Electronic Devices

• p-n junction diode is a unidirectional electronic device:
• In forward bias (p-side connected to positive terminal, n-side to negative), the depletion region narrows, allowing significant current to flow through the circuit.
• When the battery polarity is reversed (reverse bias), the depletion region widens, barrier potential increases, and current drops to a tiny reverse saturation current (almost zero).
• Ordinary p-type or n-type semiconductors behave as linear ohmic materials in either direction.
• capacitor blocks steady direct current (DC) entirely in both polarities once charged.

• p-type semiconductor is created by doping pure tetravalent silicon or germanium with trivalent impurity atoms (Group 13 elements such as Boron, Aluminum, or Indium).
• Each trivalent atom shares three valence electrons with neighboring silicon atoms, leaving a deficiency (vacancy) of an electron, which creates a hole.
• Consequently, holes are the majority charge carriers ($n_h \gg n_e$), and thermally generated electrons are the minority charge carriers.

Silicon belongs to Group 14 (tetravalent):

• To create an n-type semiconductor, pentavalent impurities (Group 15 elements with 5 valence electrons) are added. The fifth electron is loosely bound and easily enters the conduction band. Examples include Phosphorus (P) and Arsenic (As).
• Trivalent impurities (Group 13 elements with 3 valence electrons) such as Boron (B) and Indium (In) create holes and produce p-type semiconductors.

Analyzing the V-I characteristic curves of the semiconductor devices shown in the diagram:

• Forward biased p-n junction diode: Operates in the 1st quadrant, showing a threshold (knee) voltage followed by a steep exponential rise in forward current → Graph (D).
• Zener diode: Operates under reverse bias in the 3rd quadrant, displaying a sharp, almost vertical breakdown at the Zener breakdown voltage $V_Z$ where current increases dramatically while voltage remains constant → Graph (C).
• Photodiode: Operates under reverse bias in the 3rd quadrant, where the reverse saturation current is directly proportional to the incident optical power (illumination intensity) → Graph (A).
• Solar cell: Operates without external bias in the 4th quadrant (generating power where voltage is positive and current is negative, or photovoltage across open terminals $V_{oc}$ and short circuit current $I_{sc}$) → Graph (B).
  
  Matching the requested sequence in exact order:
  • Forward biased p-n junction → (D)
  • Zener diode → (C)
  • Photodiode → (A)
  • Solar cell → (B)