The junction diode is the basic semiconductor device and the simplest one in ngspice, but its model is quite complex, even when not all the physical phenomena affecting a pn junction are handled. The diode is modeled in three different regions:

• Forward bias: the anode is more positive than the cathode, the diode is `on' and can conduct large currents. To avoid convergence problems and unrealistic high current, it is prudent to specify a series resistance to limit current with the rs model parameter.
• Reverse bias: the cathode is more positive than the anode and the diode is `off'. A reverse bias diode conducts a small leakage current.
• Breakdown: the breakdown region is modeled only if the bv model parameter is given. When a diode enters breakdown the current increases exponentially (remember to limit it); bv is a positive value.