Circuit Models

The equation for current under dynamic conditions can be expressed in the form an equivalent circuit as shown below:




The model can be made more accurate by including a parameter n to model the nonidealities in the current caused by modification of barrier height due to image force lowering, tunneling etc

The effects of voltage drop across the ohmic contact and the neutral N-region can be taken into account through a series resistance as illustrated as in the Figure:

  • This simple circuit model is characterized by five parameters:

  • This model is more useful for use with circuit simulators than for “hand analysis” of circuits. A variety of simplified models appropriate for different situations are used for the latter. These are discussed in detail in the context of PN junctions.

  • Among the models is the small signal representation of the device representing the response of the diode to incremental changes in applied voltage around a particular bias point:

    • are the dc voltage and current across the diode. is a small signal superimposed on the dc diode voltage and is the small signal response of the diode. The relationship between and is known as the small signal model.

  • The small signal model can be obtained through linearization of the general large signal model described earlier. This procedure is again described in detail with respect to PN junctions and only the result is shown below:

    ; small signal resistance of the diode. The usefulness of the small signal model stems from its linear nature.
Example 4.1 :Determine the small signal resistance and capacitance of a Schottky diode formed on an N-type Silicon of doping at a current density of
Assume that Repeat your calculations for a current density of

Solution : Using expressions derived earlier in the text, the built in voltage of the diode is calculated to be 0.6 Volts and the forward On voltage to be 0.33 Volts. Assuming an ideality factor of 1, the small signal resistance is calculated to be The junction capacitance is calculated to be 17.5 pF.
The RC response time of the diode will be

If the current density is increased by three orders of magnitude, then the small signal resistance will decrease by but the capacitance as seen earlier will remain relatively unchanged. As a result, the response time of the diode will decrease to a few picoseconds.

  • This shows that the response time of the Schottky diodes is very small and they can work upto frequencies exceeding 1Ghz !


  • To summarize, Schottky diodes have the advantages of small turn on voltage, fast response time but suffer from high leakage current and low breakdown voltage.