SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

318 CHAPTER 7. TEMPORAL RESPONSE OF DIODES AND BIPOLAR TRANSISTORS

Cgeom

C Ls

d

Rd

Rs

Figure 7.6: Equivalent circuit of a Schottky diode.

and the differential capacitance of the depletion region. The depletion capacitance has the form:

Cd=A

[

eNd

2(Vbi−V)

] 1 / 2

(7.3.2)

As noted earlier, there is no diffusion capacitance. These circuit elements are in series with the
series resistanceRs(which includes the contact resistance and the resistance of the neutral doped
region of the semiconductor) and the parasitic inductance. Finally, one has to include the device
geometry capacitance:

Cgeom=

A

L

(7.3.3)

whereLis the device length. Theabsenceofthediffusioncapacitancethatdominatesthe
forward-biascapacitanceofap-ndiode allows a very fast response of the Schottky diode.

7.4 BIPOLAR JUNCTION TRANSISTORS:

A CHARGE-CONTROL ANALYSIS

In our dc analysis of the bipolar transistor , we saw that when the device is under bias, current
flows through each of the terminals, and a stored charge profile is established within the structure
(figure 6.8). When an ac signal is applied, the stored charge and the current are modulated.
However, the stored charge cannot be modulated instantaneously; once a signal is applied, a
finite amount of time is required for the corresponding charge distribution to be established.
Determining the switching behavior of a bipolar transistor essentially boils down to finding the