Every electronic or electrical device is frequency sensitive.
That is, the terminal characteristics of any device will change with frequency. Even the resistance of a basic resistor, as of any construction, will be sensitive to the applied frequency. At low to mid-frequencies most resistors can be considered fixed in value. However, as we approach high frequencies, stray capacitive and inductive effects start to play a role and will affect the total impedance level of the element.
In the p – n semiconductor diode, there are two capacitive effects to be considered. Both types of capacitance are present in the forward- and reverse-bias regions, but one so outweighs the other in each region that we consider the effects of only one in each region.
Recall that the basic equation for the capacitance of a parallel-plate capacitor is defined by C = ε A /d, where ε is the permittivity of the dielectric (insulator) between the plates of area A separated by a distance d.
In a diode the depletion region (free of carriers) behaves essentially like an insulator between the layers of opposite charge. Since the depletion width ( d ) will increase with increased reverse-bias potential, the resulting transition capacitance will decrease. The fact that the capacitance is dependent on the applied reverse-bias potential has application in a number of electronic systems.
This capacitance, called the transition ( CT ), barriers, or depletion region capacitance, is determined by
where C (0) is the capacitance under no-bias conditions and V R is the applied reverse bias potential. The power n is 1 ⁄ 2 or 1 ⁄ 3 depending on the manufacturing process for the diode.
Although the effect described above will also be present in the forward-bias region, it is overshadowed by a capacitance effect directly dependent on the rate at which charge is injected into the regions just outside the depletion region. The result is that increased levels of current will result in increased levels of diffusion capacitance ( CD ) as demonstrated by the following equation:
where τ is the minority carrier lifetime—the time is world take for a minority carrier such as a hole to recombine with an electron in the n -type material. However, increased levels of current result in a reduced level of associated resistance (to be demonstrated shortly), and the resulting time constant (τ=RC ), which is very important in high-speed applications, does not become excessive.
In general, therefore, the transition capacitance is the predominant capacitive effect in the reverse-bias region whereas the diffusion capacitance is the predominant capacitive effect in the forward-bias region.