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  Semiconductor Physics, Part 3 
  May 09, 2003, 10:00am EDT 

Breaking Down the Walls

By: Dan Mepham

Suppose I connect my pn junction to a 0.5V battery. I’ll connect the positive terminal to the p side, and the negative terminal to the n side. The battery forces a more positive potential on the p-type side, which acts to lower its energy relative to the n-side. In other words, the applied voltage will act against the built-in voltage, and act to compress the bands back together! This is called forward biasing the junction." alt="Semiconductor Physics">
Fig. 12 - A positive voltage applied to the p-type side acts to decrease the difference in the bandgaps.

By applying 0.5V to the p-type side, we’ve forward biased our junction, and effectively cancelled out 0.5V of the built-in voltage, leaving only 0.2V. The hill is that much smaller, but we’re not quite there yet. The magic happens when we apply a voltage equal to, or greater than the built-in voltage. Watch what happens if we connect a 0.8V battery." alt="Semiconductor Physics">
Fig. 13 - One a strong enough forward bias is applied, the bandgaps level out, effectively cancelling out the internal electric field, and allowing the diffusion currents to take over. Conduction goes through the roof at this point.

The flood gates are open, and electrons start to pour across the barrier in huge quantities. Not only are they diffusing across the depletion region at a high rate, but now that the electric field is reversed, they’re also being accelerated across! Because there’s such a high concentration of electrons in the n-type region, the diffusion currents are extremely strong, and electrons flow at an enormous rate. Holes do the same, in the opposite direction, of course. The current through the sample will increase exponentially as the voltage (forward biasing) increases; so fast, in fact, that if there is no external circuit in place to control the flow (a resistance), the current would increase so much that the junction would burn and destroy itself.

Essentially, we now have a device that, if subjected to any voltage under 0.7V, will not conduct, but once exposed to greater than 0.7V, will conduct like crazy. This includes negative voltages as well; applying a negative voltage to the p-type side will do the opposite of above, pulling the bands further apart, and resisting conduction. This is known, unsurprisingly, as reverse biasing. Theoretically, we could apply an infinite negative voltage to the p-side, and the device would not conduct**.

So basically we now have a device that conducts only in one direction, and only after a certain voltage. This, ladies and gentlemen, is a diode, and its practical uses are vast. It doesn’t take a Ph.D to realize the usefulness of a device that’s essentially a current ‘switch’, flowing only in one direction, and that’s essentially always on or off, never in between. Apply a certain voltage across it, and the gate is open; otherwise it’s shut. This simple structure has allowed the design of hundreds of devices.

We’ve come a long way, but we’re not quite there yet. Diodes are great, and extremely useful, but they’re basically just on/off valves. They can’t do one important thing that’s Crucial to our high-tech devices – they can’t amplify. For that, we need a transistor. Stay tuned.

**In fact, after a certain degree of reverse bias, the junction will go into what's known as 'breakdown'. This typically occurs at voltages in the neighborhood of 250V (reversed biased). Diodes that aren't designed for breakdown can be damaged, but there are certain diodes that are actually designed to operate in the breakdown region. For more information, see Zener Diodes.

1. Introduction
2. Diffusion, How Do You Take Your Coffee?
3. Putting It Together
4. It Works Both Ways
5. A Look at What's Inside
6. Breaking Down the Walls
7. Summary

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