Distinguish between reverse and forward biasing of a diode, and illustrate with circuit diagrams and band energy diagrams.

Here are the circuit diagrams and the description of the band energy diagram:

4(a) (i) Distinction between Reverse and Forward Biasing of a Diode

Forward Bias:
In forward bias, the positive terminal of the voltage source is connected to the anode (P-side) of the diode, and the negative terminal is connected to the cathode (N-side). This reduces the width of the depletion region, allowing current to flow easily through the diode.

A more detailed representation:

      + V_source -
      |          |
      R          |
      |          |
      |>|        |  (Diode symbol: triangle points to N-side, bar is N-side)
      |          |
      ------------

In this configuration, the diode conducts current.

Reverse Bias:
In reverse bias, the positive terminal of the voltage source is connected to the cathode (N-side) of the diode, and the negative terminal is connected to the anode (P-side). This increases the width of the depletion region, significantly reducing current flow (ideally, no current flows, but a small leakage current exists).

A more detailed representation:

      - V_source +
      |          |
      R          |
      |          |
      |>|        |  (Diode symbol: triangle points to N-side, bar is N-side)
      |          |
      ------------

In this configuration, the diode blocks current.

4(a) (ii) Band Energy Diagram of a Typical Diode (at Equilibrium)

A typical band energy diagram for an unbiased P-N junction shows the energy levels across the junction:

• On the P-side, the valence band ($E_V$) is close to the Fermi level ($E_F$), indicating a high concentration of holes. The conduction band ($E_C$) is higher in energy.
• On the N-side, the conduction band ($E_C$) is close to the Fermi level ($E_F$), indicating a high concentration of electrons. The valence band ($E_V$) is lower in energy.
• Across the depletion region (the junction), the energy bands bend. The conduction band ($E_C$) and valence band ($E_V$) are higher in energy on the P-side and lower on the N-side.
• The Fermi level ($E_F$) remains constant across the entire P-N junction at equilibrium, indicating no net flow of charge carriers.
• The difference in energy between the conduction band on the P-side and the conduction band on the N-side (or similarly for the valence band) represents the built-in potential barrier ($V_0$).