The Quantum Universe_ Everything That Can Happen Does Happen - Brian Cox [78]
Figure 9.4 is a sketch of the device that changed the world – the transistor. It shows what happens if we make a sandwich, with a layer of p-type silicon in between two layers of n-type silicon. Our explanation of a diode will serve us well here, because the ideas are basically the same. Electrons drift from the n-type regions into the p-type region and holes drift the other way until this diffusion is eventually halted by the potential steps at the junctions between the layers. In isolation, it is as if there are two reservoirs of electrons held apart by a barrier, and a single reservoir of holes that sits brim-full in between.
Figure 9.4: A transistor.
The interesting action occurs when we apply voltages to the n-type region on one side and the p-type region in the middle. Applying positive voltages causes the plateau on the left to rise (by an amount Vc) and likewise the plateau in the p-type region (by an amount Vb). We’ve indicated this by the solid line in the middle diagram in the figure. This way of arranging the potentials has a dramatic effect, because it creates a waterfall of electrons as they flood over the lowered central barrier and into the n-type region on the left (remember, electrons flow ‘uphill’). Providing that Vc is larger than Vb, the flow of electrons is one-way and the electrons on the left remain unable to flow across the p-type region. This all might sound rather innocuous, but we have just described an electronic valve. By applying a voltage to the p-type region we are able to turn on and off the electron current.
Now comes the finale – we are ready to recognize the full potential of the humble transistor. In Figure 9.5 we illustrate the action of a transistor by once again drawing parallels with flowing water. The ‘valve closed’ situation is entirely analogous to what happens if no voltage is applied to the p-type region. Applying a voltage corresponds to opening up the valve. Below the two pipes, we have also drawn the symbol that is often used to represent a transistor and, with a little imagination, it even looks a little like a valve.
What can we do with valves and pipes? The answer is that we can build a computer and if those pipes and valves can be made small enough then we can make a serious computer. Figure 9.6 illustrates conceptually how we can use a pipe with two valves to construct something called a ‘logic gate’. The pipe on the left has both valves open and as a result water flows out of the bottom. The pipe in the middle and the pipe on the right both have one valve closed and obviously no water can then flow out of the bottom. We have not bothered to show the fourth possibility, when both valves are closed. If we were to represent the flow of water out of the bottom of our pipes by the digit ‘1’ and the absence of flow by the digit ‘0’, and if we assign the digit ‘1’ to an open valve and the digit ‘0’ to a closed valve, then we can summarize the action of the four pipes (three drawn and one not) by the equations ‘1 AND 1 = 1’, ‘1 AND 0 = 0’, ‘0 AND 1 = 0’ and ‘0 AND 0 = 0’. The word ‘AND’ is here a logical operation and it is being used in a technical way – the system of pipe and valves we just described is called an ‘AND gate’. The gate takes two inputs (the state of the two valves) and returns a single output (whether water flows or not) and the only way to get a ‘1’ out is to feed a ‘1’ and a ‘1’ in. We hope it is clear how we can use a pair of transistors connected in series to built an AND gate – the circuit diagram is illustrated in the figure. We see that only if both transistors are turned on (i.e. by applying positive voltages to the p-type regions, Vb1 and Vb2) is it possible for a current to flow, which is just what is needed to implement an AND gate.
Figure 9.5. The ‘water in a pipe’ analogy with a transistor.
Figure 9.6. An ‘AND’ gate built using a water pipe and two valves