This Java applet simulates the energy bands interaction in semiconductor junctions. Semiconductor junctions or diodes consist of two semiconductor regions in contact :

• symetrical with equal doping concentrations in the N and P regions,
• assymetrical with doping of one region exceeds that in the other region.
• symetrical with heavy doping in both regions (tunnel - Esaki diode).

When uploaded with the browser the applet initially displays separated materials. No interaction between the energy levels occurs. By clicking on the button the junction is created.

When making the contact the Fermi levels of both regions align. During this process mobile carriers from each region migrate to the other region. These charges recombine in the "host region" creating a space charge / depletion region (region that is free from mobile charges). A voltage between the regions called built-in voltage (Vbi) is then created opposing further migration of the carriers. Equilibrium is than reached when carriers cease to migrate.

This barrier can be increased further (reverse biased diode) or reduced to enable the carriers to cross the junction (forward biased diode) by applying a voltage (supply). In reversed biased mode, the space charge created forms the parasitic capacitance of the junction. This capacitance increase with decreasing the supply voltage. Minority and majority charges ( "e" for electrons and "o" for holes) are shown in their correspending energy band. Minority charges are swept by the electric field in the barrier and their current is independant from the supply voltage. Minority current forms the reverse current also known as leakage current. Increasing the forward voltage the barrier is reduced and higher energy majority carriers cross the barrier. Reducing further the barrier, the least energy carriers cross the barrier.

In this applet, the built-in voltage can be exaggerated because the diode is considered abrupt with heavy dose in one region. In reality the profile in the heavy concentration region is tapered toward the junction leading to lower Vbi. Also, the forward supply voltage is typically 700mV at room temperature, which is sufficient in most diode applications (small signal). However, it is worth noting the shift of the Fermi level toward the intrinsec level leading to lower barrier and therefore lower voltage is needed to forward bias the junction. Typically this barrier decreases by roughly 2mV/C (1.5mV calculated) when the temperature increases.