Table of contents 
Diode Resistance 
1 Crore+ students have signed up on EduRev. Have you? 
The static resistance R of a diode is defined as the ratio V/I of the voltage to the "Current Typical values for a silicon planar epitaxial diode are V_{F} = 0.8V at I_{F}  10mA (corresponding to R_{F} = 80Ω) and I_{R} = 0.1μA at V_{R} = 50V (corresponding to R_{R} = 500M).
For small signal operation the dynamic, or incremental, resistance r is an important parameter and is defined as the reciprocal of the slope of the voltampere characteristic, r = dV/dl. The dynamic resistance is not a constant, but depends upon the operating voltage.
The dynamic conductance g = 1/r is
for V/η)V_{T} >> 1, g is small and r is very large.
For forward bias, I >> I_{0}, and r is given by
r ≈ (ηVT) /I ....(15)
The piecewise linear characterization of a semiconductor diode.
For an avalanche diode, V_{y} = V_{z}, and R_{f} is the dynamic resistance in the breakdown region.
The parallel layers of oppositely charged immobile ions on the two sides of the junctionform the transition capacitance, C_{r}, which is given by
C_{T }= (εA)/W
Where ε(= ε_{0}ε_{r}) is the permittivity of the material, A is the crosssectional area of the junction and W is the width of the depletion layer over which the ions are uncovered.
The net charge must be zero across the depletion region will satisfy the condition
N_{A}W_{p} = N_{D}W_{n} .....(16)
Where
N_{A} = acceptor concentration ; N_{D}donor concentration;
W_{p} = width of the depletion region in pside;
W_{n} = width of the depletion region in nside;
The effective barrier potential across the junction is
V_{B}= (qN_{A} W^{2}) / 2ε ....(17)
if N_{A}<< N_{D}, then W_{P}>>W_{n}
The total depletion width, W is given by
....(18)
where,
V_{B} = (V_{0}  V), V is the applied voltage and V_{0} is the barrier potential, or the contact
potential.
The capacitance that exists in a forward biased junction is called a diffusion or storage capacitance (C_{D}), whose value is usually much larger than C_{T,} which exists in a reversebiased junction. This is also defined as the rate of change ofinjected charge with applied voltage, i.e., C_{D}  (dQ/dV), where dQ represents the change in the number of minority camel's stored outside the depletion region when a change in voltage across the diode, dV is applied. Diffusion capacitance C_{D} is proportional to diode forward current, I. Therefore,
C_{D} = τI /(ηVt), where τ is the mean life time for holes and electrons. The values of C_{D} range from 10 to 1000pF, the larger values being associated with the diode carrying a larger anode current I.
The effect of C_{D} is negligible for a reversebiased pn junction. As the value of C_{D} is inversely proportional to frequency, it is high at low frequencies and it decreases with the increase in frequency.
For a reverse bias ‘g’ is very small and C_{D} may be neglected compared with C_{T}. For a forward current, C_{D} is usually much larger than C_{T}. Despite the large value of C_{D}, the time constant r^{C}D may not be excessive because the dynamic forward resistance r = 1/g is small.
r^{C}D = τ
Hence the diode time constant equals the mean lifetime of minority carriers, which lies in range of nanoseconds to hundreds of microseconds.
ChargeControl Description of a Diode:
I = (Q D_{p}) / Lp^{2} = Q/τ
This is very important equation states that the diode current is proportional to the stored charge of excess minority carriers.
Thus, in the steady state, the current I supplies minority carriers at the rate at which these carriers are disappearing because of the process of recombination.
An ideal pn junction diode is a two terminal polarity sensitive device that has Zero resistance (diode conducts) when it is forward biased and infinite resistance (diode does not conduct) when reverse biased. Due to this characteristic the diode finds number of applications as given below.
The same PN junction with different doping concentration finds special applications as follows:
Diodes which are designed with adequate power dissipation capabilities to operate in the breakdown region may be employed as voltagereference or constant voltage devices. Such diodes are known as avalanche, breakdown, or Zener diodes.
The reverse voltage characteristic of a semiconductor diode, including the breakdown region is shown in fig.7 (a). By reverse biasing, the diode is operating in the breakdown region as shown in fig.7 (b)
The diode will regulate the load voltage against variations in load current and against variations in supply voltage V because, in the break down region, large changes in diode current produce only small changes in diode voltage.
The upper limit on diode current is determined by the power dissipation rating of the diode. The thermally generated electrons and holes acquire sufficient energy from the applied potential to produce new carriers by removing valence electrons from their bonds. These new carriers, in turn, produce additional carriers again through the process of disrupting bonds. This cumulative process is referred to as “avalanche multiplication”. It results in the flow of large reverse currents, and the diode finds itself in the region of “avalanche breakdown”.
Even if the initially available carriers do not acquire sufficient energy to disrupt bonds, it is possible to initiate breakdown through a direct rupture of the bonds because of the existing of the strong electric field. Under these circumstances the break down is referred to as the “Zener Breakdown”. This Zener effect is now known to play with breakdown voltages below about 6V.
If the reference voltage is above 6V, where the physical mechanism involved is avalanche multiplication, the temperature coefficient is positive.
However, below 6V, where true Zener breakdown is involved, the temperature coefficient is negative.
The Zener breakdown voltage decreases with temperature. The value of the avalanche voltage must increase with increased temperature.
The capacitance across a breakdown diode is the transition capacitance. High power avalanche diodes have very large capacitances. C_{r} is from 10 to 10,000 pf are common.
(a) The voltampere Characteristic of an avalanche, or Zener, diode.
(b) A circuit in which such a diode is used to regulate the voltage across R_{L} against changes due to variations in load current and supply voltage.
A pn junction diode has an impurity concentration of about 1 part in 10^{8}. With this amount of doping, the width of the depletion layer, which constitutes a potential barrier at the junction, is of the order of 5 microns (5 x 10^{6} m). If the concentration of impurity atoms is greatly increased, say, to 1 part in 10^{3} (corresponding to a density in excess of 10^{19} /cm^{3}), the device characteristics are completely changed. This new diode was announced in 1958 by Esaki.
Tunneling Phenomenon: The width of the junction barrier varies inversely as the square root of impurity concentration and therefore is reduced from 5 microns to less than 100A
(10^{6} cm). This thickness is only about onefifth the wavelength of visible light.
Classically, a particle must have energy at least equal to the height of a potential barrier if it is to move from one side of the barrier to the other. However, for barriers as thin as those estimated above in Esaki diode, the Schrodinger equation indicates that there is a large probability that an electron will penetrate through the barrier. This quantum mechanical behavior is referred to as “tunneling”, and hence these highimpurity density pn junction devices are called “tunnel diodes”.
(a) symbol for a tunnel diode (b) smallsignal model of a tunnel diode in the negative resistance region (c) The voltampere characteristic of a tunnel diode.
The tunnel diode exhibits a negative resistance characteristic between the peak current I_{p} and the minimum value I_{v}, called the valley current. The tunnel diode useful in pulse and digital circuitry. The smallsignal model for operation in negative resistance region is indicated in fig.8.(b)
Tunnel diode is used as very high speed switch. Switching times of the order of a — nanosecond are reasonable and times as low as 50 p sec have been obtained. A second application is as a high frequency (microwave) oscillator. The voltages V_{v} and V_{F} have negative temperature coefficients of about 1.0 mV/^{0}C:
Tunnel diode Applications:
Advantages
Disadvantages
10 docs5 tests

Use Code STAYHOME200 and get INR 200 additional OFF

Use Coupon Code 
10 docs5 tests
