# diode4

## Which diffusion current dominates the total current under low level minority carrier injection?

Under the assumption of low level injection diffusion current dominates over the drift current.

## What is the Electric field across the open circuited diode?

In an open circuited diode the total current flowing through the junction will be zero. Hence there should exist a electric filed to drive drift current to counter act diffusion current as the net current flowing through the junction should be zero. Equating the hole current density to zero

Jp= -Dp*q* dp/dx+ q*μp*p*E

Rearranging the terms we get Eoc = (Dp/ P*μp)* dp/dx, from Einstein’s relationship  Hence Eoc = (  / P)* dp/dx.

## What is abrupt junction or step graded junction?

A PN junction in which the doping changes abruptly from acceptor to donor is defined as abrupt junction. The variation charge density in abrupt junction with Na = Nd is shown below

Variation charge density with distance in abrupt junction

## What is linear junction?

A PN junction in which the doping changes abruptly from acceptor to donor is defined as abrupt junction. The variation charge density in abrupt junction with Na = Nd is shown below

Variation charge density with distance in linear junction

## What is the relation between capacitance and reverse bias voltage?

The capacitance of the reverse bias diode decreases with increase in reverse bias voltage. When a reverse bias voltage increases then the depletion layer width increases which intern decreases the capacitance.

C= ε*A/W

Where W= width of depletion layer.

ε = permittivity of depletion region

A = area of cross section

In general capacitance depends on reverse bias voltage as

C=K (1/ (-V+Vj)^m)

where k is a constant and Vj is built-in potential, V is reverse bias voltage

Where m =1/2 for abrupt junction

m= 1/3for linear junction.

## What is the effect of doing on penetration of depletion region?

The depletion rejoin in a PN junction diode penetrates more in to the lightly doped semiconductor (P or N). This can be understood based on charge neutrality principle as follows

Consider a semiconductor junction with P type material having doping concentration Na and N type material with doping concentration of Nd. Let Wp and Wn are the depletion layer widths in P type and N type materials. Then based on law of conservation of charge, The total uncovered charge in a depletion rejoin should be zero. Accordingly

-Na*Wp*Ap +Nd*Wn*An = 0;

Assuming the area of cross sections is equal on either side

Na * Wp = Nd*Wn

From the above equation if Na>> Nd then Wp << Wn hence the depletion penetrates more in to lightly doped N type material.

## What are piece wise linear characteristics?

The non liner characteristics of diode can be approximated to piece wise linear characteristics i.e. the relation between voltage and current is defined by two expressions where each expression holds linear for unique interval of range of voltages. The piece wise characteristics of diode are shown below.

piece wise characteristics of diode

I= 0 for V< 0.7

I= V/Rd for V> 0.7

What are ideal characteristics of a diode?

The following are the ideal characteristics of a diode:

a)      The cut-in voltage should be zero.

b)      Diode dynamic resistance should be zero

c)      Diode capacitance’s should be zero

d)      Break down voltage should be infinite e.t.c

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