The unneutralized ions in the neighborhood of the junction are referred to as uncovered charges. it is called the depletion region, the space-charge region, or the transition region.

In order to have a steady flow of holes to the left, these holes must be supplied across the junction from the n-type silicon. And there are very few holes in then-type side. Hence, nominally, zero current results. Actually, a small current does flow because a small number of hole-electron pairs are generated throughout the crystal as a result of thermal energy. The holes so formed in the n-type silicon will wander over to the junction. A similar remark applies to the electrons thermally generated in the p-type silicon. This small current is the diode reverse saturation current, and its magnitude is designated by Io.

An external voltage applied with the polarity shown in Fig. 3.3 is called a forward bias.

If V were equal to Vo, the barrier would disappear and the current could be arbitrarily large, the bulk resistance of the crystal, as well as the resistance of the ohmic contacts, will limit the current.

The symbol h is unity for germanium and is approximately 2 for silicon at rated current.

At room temperature (T = 300°K), VT = 0.026 V = 26 mV.

There exists a cutin, offset, break-point, or threshold, voltage Vg below which the current is very small (say, less than 1 percent of maximum rated value). Vγ is approximately 0.6 V for silicon.

Temperature Dependence of V/I Characteristics:From experimental data, it has been observed that the reverse saturation current approximately doubles for every 100C rise in temperature.

If I = I01 at T = T1, then at a temperature T, I0 is given by10/)(

01012)( TTITI −×=

It is found that for either silicon or germanium (at room temperature)

CmVdTdV 0/5.2−≈

in order to maintain a constant value of I.

For small-signal operation the dynamic, or incremental, resistance r is an important parameter, and is defined as r ≡ dV/dI.

For I >> I0

Two mechanisms of diode breakdown for increasing reverse voltage are recognized.avalanche multiplication

LightLight--emitting Diodesemitting Diodes

• LED is a diode that gives off visible visible or invisible(infrared) light when energized

• In any forward biased pn-junction, there is, within the structure and primarily close to the junction, a recombination of holes and electrons takes place

• This recombination requires that the energy possessed by the unbound free electrons be transferred to another state. In all pn-junctions some of this energy is given off in the form of heat and some in the form of photons

• In Si and Ge diodes the greater percentage of the energy converted during recombination is dissipated in the form of heat within the structure, and the emitted light is insignificant. For this reason Si and Ge are NOT used in the construction of LED devices

• Diodes constructed of GaAs emit light in the infrared(invisible) zone during the recombination process at the pn-junction

• Through other combinations of elements a coherent visible lightcan be generated, like GaN, GaP, GaAsP, AlInGaP etc.

• The frequency spectrum for infrared light extends from about 100 THz to 400 THz, with the visible light spectrum extending from about 400 to 750 THz

The amount of energy involved is given by

λhcEg =

h = Planck’s constant = 6.626 × 10-34 J s

PhotodiodesPhotodiodes

• Light energy, transmitted as discrete packages called photons, has a level directly related to the frequency of the traveling light as: W = hf joules

• The frequency is, in turn, directly related to the wavelength. The wavelength is important because it determines the material to be used in the optoelectronic device.

• The number of free electrons generated in each material is proportional to the intensity of incident light (measured in lm/ft2, footcandles (fc), or W/m2

1lm/ft2 = 1 fc = 1.609 × 10-9 W/m2

1 lm = 1.496 × 10-9 W

• The photodiode is a semiconductor pn-junction device whose region of operation is limited to the reverse bias region. It conducts when light is applied to the junction

Recall: The reverse saturation current is due solely to the thermally generated minority carriers in the n- and p-type materials.

• The application of light to the junction will result in a transfer of energy from the incident traveling light to the atomic structure, resulting in an increased number of minority carriers and an increased level of reverse current.

ApplicationsApplications

• Instrumentation circuits as a sensor• Alarm system sensor• Detection of objects on a conveyor belt

OperationOperation

The photodiode is operated in reverse bias. When light of a particular wavelength strikes the junction it conducts. The higher the intensity of light (measured in foot-candles), the higher the conduction through the photodiode.

Note that the diode conducts somewhat with no light applied, this is called the dark current.

1919

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