2015-08-21
351
When applied voltage increases or comes down, the charge in a diode will be accumulated or cleaned out. It means that a diode has
properties of a capacitor.
The diode capacitance consists of two components: the barrier 
and the diffusion 
capacitance
.
The barrier capacitance displays processes of reducing non-compensated charges directly in a p-n junction. When voltage is changed, the barrier layer is changed too. The semiconductor p-n structure is similar to an electrical parallel-plate capacitor. The plates are formed by p- and n-regions, and the depletion layer serves as a dielectric. When reverse voltage increases, the depletion (barrier) layer spreads out and diode capacitance decreases (as if the plates are spaced more apart). Thus, barrier capacitance has non-linear characteristic and is a very important parameter of a diode at reverse voltage. The capacitance dependence uponapplied voltage (Fig. 3.3) has great practical importance in a number of applications.
Fig.3.3. Diode capacitance as a function of reverse voltage
Such a diode is called a varicap. In the region of positive voltage not only the barrier capacitance increases but the diffusion capacitance also increases. It is determined by non-equilibrium charges accumulated in p- and n-regions of a structure. Even at a low forward direction current, diffusion capacitance can overreach 10.000 
, and it exceeds considerably the barrier capacitance . So, at positive voltage, you have to take into consideration only . At reverse voltages, you should consider only 
, because 
. The operating frequency range of a diode will be wider when 
is less.
