carriers cannot cross the junction. Only minority carriers (few in number) are swept across by the field, producing a tiny reverse current.
This tiny, nearly constant current in reverse bias is called the reverse saturation current (I₀ or I_s). It is called "saturation" because it barely changes as reverse voltage increases (it saturates at a low value). For Si diodes: I₀ ≈ a few nanoamperes. For Ge: I₀ ≈ a few microamperes. This is what Statement B INCORRECTLY calls the current in forward bias — the forward current is hundreds of thousands of times larger.
I = I₀(eV/ηV_T − 1)
I₀ = reverse saturation current
η = ideality factor (1 for Ge, 2 for Si)
V_T = kT/e ≈ 26 mV at 300 K (thermal voltage)
For forward bias (V positive and large): I ≈ I₀ × eV/ηV_T — exponential increase. For reverse bias (V negative and large): I ≈ −I₀ — constant reverse saturation current. This equation explains the characteristic shape of the V-I curve of a p-n junction.
A Zener diode is specially designed to work in reverse breakdown. When reverse voltage exceeds the Zener voltage (V_Z), the diode breaks down and current increases steeply while voltage remains constant at V_Z. This property makes Zener diodes ideal for voltage regulation — they maintain a constant output voltage despite variations in input voltage or load current.
Two breakdown mechanisms: Zener breakdown (dominant at V_Z < 5V — due to quantum tunnelling of electrons through narrow depletion region) and Avalanche breakdown (V_Z > 5V — due to impact ionisation by accelerated carriers). Despite both being called "Zener breakdown" colloquially, only the former is true Zener effect.
📌 Half-wave rectifier: single diode converts AC to pulsating DC
📌 Full-wave rectifier: bridge (4 diodes) converts both half-cycles
📌 Voltage regulation: Zener diode maintains constant voltage
📌 Clipping circuits: cuts off signal above/below a level
📌 Clamping circuits: shifts signal level
📌 LED (Light Emitting Diode): forward biased diode emits light
📌 Photodiode: reverse biased, generates current when illuminated
📌 Solar cell: p-n junction converts light to electricity (photovoltaic effect)
A transistor consists of two p-n junctions. In a npn transistor: emitter (n) | base (p) | collector (n). Emitter-base junction is forward biased; collector-base junction is reverse biased. A small base current controls a large collector current — this is current amplification. Current gain β = I_C/I_B (typically 50–500). The transistor can act as an amplifier or a switch.
⚠️ Forward current (large, mA-A) ≠ Reverse saturation current (tiny, nA-μA)
⚠️ Knee voltage: Si = 0.7V, Ge = 0.3V — memorise both!
⚠️ Reverse saturation current flows in REVERSE bias — not forward
⚠️ Zener diode works in REVERSE breakdown — not forward