LIGHT SEMICONDUCTORS

In 1907, English engineer Round, who worked at the world famous Marconi laboratory, accidentally noticed that running the detector around a point of contact there is a glow. Also seriously interested in this physical phenomenon and tried to find the practical application of “inconceivably talented Russian” — Oleg Vladimirovich Losev.

 

Found in 1922 during their nightly radioact glow of the crystal detector, this then 18-year-old, ham radio was not limited to the statement “strange” fact, and immediately moved to the original experiments. In an effort to obtain a stable generation of the crystal, it passes through a point contact diode detector current from the batteries. That is dealt with nothing else, like a prototype semiconductor device, later called led. The whole world started talking about “Losev effect” on the practical application which the inventor had to (before his death in the war in 1942), four (!) patent.

 

Since 1951, the center for the development of “semiconductor lamps”, operating on the basis of “Losev effect” has moved to America, where it was headed by K. Lehovec (USA). In the study of problems associated with LEDs, and took an active part and “father of transistors” physicist W. Shockley.

 

It soon became clear that germanium (Ge) and silicon (Si), on the basis of which to make a semiconductor triode, hopeless for the LEDs because of too much “work output” and, accordingly, weak emission of photons on p-n junction (more about this in the journal “modelist-Konstruktor” No. 5 and 6 for 1996). The success accompanied the single crystals of complex composite semiconductors — compounds of gallium (Ga), Arsenicum (arsenic — As), phosphorus (P), indium (In), aluminum (Al), other elements of the periodic system. As a result, in the late 60-ies LEDs, becoming quite technologically advanced devices with stable parameters, began to increasingly enter our daily life.

 

 

 

 

Structural diagram homemade sweetelena (A), schematics of the transmitter (B) family of output characteristics (Y) and the receiver (In)

 

In the cross section of a crystal of any of the modern LEDs can be divided into two region: p-and n-conductivity. Led light is the result of recombination (connection) of electrons and holes on the surface of the p-n junction during the occurrence of a strong direct current (“Losev effect”). Minority carriers (for the recombination process is the electrons) come into the p region of the led from the electrical power source. The more current in the “forward” direction PR, the more comes electrons and the stronger the flow of light emitted from the surface of the p-n junction.

 

As for the brightness, then it is not indifferent and optical clarity n-region (ultrathin films of semiconductors are quite transparent). Well, the color (frequency) of radiation, having a clear functional relationship with the energy of the emitted photons depends on the semiconductor materials p-n junctions. In particular, a pure monocrystal of GaAs gives the infrared beam, a small additive of Al and/or P changes the emission color to red. The green light emitting GaP. The use of p-n junction on the basis of the composition allows to obtain dual color mini-yellow or orange light. Blue LEDs were late for twenty years — used components of a complex and even now are very expensive. Well, the LEDs are white (the real “semiconductor light bulbs”), there are still only in draft.

 

The active element is a semiconductor monocrystal in most modern LEDs used in the form of a die (chip) size 0,3×0,3×0,25 mm, containing the p-n or heterojunction and ohmic contacts. However, LEDs are also available with two chips (in different color of light) or one, but having a pair of p-n junctions that emit in two spectral bands. In this case you can control the glow color. There are more complex structures with embedded electronic components or chip, which allows to control the supply voltage. Very often all combined in a transparent plastic casing (with the cap in the form of microlenses or contain other elements concentration of radiation, enhancing the exterior of the optical efficiency and the necessary orientation of the radiation).

 

All gallium arsenide LEDs have similar electrical parameters. For these semiconductor devices characterized by a sufficiently large direct (forward) voltage drop Ypres = 1,8…2,2 V. But, unfortunately, a valid reverse (breakdown) voltage Uo6p also is a few volts. And it is necessary to consider protecting the LEDs from dangerous overvoltage additional input silicon diode in the “reverse” is turned on.

 

As a rule, led working indicator, it is enough 1-2 mA. Well, where is required high brightness of illumination (concentrated, say, in a narrow beam) use a special semiconductor (including a laser!) devices or increase the current a standard led with a built-in winsocki, not forgetting the observance of the thermal mode. When SD equal to, for example, 50 mA, the led average brightness will dissipate about 100 mW. And the cost of radiation are only a few mW, the remaining power necessary from the led can take without overheating the crystal.

 

From the first LEDs were able to light force not exceeding a few tens of microchannel (candela candle). It’s not a deficit, even super-bright LEDs, which is a 3.5 KD is not the limit. Based on the use is quite common semiconductor devices are designed, in particular, and original homemade sweetlife. Using two of these devices are easy to organize, for example, a private light connection, the type of industrial laser.

 

Analysis the circuit diagram of the transmitting channel sweetelena better to start with a light emitter VD4 — a relatively inexpensive led L-1513SRC-E, manufactured by Knightbright. This led of 5 mm diameter is able to create directional red light with a light intensity of 3.5 CD. Its linear modulator brightness is performed on the operational amplifier OPAMP of the type К140УД7 (К153УД2). Moreover, in the embodiment of the structure imposed on the readers loved the magazine, this “operazioni” works as a voltage Converter of the signal Uc to the load current IR equal to SD of the light emitter.

 

Indeed, for the high quality of OU control voltage observed between the inputs, can be considered zero. This means that the voltage drop across the resistor-sensor RD (R10) when the flow of the load current must be almost equal to the signal voltage, that is, Uc = Upд = lсдRpд. It is easy to calculate the current flowing through the led: SD = Uc/Rрд. The steepness of amplification of the cascade, respectively, will be S = I/Is = 1/Rрд.

 

Family typical performance characteristics led L-1513SRC-E can be found and the optimal power mode VD4 light emitter current SD. When Rрд about 1 Ohm (that is the nominal value of R10 from the circuit) the steepness of the S will be 1 A/V. the resistor-current limit R9 is 45 Ohms, minimum (threshold) current Min = 30 mA, maximum current Imax = 80 mA. The working point RT is selected in the middle of the linear section of the graph. To enable this mode on the+input of op-amp DA2 was required to file a potential shift of Uсдв = 55 mV overlay on it signal with amplitude Uc(t) = ±25 mV. And since the output current OPAMP for General use may not exceed 25 mA, the feedback loop added current amplifier — medium power transistor VT1.