Thursday, August 18, 2011
A diode has ineternal body resistance and the extermal contact resistance.
The actual reverse saturation current of a commercially available diode will normally be measurably larger than that apperaing in the Shockley’s equation.
The contact area at the junction has a strong effect on the magnitude of the reverse saturation currnet.
The maximum reverse bias potential that can be applied before enetering the zener region is called the peak inverse voltage or PIV rating or peak reverse voltage or PRV rating.
At a fiuxes temperature the reverse saturation current of a diode increases with an increase in the reverse bias potential.
For commercially available diodes, for GaAs the reverse saturation current is typically about 1pA, compared to 10pA for Si and 1uA for Ge.
Breakdown voltages – Maximum GaAs, exceeds by 10% that Si devices which has 50V to 1kV. (However some Si Power Diodes have breakdown voltage 20kV). Ge typically has minimum breakdown voltage, having its maximum value 400V, generally less than 100V.
In the reverse bias region the reverse saturation current of a Si diode doubles for every 10 degree rise in temperature.
Thursday, August 18, 2011 0
Tuesday, August 16, 2011
The current that exists under reverse bias conditions is called reverse saturation current and may be represented by Is.
The general characteristics of a semiconductor diode can be represented by the Shockleys’s equation for forward bias and reverse bias regions.
ID = IS ( eVd/nVt - 1 )
Where ID is the reverse saturation current. VD is the applied forward bias voltage. N is an ideality factor, function of operating conditions and physical construction, range is from 1 to 2 depending upon a wide variety of factors. VT is the thermal voltage and is detemined by,
VT = kT/q, where k is Boltzmann’s constant which is equal to 1.38 * 10-23 J/K, T is the absolute temperature in kelvns, q is the magnitude of electronic charge.
ID = IS eVd/nVt - IS
Case 1. When VD is positive, ID =~ IS eVd/nVt
Case 2. When VD is negative, ID =~ -IS
Case 3. Wheb VD is zero, ID is also zero..
These quations can be easily plotted on a graph, because they contain only exponential function. So the semiconductor diode’s forward and reverse characteristics can be easily determined.
Tuesday, August 16, 2011 0
Monday, August 15, 2011
1. The first integrated circuit (IC) was developed by Jack Kilby while working at texas Instrumentsin 1958. It was a phase shift oscillator.
2. The three semiconductors used most in the construction of electroninc devices are Ge, Si and GaAs.
3. Diode was disvovered in 1939 and transistor was descovered in 1947.
4. In 1954 first silicon transistor was introduced.
5. Silicon is less temprature sensitive and one of the most abundant m,aterials on Earth.
6. First GasAs transistor was developed in early 1970s which had speed of operation five times that of silicon transistor. Today it is used consistently as base material for new high speed very large integrated circuits designs (VLSI).
7. At room temperature there are approximately 1.5 * 1010 free electrons in 1cm3 of intrinsic silicon material that is 15000000000 (15 billion) electrons in a space smaller than a small sugar cube – an enormous number.
8. Free carriers in GaAs have more than five times the mobility of free carriers in Si, a factor that results in response time using GaAs electronic devices that can be upto five times those of same devices made from Si. (Relative Mobility Factor of Si is 1500, Ge is 3900 and GaAs is 8500.)
9. Semiconductors have a negative temperature coefficient. They exhibit an increased level of conductivity with the application of heat.
10. Ge has the smallest band gap and GaAs has the largest gap. An electron in valence band of GaAs must gain more energy than one in Si or Ge to enter into conduction band.
11. For onductors, the overlapping of valence and conduction bands essentially results in all additional energy picked up by the electrons being dissipated in the form of heat. But for GaAs, the band gap is suficiently large to result in significant light radiation.
12. For LEDs, the level of doping and materials chosen determine the resulting colour.
13. Substiituting the charge of one electron and a potential difference of one volt results in an energy level reffered to as one electron volt.
Monday, August 15, 2011 0
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