Diodes and transistors: controlling current since 1874 and the 1950s
Purpose: Provide an explanation of transistors appropriate for placement in a transistor radio kit for adults.
Audience: Curious hobbyists who have read previous sections of the leaflet found in the kit, which explained electrons and holes, as well as some basic properties of semiconductors.
Context: This is part two of a two-part series. Part one explained semiconductors.
What happens if you dope one semiconductor crystal with n-type atoms on one side and p-type atoms on the other side? There is no physical barrier, so both the electrons and the holes can move where they please. Near the junction, some of them will mix and combine, annihilating each other. This will leave a bit of excess charge in the middle – a bit of positive charge on the n-doped side, and a bit of negative charge on the p-doped side. These extra bits of charge will create a no-man’s land – a place where electrons and holes don’t settle. We call this the depletion region. You can see this in the diagram below. The black circles are crystal sites filled with semiconductor atoms. Each n-dopant atom is represented by a filled red circle at the crystal site and a smaller blue filled circle – the electron. Together, their net charge is zero, but when the negatively charged electron moves around, it leaves behind a crystal site with a positive charge. The p-dopant atoms are represented by a filled blue circle at the crystal site and a smaller hollow red circle – the hole.
This depletion region can be manipulated by applying a voltage across the semiconductor. You can attach a battery in one of two ways, so we’ll look at both. If you attach the positive terminal of a battery to the p-doped side and the negative terminal to the n-doped side, then we would say the voltage on the p-doped side is higher than that on the n-doped side. Each terminal pushes away like charges, so both electrons and holes pile up near the junction. As soon as the charge carriers cross and combine, the battery can supply another hole and another electron to push toward the junction. This keeps the depletion region very small. In addition, because the movement of holes is essentially electrons moving in the opposite direction, we can see that electrons are flowing through the semiconductor, creating current. This configuration is called forward-biased – charge carriers just keep moving forward and across the junction.
The opposite situation (negative terminal to the p-doped side and positive terminal to the n-doped side) is reverse-biased. In this case, opposite charges attract, so the electrons will cluster near the positive contact while the holes cluster near the negative contact. This increases the width of the depletion region – this time though, the lack of charge carriers is less due to recombination than to the pull of the battery. There is no need for a charge carrier to be supplied from the battery in this configuration; current doesn’t flow in a reverse-biased configuration. This semiconductor, in which current can only flow one way, is called a diode.
Bipolar junction transistor physics
To understand our bipolar junction transistor, we will start with a forward-biased diode with a small p-doped side. We’ll also move the contact from the end to the side. In this configuration, electrons will flow from the n-doped side across the junction to the contact on the p-doped side.
Next, add another n-doped section to the other side of the p-doped section, making a sandwich (though the filling on this one is very thin). This junction between this new n-doped section and the p-doped section is set up as reverse-biased. In this new set-up, only a few of the electrons will follow the same path they followed before. The rest will actually make it through the p-doped layer, arriving in the n-doped section of the depletion region of the other junction. Once in the depletion region, the electrons feel a strong push toward the other end of the semiconductor, toward the final contact.
When you use the transistor as an amplifier for your radio, a small signal is sent to the contact on the p-doped section (the base). Changes in this signal create corresponding but larger changes in how many electrons (how much current) makes it through to the other side. A small increase in the voltage in the base can significantly increase the rate at which electrons are pulled from the first n-doped section (called the emitter) before arriving at the final contact (the collector). It’s like filling a cup of tap water – a small turn of the knob (the base) releases water from the pipes (the collector) and it exits through the faucet (the emitter).