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 (ie. metallurgical junction) the Fermi levels of both regions align. During this process mobile carriers from each region migrate to the other region and leave behind ionized regions called space charge or depletion regions on both side of the contact. When no voltage is applied to the diode, mobile charges that are in the host region (now minority carriers) gradually recombine with the majority carriers of the host region outside the space charge. During this process 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). 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. This minority charge transfer forms the reverse current also known as leakage current of the diode. In forward mode the barrier is reduced and higher energy majority carriers cross the barrier first. Reducing further the barrier, the least energy carriers cross the barrier.

The space charge created forms the parasitic capacitance of the junction. This capacitance decreases with increasing barrier voltage and increases during forward bias.

In this applet, the built-in voltage is 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.