JNTU B.Tech II ANALOG AND DIGITAL ELECTRONICS Previous Question Paper (October - 2020 - R18)

Diffusion capacitance (C_D) is the capacitance that arises in a forward-biased p-n junction diode due to the storage of minority charge carriers near the depletion region. As the forward bias increases, more carriers are injected across the junction, leading to a change in stored charge, which acts like a capacitor.

To derive the expression, consider the stored charge Q in the n-region for holes: Q = I_pτ_p, where I_p is the hole current and τ_p is the mean lifetime of holes. Since the total diode current I ≈ I_p, we have Q = Iτ_p.

The diffusion capacitance is defined as the rate of change of charge with respect to voltage:

Since the dynamic conductance g = dI/dV = I / ηV_T (where V_T is thermal voltage and η is the ideality factor), the expression becomes:

A tunnel diode is a heavily doped p-n junction diode that exhibits negative differential resistance. Its V-I characteristics are divided into four main regions:

1. Tunneling Region: As forward voltage increases from zero, current rises rapidly to a peak point (I_p) due to quantum mechanical tunneling of electrons through the narrow depletion barrier.
2. Negative Resistance Region: Beyond the peak point, as voltage increases further, the tunneling current decreases, creating a region where current drops as voltage rises. This is the region where the diode can function as an oscillator.
3. Valley Point: The current reaches a minimum value (I_v) at the valley voltage.
4. Normal Diode Region: Beyond the valley point, the characteristic resembles a standard p-n junction diode where current increases exponentially with voltage due to thermionic emission.