Diodes - Simple Applications Why would you want the equivalent of an electronic check-valve?
Diode Current-Voltage relationship The characteristics of all silicon based diodes are effectively the same, showing a sharp exponential increase in current at forward bias and only a few As at reverse bias. Note that this “knee” is temperature-dependent as observed in lab 1. The non-ideal behavior shows a slower than exponential increase due to a series resistance, like the inductor. At large reverse-bias (not shown) current starts to flow. We will discuss this region later. 1N4148 characteristics
Diode models A diode is often approximated as a 2-terminal device with ~0.7 v drop as long as 1-100 mA are flowing in the forward direction. The curves for a typical switching signal diode show why this is OK (note the log scale for current). When it is either reverse or weakly forward biased it is modeled as an open circuit. Real diodes and the symbol are shown. The arrow points in the direction of forward current flow.
More Diode Physics Pure semiconducting materials like Silicon have very high resistivity at room temperature. However it can be made conductive by adding extra electrons. In n-type material this is done via atoms with one more electron than silicon (phosphorus). It can also be more conductive if one electron is removed, allowing the resulting “holes” to move about (they are really absences of valence or bonding electrons that hop from one site to the next). This is done in p-type material by adding elements with one less electron (aluminum). This is known as doping. An interesting thing happens if p- and n-type material are in contact. A region that has no holes on the p-side and no electrons on the n-side forms, called the depletion region. This area is like the original pure semiconductor and does not conduct, making the junction a very poor conductor again. However if it is biased (meaning a voltage is applied) with the p-type positive relative to the n-type, the depletion region shrinks. This is the origin of the one-way flow. The next slides describe why this is, using images from the website http://jas.eng.buffalo.edu/
Diode Current-Voltage relationship (1) When material with lots of electrons (n-type) is brought into contact with material with lots of holes (p-type) the electrons and holes diffuse, resulting in a large current. However the loss of these carriers leaves a depleted region that is charged (because of the P and Al ions left behind) and reduces the current since the electric field repels the carriers.
Diode Current-Voltage relationship (2) At zero bias electrons (holes) flow left (right) because of diffusion and an equal amount flow right (left) due to the electric field, resulting in no current. As the depletion region increases with reverse bias (not shown), the diffusion current to the left decreases leaving only a small reverse saturation current due to the field. Notice that for these doping levels the depletion region is 0.15 microns thick.
Diode Current-Voltage relationship (3) At positive bias the depletion region is reduced and many electrons (holes) diffuse left (right) while only a small amount flow right (left) due to the electric field, resulting in a large current (sum of holes and electrons) flowing right. When an electron gets to the p-type material it recombines with a hole. If it does this in the depleted region the ideality factor N is 2, while it is 1 if recombination occurs outside. The depletion region is now only 0.05 microns thick.
Types of diodes Power diodes (or rectifiers) - designed to handle large currents, typically used on power supplies or for switches. Typical values (for 1N4002) Max Average current: 1 A Peak current: 30 A Reverse voltage: 100 v Signal diodes - low powered, often faster switching (smaller depletion region). Typical values (for 1N4148) Max Average current: 100 mA Peak current: 450 mA Reverse voltage: 75 v Recovery time: 4 ns Zener diodes - voltage regulation, similar to power diodes but has any important specification: the zener voltage, V z (more later). Designed to breakdown at precise voltage (range 1-400 v).
Clamp (or clipper) What does this circuit do? As mentioned in the text it “clamps” or “clips” the output voltage. If the magnitude of the input exceeds V batt +0.7 v, either negative or positive, the diode starts conducting. This limits the maximum voltage (this is similar to surge protectors). V bat One limitation of this is that by design it will distort a time-varying voltage input, cutting it off at a max and a min value. V in R V out
Stiff Clamp V in C V out This circuit on the other hand works very well at higher frequencies in limiting the minimum voltage on the output. Treating the diode like a simple one-way switch we see that the output can never fall below -0.7 v, because then the diode begins conducting. How does the circuit do this? If the input voltage is less than this minimum, current flows counterclockwise, charging the left capacitor plate negative and the right plate positive. This capacitor voltage adds to the input, raising it to a minimum of -0.7 v, when current stops flowing. Note that it is the opposite if the diode is reversed because the currents flow clockwise.
Zener diode - IV characteristics All diodes show a breakdown under reverse bias when the internal electric field gets very high, ripping electrons from atoms. This avalanche breakdown occurs typically at -50 to -1000 v. If the depleted region that acts like a barrier to current is thin there is another possibility. The electrons can quantum mechanically tunnel from the n- to the p-side, resulting in a reverse current. This can be engineered to occur at a precise voltage called the Zener voltage, V Z . V I V Z I II III The graph shows three distinct regions, as labeled. Region III is where zener diodes normally operate.
Zener diode - voltage regulator (1) How can a zener regulator act to control the output voltage? In the circuit shown (note that the zener is connected so that it is reverse biased ) current flows through the zener and it is in region III, so the voltage drop across it is V Z . A zener only works as a regulator because it ensures that enough current is drawn through the series resistor so that the voltage drop results in V out =V Z . If no current flows through the zener (actually less than a few mAs) it no longer regulates because it is in region II. V out Unregulated V in
Zener diode - voltage regulator (2) The voltage across the load is V Z since there is current through the zener (Region III) R R The voltage on the load is R load /(R+R load )V in because there is no current through the zener. R load Unregulated V in R load Unregulated V in