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ANALOG AND DIGITAL ELECTRONICS
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MODULE – 1
ANALOG ELECTRONIC CIRCUITS
OPTOELECTRONIC DEVICES
Optoelectronic Devices is the field that deals with study of devices that emit, detect and control light in
the wavelength spectrum ranging from ultraviolet to far infrared. They include electrical-to-optical
(convert electrical energy into light energy) and optical-to-electrical (convert light energy into electrical
energy) transducers. Optocouplers also come in this broad category.
PHOTODIODES:
Photodiode is a light detector semiconductor device that converts light energy into electric current or
voltage which depends upon the mode of operation.
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The upper cut-off wavelength of a photodiode is given by; 𝜆𝜆𝑐𝑐 =
𝐸𝐸𝑔𝑔
where, 𝜆𝜆𝑐𝑐 is the cut-off wavelength in nm and 𝐸𝐸𝑔𝑔 is the bandgap energy in eV.
A normal p-n junction diode allows a small amount of electric current, under reverse bias, due to minority
charge carriers. To increase the electric current under reverse bias condition, we need to generate more
minority carriers. The external reverse voltage applied to the p-n junction diode will supply energy to the
minority carriers, but it will not increase the population of minority charge carriers.
A small number of minority carriers are generated due to external reverse bias voltage. The minority
carriers generated at n-side or p-side will recombine in the same material, before they cross the junction.
As a result, no electric current flows due to these charge carriers. For example, the minority carriers
generated in the p-type material experience a repulsive force from the external voltage and try to move
towards n-side. However, before crossing the junction, the free electrons recombine with the holes within
the same material. As a result, no electric current flows.
To overcome this problem, we need to apply external energy directly to the depletion region to generate
more charge carriers. A special type of diode called photodiode is designed to generate more number of
charge carriers in depletion region. In photodiodes, we use light or photons as the external energy to
generate charge carriers in depletion region.
Construction:
The typical construction of a photodiode is illustrated in the following Figure. This example uses a
construction technique called ion implantation where the surface of a layer of N-type is bombarded with
P- type silicon ions to produce a P-type layer of about 1 µm (micrometre) thick. During the formation of
the diode, excess electrons move from N-type towards P-type and excess holes move from P-type towards
MAHESH PRASANNA K., VCET, PUTTUR
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N-type; this process is called diffusion, resulting in the removal of free charge carriers close to the PN-
junction, so creating a depletion layer as shown in the following Figure.
The (light facing) top of the diode is protected by a layer of Silicon Dioxide (SiO2) in which there is a
window for light to shine on the semiconductor. This window is coated with a thin anti-reflective layer of
Silicon Nitride (SiN) to allow maximum absorption of light and an anode connection of aluminium (AI)
is provided to the P-type layer. Beneath the N-type layer, there is a more heavily doped N+ layer to
provide a low resistance connection to the cathode.
Working Principle:
When the conventional diode is reverse biased, the depletion region starts expanding and the current starts
flowing due to minority charge carriers. With the increase of reverse voltage, the reverse current also
starts increasing. The same condition can be obtained in photodiode without applying reverse voltage.
The following Figure shows photo diode bias symbol. The junction of Photodiode is illuminated by the
light source; the photons strike the junction surface. The photons impart their energy in the form of light
to the junction. Due to which electrons from valence band get the energy to jump into the conduction
band. This leaves positively charged holes in the valence band, so producing 'electron-hole pairs' in the
depletion layer. Some electron-hole pairs are also produced in P and N layers, but apart from those
produced in the diffusion region N layers, most will be re-absorbed within the P and N materials as heat.
The electrons in the depletion layer are then swept towards the positive potential on the cathode, and the
holes swept towards the negative potential on the anode, so creating a photo current. In this way, the
photodiode converts light energy into electrical energy.
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V-I Characteristics of Photodiode:
The characteristics curve of the photodiode can be understood with the help of the following Figure. The
characteristics are shown in the negative region because the photodiode can be operated in reverse biased
mode only.
The reverse saturation current in the photodiode is denoted by I0, It varies linearly with the intensity of
photons striking the diode surface. The current under large reverse bias is the summation of reverse
saturation current and short circuit current.
𝐼𝐼 = 𝐼𝐼𝑠𝑠𝑠𝑠 + 𝐼𝐼0 (1 − 𝑒𝑒 𝑉𝑉/∆𝑉𝑉𝑉𝑉 )
Where Isc is the short circuit current, V is positive for forward voltage and negative for reverse bias, Vt is
volt equivalent for temperature, ∆ is unity for germanium and, 2 for silicon.
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Applications:
• Photodiodes are used in consumer electronics devices like smoke detectors, compact disc players,
and televisions and remote controls in VCRs.
• In other consumer devices like clock radios, camera light meters, and street lights,
photoconductors are more frequently used rather than photodiodes.
• Photodiodes are frequently used for exact measurement of the intensity of light in science and
industry. Generally, they have an enhanced, more linear response than photoconductors.
LIGHT EMITTING DIODE (LED):
The LED is a PN-junction diode which emits light when an electric current passes through it in the
forward direction. A P-N junction can convert absorbed light energy into a proportional electric current.
The same process is reversed here (i.e. the P-N junction emits light when electrical energy is applied to
it). This phenomenon is generally called Electroluminescence.
Electroluminescence is the properly of the material to convert electrical energy into light energy and
later it radiates this light energy. Different sizes of light emitting diodes are available in market form
lmm2 to onward.
Construction:
The semiconductor material used in LED is Galliurn Arsenide (GaAs), Gallium phosphide (GaP) or
Gallium Arsenide Phosphide (GaAsP). Any of the above-mentioned compounds can be used for the
construction of LED, but the color of radiated light changes with the change in material (for example,
GaP material gives green/red color with forward voltage of 2.2V).
The semiconductor layer of P-type is placed above N-type because the charge carrier recombination
occurs in P-type. Besides, it is the surface of the device, and thus, the light emitted can be easily seen on
the surface. If P-type is placed below the N-type, the emitted light cannot be seen. The following Figure
shows cross sectional view of diffused LED.
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