Semiconductor Sandbox
Exploring the foundational principles of solid-state electronics.
Exploring the foundational principles of solid-state electronics.
Discover how electron energy bands and the Fermi level determine whether a material conducts electricity.
$Carrier\ conc. \propto e^{-E_g / (2k_B T)}$
In a solid, atomic energy levels form bands. The Valence Band is the highest band typically filled with electrons. The Conduction Band is the next band, which is mostly empty. The Band Gap ($E_g$) is the energy between them. Crucially, the Fermi Level ($E_F$) represents the highest energy an electron can have at 0K. Its position relative to the bands dictates the material's properties.
The fundamental building block of modern electronics. See how joining P-type and N-type silicon creates a one-way gate for current.
$I = I_0 (e^{qV/k_BT} - 1)$
When P-type (excess holes) and N-type (excess electrons) materials meet, mobile carriers diffuse across the junction and cancel out, leaving behind fixed charged ions. This creates a Depletion Region with a built-in electric field, forming a potential barrier or "energy hill." Applying a voltage can either shrink this barrier (Forward Bias) allowing current to flow, or grow it (Reverse Bias) blocking the flow.
See how a diode's one-way current property can be used to convert alternating current (AC) to direct current (DC).
A rectifier circuit uses a diode to block one half of the AC waveform. The result is a pulsed DC signal. To create smooth DC voltage, a Filter Capacitor is added in parallel with the load. It charges when the voltage is high and discharges when the voltage is low, smoothing out the pulses and reducing the "ripple".
An LED is a P-N junction that emits light when forward biased. See how different semiconductor materials create different colors.
$E_{photon} = hf \approx E_g$
When a P-N junction is forward biased, electrons and holes are injected into the depletion region where they recombine. In certain semiconductor materials (like Gallium Arsenide), the energy lost by the electron as it recombines with a hole is released as a photon of light. The energy of this photon—and thus its color—is determined by the material's band gap energy ($E_g$).
A photodiode converts light energy into electrical current. It's operated under reverse bias.
$I_{photo} \propto \text{Intensity (if } E_{photon} > E_g)$
When a photon with energy greater than the band gap ($E_g$) strikes the depletion region of a reverse-biased P-N junction, it creates an electron-hole pair. The strong electric field in the depletion region sweeps these new carriers out, creating a measurable photocurrent. The magnitude of this current is proportional to the light intensity.
A Zener diode is designed to operate in reverse breakdown, making it perfect for voltage regulation.
Unlike a normal diode, a Zener diode is heavily doped to have a precise and sharp reverse breakdown voltage (the "Zener Voltage", $V_Z$). When the reverse voltage reaches $V_Z$, a large current can flow without damaging the diode. This unique property allows it to maintain a constant voltage across its terminals, even if the current changes, making it an ideal voltage regulator.
Discover how a small base current can control a large collector current, the principle behind amplification.
$I_C = \beta \cdot I_B$
A BJT consists of three semiconductor layers (N-P-N or P-N-P) forming two P-N junctions. It has three terminals: the **Emitter**, **Base**, and **Collector**. By injecting a small current into the base terminal ($I_B$), we control a much larger current flow from the collector to the emitter ($I_C$). This current gain, or **Beta ($\beta$)**, is the key to amplification. The transistor has three main operating regions: cut-off (off), active (amplifier), and saturation (on).