semiconductors
Energy Bands in Solids
Energy Levels: In solids, energy levels exist due to a systematic arrangement of atoms. The closeness of atoms leads to intermixing of valence electrons, resulting in increased permissible energy levels, forming energy bands.
Conduction Band, Valence Band & Forbidden Energy Gap
Valence Band: Occupied by valence electrons; it represents the highest energy level with electrons.
Conduction Band: The lowest unfilled band of energy levels.
Forbidden Energy Gap: The energy range that has no permissible states for electrons. Energy gap between VB and CB. It determines the electrical conductivity of a semiconductor, influencing whether it behaves as a conductor, insulator, or semiconductor under varying conditions.
Conductors, Semiconductors, and Insulators
Conductors: Materials that conduct charge carriers easily, with overlapping conduction and valence bands (no forbidden gap).
Insulators: Materials with a wide forbidden band, preventing electron transfer from valence to conduction band.
Semiconductors: Have a small forbidden band (4 valence electrons) (e.g., silicon, germanium), allowing electrons to jump from valence to conduction band when provided with energy (e.g., heat).
Mechanism of Electron and Hole Conduction
Thermal Generation:
Low energy: At low temp., VB is fully occupied and CB is empty, hence the semiconductor act as an insulator.
High energy: At high temp. Valence band electrons gain enough thermal energy to jump into the conduction band, creating free electrons and holes that contribute to electrical conductivity. This phenomenon increases the number of charge carriers, thereby enhancing the material's conductivity significantly.
Drift of Holes and Electrons: Under an electric field, electrons drift opposite the field direction, while holes drift in the direction of the field. The number of electrons equals the number of holes created.
Generation and Recombination of Carrier
Recombination: A free electron can recombine with a hole. This process emits energy as heat, which can re-excite other electrons, creating more electron-hole pairs, particularly under increased temperatures, enhancing conductivity.
Intrinsic and Extrinsic Semiconductors
Intrinsic Semiconductors: Pure form, with free electron concentration equal to hole concentration.
Extrinsic Semiconductors: Doping with impurities modifies conductivity; can be N-type (with pentavalent atoms (donor atoms) increasing electrons) or P-type (with trivalent atoms increasing holes (creates vacancy for electrons)).
N-type Semiconductors: Characterized by an abundance of free electrons, which are contributed by the dopant atoms, leading to higher electrical conductivity. majority charge carriers- electrons and minority charge carriers- holes.
P-type Semiconductors: Characterized by a predominance of holes, allowing for the flow of positive charge, which enhances conductivity in the presence of an electric field. Majority charge carriers- holes and minority charge carriers- electrons
P-N Junction Diode
Definition: Formed by joining P-type and N-type materials, behaves differently in forward and reverse bias conditions.
Forward Bias: Allows current flow; P-side connected to positive.
Reverse Bias: Prevents current flow; P-side connected to negative. **P-N Junction Diode** **Definition:** A P-N junction diode is a semiconductor device formed by the direct joining of P-type and N-type materials. This junction creates an interface that exhibits distinct electrical properties based on the application of an external voltage, leading to unique current flow characteristics under different biasing conditions. The diode primarily functions to control the flow of electrical current, allowing it to conduct in one direction while inhibiting it in the opposite direction. **Forward Bias:** When the P-side (anode) of the diode is connected to a positive voltage relative to the N-side (cathode), the junction is said to be forward-biased. In this state, the electric field created by the external voltage reduces the potential barrier of the depletion region at the junction, which allows charge carriers (holes from the P-side and electrons from the N-side) to recombine. As a result, current flows easily through the diode, with a small forward voltage drop, typically around 0.7 volts for silicon diodes and about 0.3 volts for germanium diodes. **Reverse Bias:** In contrast, when the P-side is connected to a negative voltage, the diode is reverse-biased. This configuration increases the potential barrier at the junction and widens the depletion region, thereby preventing the flow of current. The only current that can flow in reverse bias is a very small leakage current, which can vary depending on temperature and the specific characteristics of the diode. If the reverse voltage exceeds a certain threshold (the breakdown voltage), the diode may enter a breakdown region, leading to significant current flow and potentially damaging the diode unless protective measures are in place.
Depletion layer width increases, making it more difficult for charge carriers to cross the junction, thus enhancing the diode's ability to block reverse current.
Rectifiers
Junction Diode Rectifier: Converts AC to DC; operates by conduction during the forward bias cycle and non-conduction during the reverse bias cycle.
Half wave rectifier: This type of rectifier allows current to flow only during one half of the AC cycle, effectively blocking the negative half and resulting in a pulsating DC output. One diode in a half wave rectifier configuration is sufficient to convert the AC signal to a pulsating DC output, making it a simple and cost-effective solution for low-power applications.
Full wave rectifier: This type of rectifier utilizes both halves of the AC cycle, allowing current to flow during both the positive and negative halves, resulting in a smoother and more efficient DC output. Two diodes
Full Wave Rectifier: Utilizes both halves of AC cycles to produce a continuous output.