This simulator puts an adjustable supply across a PN junction and shows what happens inside and outside the diode. Forward bias narrows the barrier and lets current grow exponentially; reverse bias widens the depletion region, lowers capacitance and leaves only a small saturation current.
• A diode test fixture, a magnified depletion section, an adjustable voltage source, an electron barrier-energy ribbon and injection and reverse-leakage markers. • Controls for applied p-to-n voltage (-5 V to 0.55 V), acceptor and donor concentration exponents (10¹⁵ to 10¹⁷ cm⁻³), junction area (0.01-1 mm²) and an automatic sweep from -2 V to +0.5 V. • Readouts for applied bias, ideal diode current, depletion width, electrostatic barrier, depletion capacitance and reverse saturation current. • Experiments for reverse bias (depletion widens, capacitance falls, current approaches -Is) and forward bias (the barrier narrows and current rises exponentially).
The ideal diode equation is I = Is[exp(Va/VT) - 1], with Is = q A ni² [Dp/(Lp ND) + Dn/(Ln NA)], using Dn = 35 and Dp = 12 cm²/s and Ln = Lp = 0.01 cm. The barrier becomes Vbarrier = Vbi - Va, which sets the depletion width, and Cj = εsi A / W. Reverse bias therefore raises the barrier and width while dropping capacitance; forward bias does the opposite and current grows by about a decade for every 60 mV.
This is an ideal abrupt silicon diode at 300 K with an ideality factor of one. It has no series resistance, high-level injection, generation-recombination current or breakdown, and bias is limited to values below the built-in voltage. The animated sweep overrides manual voltage while enabled.
Forward bias lowers the potential barrier by the applied voltage, so the number of carriers able to cross rises as exp(Va/VT). That is the exponential term in the ideal diode equation.
Reverse bias raises the electrostatic barrier to Vbi plus the reverse voltage, widening the depletion region. A wider region means lower junction capacitance, since Cj = εsi A / W.
It is the small leakage current that remains in strong reverse bias, set by minority-carrier diffusion near the depletion edges. In this model current approaches -Is as reverse bias grows.
Series resistance, generation-recombination current, high-level injection and breakdown are not included, so real diodes differ at high current and large reverse voltage.