E.D.1.V3 - 303.1.V4 Drexel University Electrical and...

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303.1.V4 1-1 Drexel University Electrical and Computer Engr. Dept. Electrical Engineering Laboratory III, ECEL-303 E. L. Gerber ELECTRONIC DEVICES Object The object of this experiment is to introduce you to several semiconductor electronic devices and their properties. You will be able to identify these devices by name and symbol and you will be able to utilize them in several applications. Theory Silicon is the most common semiconductor material used in fabricating electronic devices. Very pure silicon is “doped” with either positive-ion or negative-ion atoms to create P-type or N-type semiconductor regions in the device. The conductivity of the structure is controlled by the amount of doping that is added to the semiconductor. The conductivity is given as σ = qμN, where q = 1.6 x 10 -19 C, μ is the mobility of the carrier (electrons or holes), and N is the doping concentration. A semiconductor resistor can be formed using this method, where R = L/A σ . 1 - PN Junction: A PN junction is formed when a pure semiconductor is doped with P-type ions in one region and N-type in an adjacent region. See Fig. 1. Fig. 1. PN Junction Structure and Symbol. The interface of PN material defines the basic PN junction, W J . When a PN junction is forward biased (P-side is connected to a positive voltage), large amounts of current will flow. V is a positive value in the equation below when it is forward biased.
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303.1.V4 1-2 I = I o [exp(qV/kT) - 1] This equation is an accurate model of the PN junction. The current increases exponentially with the positive voltage, V, applied. When the junction is reversed biased (P-side is connected to a negative voltage), very little current flows; the equation gives, I = - I o . I o is the saturation current of the junction and is very small (~10 -9 amp). At room temperature q/kT 39. The width of the actual junction is very small, in the range of 10 -5 to 10 -4 cm. The P- and N-type doped regions may be many times larger than the junction. The junction region is depleted of carriers; i.e., no free electrons or holes are in the junction structure. When the PN junction is reverse biased, large voltages can be applied across the junction, and the current remains at - I o . If the electric field in the junction, E = V/W J , exceeds the dielectric breakdown strength of the material ( E b 3x10 5 V/cm for Si) the junction electrically breaks down and current flows. The typical breakdown voltage of a junction is V B = E B W J 3x3.3x10 0 = 10 V. C. Zener first described the breakdown condition in PN junctions. The phenomenon results in a large current flow at the breakdown voltage, called V B or V Z , the Zener voltage. The voltage across the junction remains nearly constant over a wide range of currents during breakdown. All PN junctions will break down when the voltage applied is larger than V B . The condition is not catastrophic. When the voltage is reduced, the current decreases to -I 0 and the device is still usable. It is not
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E.D.1.V3 - 303.1.V4 Drexel University Electrical and...

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