646
23 Electricity-to-Light Conversion
Core–Valence Luminescence
In some materials, e.g., BaF 2 , CsF, BaLu 2 F 8 the energy gap between the valence band and the top core
band is less than the fundamental band gap. A radiative transition occurs when an electron from the
valence band fills a hole in the top core band that has been created by the radiation. The light yield is
limited to about 2 photons/keV.
Semiconductor Recombination Processes
Free excitons or excitons bound to impurities can recombine radiatively. This process is most efficient at
low temperatures. At room temperature, the emission is typically much weaker ( 10×) since excitons
become unbound or dissociated. Highly doped n-type semiconductors, e.g. CdS:In, exhibit recombination between donor-band electrons and holes. ZnO:Ga has an efficiency of about 15 photons/keV and
a fast response (with 2.4 photons/keV emitted in the first 100 ps). Luminescence can also stem from
donor–acceptor pair transitions, e.g. in PbI 2 with an efficiency of 3 photons/keV at 10 K. Isoelectronic
impurities such as nitrogen in GaP:N and tellurium in CdS:Te attract an electron and subsequently
a hole. In ZnS:Ag and ZnS:Cu (conduction) band to trap recombination is dominant. In a codoping
scheme like CdS:In,Te, In supplies electrons in an impurity band that can recombine with holes trapped
at Te.
23.3 Light-Emitting Diodes
23.3.1 Introduction
Light-emitting diodes (LEDs) are semiconductor devices in which injected carriers recombine radiatively. The recombination process leading to light emission can be of intrinsic nature, i.e. band–band
recombination, or extrinsic, e.g. impurity-bound excitons. Impurity-related luminescence can also be
excited via impact excitation. For an extensive treatment of LEDs see [1934, 1935], for a review of
the early field [1936] and for recent reviews [1937, 1938]. Mostly LEDs are pn-diodes although also
some MIS-based devices have been reported [1939, 1940].
23.3.2 Spectral Ranges
Applications for LEDs can be sorted by the color of emission. In Fig. 23.4, the standard sensitivity
V (λ) of the human eye is shown (see Fig. 23.1a). In the visible spectral region (about 400–750 nm) the
perceived brightness of the LED depends on the eye sensitivity. It is largest in the green (at 555 nm)
and drops strongly towards the red and blue.
The most important spectral regions and applications are:
• infrared (λ > 800 nm): remote controls, optocouplers, low-cost data transmission, IR interface
• visible: indicator LED, lighting
11 (room, buildings, cars), white LED for broad spectrum
• ultraviolet (λ < 400 nm): pumping of phosphors for white LEDs, biotechnology
In Fig. 23.4, potentially useful semiconductors for the various spectral regions are shown. The semiconductors that are currently used for the various colors of the visible spectrum are
• red–yellow: Ga(As,P)/GaAs, now (Al,In,Ga)P/GaP
• yellow–green: GaAsP:N, GaP:N
11 Penetration of white LEDs into the general lighting market could translate (globally) into cost savings of $ 10 11 or a
reduction of power generation capacity of 120 GW.
23 Electricity-to-Light Conversion
Core–Valence Luminescence
In some materials, e.g., BaF 2 , CsF, BaLu 2 F 8 the energy gap between the valence band and the top core
band is less than the fundamental band gap. A radiative transition occurs when an electron from the
valence band fills a hole in the top core band that has been created by the radiation. The light yield is
limited to about 2 photons/keV.
Semiconductor Recombination Processes
Free excitons or excitons bound to impurities can recombine radiatively. This process is most efficient at
low temperatures. At room temperature, the emission is typically much weaker ( 10×) since excitons
become unbound or dissociated. Highly doped n-type semiconductors, e.g. CdS:In, exhibit recombination between donor-band electrons and holes. ZnO:Ga has an efficiency of about 15 photons/keV and
a fast response (with 2.4 photons/keV emitted in the first 100 ps). Luminescence can also stem from
donor–acceptor pair transitions, e.g. in PbI 2 with an efficiency of 3 photons/keV at 10 K. Isoelectronic
impurities such as nitrogen in GaP:N and tellurium in CdS:Te attract an electron and subsequently
a hole. In ZnS:Ag and ZnS:Cu (conduction) band to trap recombination is dominant. In a codoping
scheme like CdS:In,Te, In supplies electrons in an impurity band that can recombine with holes trapped
at Te.
23.3 Light-Emitting Diodes
23.3.1 Introduction
Light-emitting diodes (LEDs) are semiconductor devices in which injected carriers recombine radiatively. The recombination process leading to light emission can be of intrinsic nature, i.e. band–band
recombination, or extrinsic, e.g. impurity-bound excitons. Impurity-related luminescence can also be
excited via impact excitation. For an extensive treatment of LEDs see [1934, 1935], for a review of
the early field [1936] and for recent reviews [1937, 1938]. Mostly LEDs are pn-diodes although also
some MIS-based devices have been reported [1939, 1940].
23.3.2 Spectral Ranges
Applications for LEDs can be sorted by the color of emission. In Fig. 23.4, the standard sensitivity
V (λ) of the human eye is shown (see Fig. 23.1a). In the visible spectral region (about 400–750 nm) the
perceived brightness of the LED depends on the eye sensitivity. It is largest in the green (at 555 nm)
and drops strongly towards the red and blue.
The most important spectral regions and applications are:
• infrared (λ > 800 nm): remote controls, optocouplers, low-cost data transmission, IR interface
• visible: indicator LED, lighting
11 (room, buildings, cars), white LED for broad spectrum
• ultraviolet (λ < 400 nm): pumping of phosphors for white LEDs, biotechnology
In Fig. 23.4, potentially useful semiconductors for the various spectral regions are shown. The semiconductors that are currently used for the various colors of the visible spectrum are
• red–yellow: Ga(As,P)/GaAs, now (Al,In,Ga)P/GaP
• yellow–green: GaAsP:N, GaP:N
11 Penetration of white LEDs into the general lighting market could translate (globally) into cost savings of $ 10 11 or a
reduction of power generation capacity of 120 GW.