The behaviour of a diode depends on its polarity in the circuit (figure 4.2). If the diode is reverse biased (positive potential on N-type material) the depletion region increases. The only charge carriers able to support a net current across the PN junction are the minority carriers and hence the reverse current is very small. A forward-biased diode (positive potential on P-type material) has a decreased depletion region; the majority carriers can diffuse across the junction. The voltage may become high enough to eliminate the depletion region entirely.
Figure 4.2: Diode circuit connections: a) reversed biased and b) forward biased.
An approximation to the current in the PN junction region is given by (shown in figure 4.3a)
where both and are temperature dependent. This equation gives a reasonably accurate prediction of the current-voltage relationship of the PN junction itself - especially the temperature variation - and can be improved somewhat by choosing and empirically to fit a particular diode. However, for a real diode, other factors are also important: in particular, edge effects around the border of the junction cause the actual reverse current to increase slightly with reverse voltage, and the finite conductivity of the doped semi-conductor ultimately restricts the forward current to a linear increase with increasing applied voltage. A better current-voltage curve for the real diode is shown in the figure 4.3b.
Figure 4.3: Current versus voltage a) in the PN junction region and b) for an actual PN diode.
Various regions of the curve can be identified: the linear region of forward-biasing, a non-linear transition region, a turn-on voltage ( ) and a reverse-biased region. We can assign a dynamic resistance to the diode in each of the linear regions: in the forward-biased region and in the reverse-biased region. These resistances are defined as the inverse slope of the curve: . The voltage , represents the effective voltage drop across a forward-biased PN junction (the turn-on voltage). For a germanium diode, is approximately 0.3 V, while for a silicon diode it is close to 0.6 V.