What is drift current?

The Flow of charge carriers under the influence of applied electric field is called as drift current. Magnitude of Drift current depends on the mobility, concentration, magnitude of applied electric field and the cross sectional area through which the current is flowing. The equation for drift current density for electrons and holes is given by

   Jn = q*μn*n*E,

for holes Jp = q*μp*p*E

Where μn, μp are mobilities of electrons and holes, n, p are electron and hole concentrations, E is applied electric field.

What s diffusion current?

The flow of charge carriers owing to the presence of concentration gradient due to diffusion is termed as diffusion current. This diffusion current flows from high concentration side to low concentration side. Diffusion current is purely a statistical phenomenon. Diffusion current density is directly proportional to concentration gradient (concentration gradient is the rate at which the concentration of carriers varies with distance dp/dx or dn/dx) and is given by

                                                           Jn= Dn*q* dn/dx for electrons

                                                           Jp= -Dp*q* dp/dx for holes

Where Dn, Dp are diffusion constants for holes and electrons, q is electron charge, dp/dx, dn/dx is concentration gradient for holes and electrons.

What is storage time, forward and reverse recovery times of diode?

The forward recovery time of a diode is defined as the time difference between the 10 % point of diode final voltage and the time when the diode voltage reaches 90 % and remains within 10 % of final value.

The terms storage and transition times are used when diode is suddenly switched from forward bias to reverse bias.

 Storage time is defined as the time interval between application of reverse bias and the time at which stored minority charge becomes zero.

 Transition time is defined as the time the diode transition capacitance takes to charge from zero (This happens when stored minority charge becomes zero) to applied reverse bias.

The total time elapsed between the application of reverse bias till the diode charge fully to applied reverse bias is called as reverse recovery time. It is the sum of storage time and transition time.

What is current equation of diode?

The current equation of a diode is given by

                                             Id = Ido*(exp (V/Vt)-1)


Ido is reverse saturation current,

V is voltage applied across diode,

Vt is the voltage equivalent of temperature.

If V is negative and V >> Vt exp (V/Vt) tends to zero and Ic = -Ico.

What is reverse saturation current in diode?

It is the current flowing through the diode when diode is reverse biased with a voltage greater than 5*Vt (0.13 volts at room temperature) where Vt is the voltage equivalent of temperature which is 26 milli volts at room temperature. Reverse saturation current is minority carrier current resulting from flow of minority carriers from either side of junction (electrons from p-side to n-side and holes from n-side to p-side). It is a function minority carrier concentrations on either side i.e. electrons in p-type semiconductor and holes in n-type semiconductor, carrier life times and diffusion constants.

How reverse current varies with temperature?

The reverse saturation current of a diode is a sensitive function of temperature and increases rapidly with increasing temperatures. It is because the reverse saturation current increases with increasing conductivity of silicon material using in manufacturing the transistor. The conductivity in turn depends on free electron and hole concentrations which increases as temperature increases due to increased thermal energies of electrons(due to increased temperatures). Approximately the reverse saturation current doubles for every 10 Deg C rise in temperature. If Ico1 is the reverse saturation current at temperature T1, then at temperature T2 the reverse saturation current is given by

                                          Ico2= Ico1*2(T2-T1)/10

What is continuity equation?

The continuity equation governs the functional relationship of carrier concentrations for applied voltage (or) current inputs in the semiconductor body with time and distance. It is based on law of conservation of charge. The continuity equation for holes is given as

                                     p/ t = ((p0-p)/tp) (1/q)* Jp/ x

Where p is hole concentration which is function of distnce and time, p0 = g* tp, g is holes generated per volume due to thermal generation, Jp is hole current.The continuity eqaution is simlar for elctrons with p replaced by n.                              

What is built in potential of diode?

ANS: The potential developed across a open circuited PN junction diode id termed as Built in potential (or) contact potential. For a PN junction diode this is given as

                                                V0 = Vt*ln (Na*Nd/ni^2)

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