8.6 Circuit Models for Junction Diodes
Two ways of modeling a diode for circuit analysis are given in this section：a ‘small-signal’ model suitable for hand calculations，and a more comprehensive，large-signal model as used in computer simulationsof circuit behavior.
Fig.8.6 Small-signal equivalent circuit of a diode.
When the diode is used in a circuit in which small alternating signal voltages and currents are superimposed on static（d. c.）values，the diode can be replaced for the purposes of calculating signal voltages and currents by the small-signal equivalent circuitshown in Fig.8.6.
Let the static voltage across the junction be V，and treat the a. c. signalas an infinitesimal voltagedV. The total current is similarly expressed as the sum of I and dI. The equivalent a. c. conductance gd is the ratio dI/dV，which is obtained by differentiating the I-Vrelation and given by Eq.（8.2）The inverse of gd is the resistance of the equivalent resistor rd in Fig.8.6，whose value according to Eq.（8.2）depends on the d. c. quiescent diode current. The series resistance RS is also included in Fig.8.6. It is often negligibly small compared to rd.
In reverse bias，leakage effects become important in determining the equivalent resistance，and the equivalent capacitance has a much lower impedance than the resistor. The two capacitors in Fig.8.6 represent the two capacitive effects. Cd called the diffusion capacitance，or storage capacitance，is a current-dependent capacitance which models the storage of injected carriers in the neutral，or diffusion，region of the diode. 
Cj is the depletion-layer，or junction capacitance given by either Eq.（8.4）or（8.5）. Both capacitances depend on the applied voltage or current in such a way that in forward bias Cd》 Cj，while in reverse bias Cj 》 Cd as seen in Fig.8.5.
An alternative way of modeling a diode is to use the full I-Vrelation. This approach is used in the standard computer simulations，of which the best known is probably SPICE（Simulation Program with Integrated Circuit Emphasis），which is widely used for simulating the behavior of integrated circuits .