CSE-Module- 3:LED
PHYSICS
CSE- Module-3: SEMICONDUCTOR LIGHT EMITTING DIODE :LED (5Lectures)
Light Emitting Diodes (LEDs) Light Emitting Diodes (LEDs) are semiconductors p-n junction operating under proper forward biased conditions and are capable of emitting external spontaneous radiations in the visible range (370 nm to 770 nm) or the nearby ultraviolet and infrared regions of the electromagnetic spectrum
General Structure LEDs are special diodes that emit light when connected in a circuit. They are frequently used as “pilot light” in electronic appliances in to indicate whether the circuit is closed or not. The structure and circuit symbol is shown in Fig.1. The two wires extending below the LED epoxy enclose or the “bulb” indicate how the LED should be connected into a circuit or not. The negative side of the LED is indicated in two ways (1) by the flat side of the bulb and (2) by the shorter of the two wires extending from the LED. The negative lead should be connected to the negative terminal of a battery. LEDs operate at relative low voltage between 1 and 4 volts, and draw current between 10 and 40 milliamperes. Voltages and Fig.-1 : Structure of LED current substantially above these values can melt a LED chip. The most important part of a light emitting diode (LED) is the semiconductor chip located in the centre of the bulb and is attached to the
1
CSE-Module- 3:LED
top of the anvil. The chip has two regions separated by a junction. The p- region is dominated by positive electric charges, and the n-region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and n-regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow and the electrons cross the junction into the p-region. In the absence of large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to flow of electrons.
LED: p-n Junction
When an n-type semiconductor is brought into the physical contact with a
p-type semiconductors, a p-n junction is created. At the boundary of the junction, electrons from the n-side diffuse to the p-side and recombine with holes and at the same time, holes from the p-side diffuse to the n-side and recombine with electrons. Thus, a finite region called the depletion or space charge region forms. Here there are no mobile electrons or holes. Since positive ions at the n-side and negative ions at the p-side within the depletion region are left without electrons or holes, these ions create an internal electric field called a contact potential. This potential is noted by
VD in Fig. 2(a).
Fig.-2 : Light radiation by the p-n junction of a semiconductor (a) Depletion region
and Depletion voltage VD (b) Light radiation as the result of electron-hole recombinations.
2
CSE-Module- 3:LED
To radiate the quantum of light-photon it is necessary to sustain electron- hole recombinations. But the depletion voltage prevents electrons and holes from penetrating into a voltage is called as forward biasing voltage, V as shown in Fig. 2 (b). Emission of Light
When sufficient voltage is applied to the semiconductor chip across the leads of the LED, electrons can move easily in one direction across the junction between the p and n-regions. In the p-region there are many more positive than negative charges. In the n-region the electrons are more numerous than the positive electric charges. When a voltage is applied and the current starts to flow, electrons in the n- region have sufficient energy to move across the junction into the p-region. Once in the p-region the electrons are immediately attracted to the positive charges due to the mutual coulomb forces of attraction between opposite electric charges. When an electron moves sufficiently close to a positive charge in the p- region, the two charge “re-combine”. Each time an electron recombines with hole, electric potential energy is converted into electromagnetic energy. For each electron-hole recombination, a quantum of electromagnetic energy is emitted in the form of a photon of light with a frequency characteristic of the semiconductor material (usually a combination of the chemical elements gallium, arsenic and phosphorus). Only photons in a very narrow frequency range can be emitted by any material. LEDs that emit different colours are made of different semiconductor materials and require different energies to light them, as given below : GaAs infra red radiation (invisible) GaP red or green light GaAsP red or yellow light
3
CSE-Module- 3:LED
Gallium Arsenide has much probabilities of radiative recombination than other semiconductor such as Ge or Si.
Energy of Emission
The electric energy is proportional to the voltage needed to cause electrons to flow across the p-n junction. The different coloured LEDs emit predominantly light of a single colour. The energy of the light emitted by an LED is given by E = electric charge ×voltage required to light the LED E = qV Joules In one knows the voltage across the leads of an LED, one can find the corresponding energy required to light the LED. Let us say that one has a red LED, and the voltage measured between the leads is 1.65 volts. So the energy required to light the LED is E = qV = 1.6×10-19 (1.65) Joules = 2.64×10-19 joule
Difference between an LED and a Regular diode
The difference between an LED and a regular diode is that in a regular diode the electron-hole recombinations release energy in the thermal rather than the visible portion of the spectrum. Due to this reason, the electronic devices warm up when you turn on. In an LED, however, these recombinations result in the release of radiation in the visible, or light, part of the spectrum. The first type of recombination is called nonradiative while the second type is called radiative recombination. In a diode both types of recombinations take place, in case of LED the majority of recombinations are radiative.
4
CSE-Module- 3:LED
Radiated Power and Input-Output Characteristic
When forward biased voltage is applied to the leads of LED, the forward current injects electrons into the depletion region, where they recombine with holes in radiative and nonradiative ways. The nonradiative recombinations take excited electrons from radiative recombinations, this results in the decrease in the efficiency of the process which is characterized
by the internal quantum efficiency, int,
(a) (b) Fig.-3 : (a) Electronic circuit of an LED under operation and (b) input-output characteristic.
Quantum Efficiency (int) and LED Power The internal quantum efficiency in the active region is fraction of the electron-hole pairs that recombine radiatively. If the radiative
recombination rate is Rr and the nonradiative recombination rate is Rnr,
then the internal quantum efficiency int is the ratio of the radiative recombination rate of the total recombination rate
Rr int (1) Rr Rnr
5
CSE-Module- 3:LED
For exponential decay of excess carriers, the radiative recombination n lifetime is r and the nonradiative recombination lifetime is Rr n nr . Rnr Thus the internal quantum efficiency can be expressed as
n r int n r n nr
1 r int 1 r 1 nr int (2) r where is the bulk recombination life time, given by the expression : 1 1 1 r nr The power of radiated light as a function of forward current can be plotted in the form of a graph known as input-output characteristic of an LED, shown in Fig.-3(b). From figure, it is clear that the greater the forward current, the greater the number of electrons that will be excited at the conduction band and the grater the number of photons (light) that will be emitted. Hence radiated light power of an LED is directly proportional to the forward current. However, for higher values of forward current one expects the saturation effect as shown by dotted line in Fig.-3(b). This takes at the point where all the available mobile electrons will be involved in radiation and further increasing the current value, will not produce additional photons hence the emitted light.
Light Power, P (energy per second), is the number of photons (np) times the
energy of an individual photon, Ep
6
CSE-Module- 3:LED
n p E p P (3) t
where, np Nint where, N is the number of excited (injected) electrons.
Nint E p P (4) t
Ne Also current I (5) t
Putting the value of N from equation (5) in (4), we get. Hence the radiated light power is
It int E p P e t
int E p P I (6) e
If E p is measured in electron volts (eV) and I in milliamperes (mA), the
PmW intEp eV ImA (7) To find the emitted power, one needs to consider the external quantum
efficiency (ext ). This is defined as the ratio of the photons emitted from the LED to the number of internally generated photons. This can be expressed as
1 c T 2 sin d ext (8) 4 0
7
CSE-Module- 3:LED
where c is the critical angle, in incidence angle, T() is the Fresnel co- efficient or Fresnel transmissivity.
The critical angle is given as
n 1 2 c sin (9) n1 where n1 is the refractive index of the semiconductor material and n2 is the refractive index of the outside material.
For normal incidence, the Fresnel transmissivity is given as
4n1n2 T0 2 (10) n1 n2
Assuming that the outside medium is air and let n1 = n
4n T0 n 12
The external quantum efficiency is then approximately given by
1 ext (11) nn 12
The optical power emitted from the LED is given by
P ext Pint
Pint P (12) n n 12
8
CSE-Module- 3:LED
LIGHT SOURCE MATERIALS
LEDs that emit different colours are made of different semiconductor materials. The semiconductor materials that are used for the active layer of an optical source must have a direct band gap. In a direct band gap semiconductor, electrons and holes can recombine directly across the band gap without needing a third particle to conserve momentum. Various binary, ternary and quaternary semiconductors are used for optical sources. Binary semiconductor, those semiconductors containing a single anion and a single cation, that are used in LED include , SiC, ZnSe, GaN, GaP, GaAs, InP, InAs and GaaSb. The first three are wide band gap semiconductors and are used to generate blue light. However, they are presently of little importance because LEDs made from them are very inefficient. Many of the visible LEDs are made with GaP containing different dopants. More lightly nitrogen-doped material is used for the yellow emitter. When GaP is doped with both zinc and oxygen, it emits red light. GaAs, InP, InAs and GaSb, unlike GaP, are direct band gap materials so that they emit light more sufficiently. However, they emit in the infrared region. Ternary semiconductors such as GaAsP, GaAlAs and InGaAs are used because one can adjust the energy gaps to a desired wavelength by choosing the appropriate composition. There are, however, no ternary substrates, and the interface defects generated by growing films on a substrate with a different lattice parameter can be very detrimental to the device characteristics. This problem can be solved by using a quaternary semiconductor such as InGAAsP so that of the substrate. Ternary LEDs have the advantage that they can be tuned to emit light at a specific wavelength as the energy gap varies with the composition. To a first approximation it varies linearly and is given by
9
CSE-Module- 3:LED
Eg A1x BxC Eg AC [Eg BC Eg AC x] (13) Whereas a more accurate representation is
Eg A1xBxC Eg AC [ Eg BC Eg AC x]bAB x1 x (14)
where x is the fraction of component B and bAB is called the bowing parameter.
It is an experimentally determined parameter that is always positive; Eg always varies sublinearly. Clearly, either the cations or the anions can be mixed.
When energy band gap for ternary semiconductor is computed, both the direct and the indirect gaps are found whichever one is smaller is the listed
energy gap of the material. For GaAs1-xPx the gap is direct for small values
of x and indirect for larger values. For example: GaAs1-xPx is the principal material used for operation in the 800-900 nm spectrum. The values of lattice parameter, direct and indirect energy gap of III-V semiconductor components are given in Table-1
Characteristics of LEDs 1. Life Time, Rise / Fall time : lifer time of the charge carriers is the time between the moment they are excited (injected into depletion region) and they are recombined. This is also known as recombination life time and it ranges from nanoseconds to milliseconds. There are two recombination
10
CSE-Module- 3:LED
lifetimes : one is radiative (r) and other is nonradiative (nr). So total carrier lifetime is given as 1 1 1 (1) r nr
Rise / Fall time : tr is defined as 10 to 90% of the maximum value of the pulse, as shown in Fig-4 (a). For an LED, this characteristic shows how an output light pulse, follows the electrical-modulating input pulse, which is illustrated in Fig-4 (b).
(a) (b)
Fig.4: Characteristics of an LED : (a) rise time and fall time and (b) modulation of pulse
Rise / fall time is determined by an LED’s capacitance (C), input step
current with amplitude (IP) and the total recombination lifetime () 1.7104 T K C tr 2.2 (2) I P
where T K is absolute temperature. 2. Modulation Bandwidth,: BW, of an LED can be defined in either electrical or optical terms. Normally, electrical terms are used since the bandwidth is actually determined via the associated electrical circuitry. Thus, the modulation bandwidth is defined as the point where the electrical signal power has dropped to half its constant value resulting from the modulated
11
CSE-Module- 3:LED
portion of the optical signal. This is the electrical 3-dB point; that is frequency at which the output electrical power is reduced by 3 dB with respect to the input electrical power, as illustrated in Fig-5
Fig.-5 : Frequency response of the optical source showing the electrical and optical bandwidth. In electronics, the general relationship between bandwidth and rise time is given as 0.35 BW tc But practical value of BW, does not match with the theoretical value, calculated from above. This discrepancy occurs because if the forward current is modulated at angular frequency, , an LED’s output light intensity, I(), will vary as follows :
1 2 2 I I0 1 Were I(0) is the LED’s light intensity at constant current. Detected electric power is proportional to I2 I 2 1 Taking I 2 0 2
12
CSE-Module- 3:LED
Which is a 3 dB decline. Hence bandwidth becomes: 1 BW From above equation we find conclusion that an LED’s modulation bandwidth is limited by the recombination lifetime of the charge carriers. Power-bandwidth product is another important characteristics of an LED. The product of LEDs output power and its modulation bandwidth is constant. BW P Constant This means that if we increase bandwidth, the output will decrease and vice- versa. 3. Radiated wavelength and spectral width : Radiated wavelength,
referred as a peak position λp is determined by an energy gap, Eg and spectral width, Δλ is measured as full width at half maximum (FWHM).
Fig.-6: Thermal broadening (Relative intensity versus emission peak
energy minus Eg at a given temperature).
Both the peak position and bandwidth are affected by the temperature. The peak moves longer wavelength and the peak broadens with rise in
13
CSE-Module- 3:LED
temperature. One can attribute the longer wavelength to a smaller energy gap which, in turn, can be attributed to less splitting between the bonding and antibonding orbitals as the atoms move further apart.
The thermal peak broadening is due to a broadened thermal distribution of the electrons in the conduction band or the holes in the valence band both. The peak broadening is shown in Fig.-6.
The peak width also increases with the doping level and this is shown in Fig.-7. This is attributed to the fact that the donor and / or acceptor bandwidth increases with the doping level ad electron in the conduction band can fall to any level in the acceptor band.
Fig.-7 : Emission peak broadening due to increase doping levels for (a) silicon doped GaAs at 77 k and (b) germanium and zinc doped GaAs at room temperature.
14
CSE-Module- 3:LED
Table-2 Typical Characteristics of LEDs
15