8.2 Small-Signal Diffusion Resistance of the pn Junction
We have been considering the dc characteristics of the pn junction diode. 
When semiconductor devices with pn junctions are used in linear amplifier circuits，for example，sinusoidal signalsare superimposed on the dc currents and voltages，so that the small-signal characteristics of the pn junction become important.
Assume that the diode is forward-biased with a dc voltage Vo producing a dc diode current IDQ. If we now superimposes small，low-frequency sinusoidal voltage as shown in Fig.8.2，then a small sinusoidal current will be produced，superimposed on the dc current. The ratio of sinusoidal current to sinusoidal voltage is called the incremental conductance . In the limit of a very small sinusoidal current and voltage，the small-signal incremental conductance is just the slope of the dc current-voltage curve，or
If we assume that the diode is biased sufficiently far in the forward-bias region，then the incremental conductance becomes
where Vt=kT/q，and is defined as the thermal voltage .
The small-signal incremental resistance is then the reciprocal function，or
where IDQ is the dc quiescent diode current.
The incremental resistance decreases as the bias current increases，and is inversely proportional to the slope of the I-Vcharacteristic as shown in Fig.8.2. The incremental resistance is also known as the diffusion resistance .
Fig.8.2 Curve showing the concept of the small-signal diffusion resistance.