Semiconductor Materials and Devices
Modern radios depend on components like diodes and transistors. They manage the flow of current in a circuit and do things like switching. These are part of a family of components called semiconductors. Like the name implies, semiconductors are partially conductive to electricity. Their conductivity falls between an insulator and a conductor. Somewhere between a pure metal like copper and an insulator like plastic or glass.
One of the materials used to create semiconductors is gallium arsenide. You’ll find it in microwave circuits and other components. Gallium arsenide is considered a “pure” semiconductor with a crystal structure.
Gallium Arsenide molecule
Semiconductors can be modified to provide different performance levels. That’s done by adding impurities to the pure component. This is called doping. “Doped” semiconductors free up electrons to move. An impurity atom that adds holes to a semiconductor crystal structure gives it acceptor impurity.
Semiconductor materials that contain excess free electrons are N-type semiconductors.
A transistor is one of the most used semiconductors in radio. It functions like a switch, using an electrical field to control the flow of currents. We’re going to focus on three types of transistors here.
The first is a field-effect transistor or FET. This is pronounced “fet” like the Star Wars character “Boba Fet.” FET’s use a single kind of charge carrier. The second is bipolar junction transistor or BJT. The BJT uses both electrons and electron holes as charge carriers. FET’s and BJT’s offer similar functionality. Which one you use in your circuit depends on the application.
Both FET and BJT transistors have three poles or connections. One of the connections is used like a control knob to manage how current passes between the other two poles. It acts like a switch or volume control. Quick distinctions between the two types:
- FETs are voltage-controlled while BJTs are current-controlled,
- The three connections of an FET are called Gate, Drain and Source
- In a BJT they are called a Base, Collector and Emitter.
Here’s a deeper dive on FET’s. A FET has higher input impedance when comparing the DC input impedance at the gate of a FET to the DC input impedance of a bipolar transistor. So, if you need high input impedance, choosing an FET is probably the right choice.
A special type of field effect transistor called a depletion-mode FET. It exhibits a current flow between source and drain when no gate voltage is applied. That is useful if you want current to flow without having to continually apply a voltage.
MOSFET schematic
Another type of field effect transistor is the MOSFET. What makes a MOSFET unique is the gate leg is insulated. That allows for minimal input current to control the switching. In many configurations a MOSFET will include a Zener diode. It’s connected between the gate and the source or drain elements. This is to protect the gate from static damage.
Let’s go through the schematics for field effect transistors. You may see these in figure E6-1 on your exam.
All these have elements labeled with G, D, and S. Remember, FETs have a Gate, Drain and Source, so we are clearly looking at FETs.
To indicate a N-channel FET, the schematic arrow is shown pointing in toward the junction. An N-channel FET is the most common type and uses electrons as the carrier. Focus on diagram number 4. It shows the arrow pointing toward the center. It also has two gates, indicated by G1 and G2. Therefore number 4 is an N-channel dual-gate MOSFET.
When the arrow is pointing away from the junction, this indicates a P-channel FET. A P-channel FET uses holes as the carrier instead of electrons. Diagram number 1 is the schematic symbol for a P-channel junction FET.
Let’s take a look at some of the elements of the bipolar junction transistor. One subtype of BJT is the silicon NPN junction transistor. It is biased on a base-to-emitter voltage of approximately 0.6 to 0.7 volts. Bias is the input level at which it passes a steady current.
There are two measurements to be aware of for BJT’s. They are the alpha and beta measurements. The Alpha cutoff frequency for a BJT indicates the frequency at which the grounded-base current gain of a transistor has decreased to 0.7 of the gain obtainable at 1 kHz. The beta of a bipolar junction transistor is the change in collector current with respect to the change in base current. In other words, how much current do you put in versus how much do you get out.
One more type of transistor semiconductor to cover. A bigger semiconductor-based switch is known as a solid-state relay. It’s a device that uses semiconductors to implement the functions of an electromechanical relay. So transistors really can be switches!
Diodes
A diode is a semiconductor which limits current flow to a single direction. Diodes can work in two ways. One is a forward bias that passes current through. The other is a reverse bias that blocks current from passing.
Silicon diode
Maybe the most popular diode ever is the LED or Light Emitting Diode. An LED is forward biased. What happens if you accidentally put an LED in backwards? It would block the current passing through the circuit instead of lighting it up. The amount of light it delivers is a function of the energy band gap. The band gap is the property of an LED’s semiconductor material that determines its forward voltage.
LED schematic
An LED is a class of PN-junction diode. These diodes don’t conduct current when reverse biased. This works because holes in P-type material and electrons in the N-type material are separated by the applied voltage, widening the depletion region. In other words, it creates a gap that the current cannot pass.
If you pass too much current through a diode, then this current can heat up and cause your junction diode to fail. In this case, the failure mechanism would be excessive junction temperature.
There are plenty of diodes that don’t light up when they have current passed through them. These diode subtypes are optimized for different functions in a circuit.
Earlier you heard how the addition of a Zener diode protects a MOSFET from static. The most useful characteristic of a Zener diode is a constant voltage drop under conditions of varying current. However, under the right conditions a Zener can pass current “backwards.”
A Schottky barrier diode has a metal-semiconductor junction. Why would you choose a Schottky diode over an ordinary silicon diode in a power supply rectifier? Because the Schottky diode has less forward voltage drop. Another common use of a Schottky diode is as a VHF/UHF mixer or detector.
For a circuit that has variable capacitance you might choose a varactor diode. This type of semiconductor is designed for use as a voltage-controlled capacitor.
Varactor diode
When you are building an RF circuit and need a switch, you might use a PIN diode. The characteristic that makes a PIN diode useful as an RF switch is low junction capacitance. Applying a forward DC bias current is used to control the attenuation of RF signals by a PIN diode.
PIN diode
Another diode in your RF circuit might be a point-contact diode which would be used as an RF detector.
Let’s wrap up diodes with another schematic symbol. The most basic diode symbol is number 4. All the others build upon that. Look at number 6 which has the arrow with a backward “S” shape. That’s the symbol for a Schottky diode. Don’t confuse that with number 3, which has the subtle Z indicating a Zener diode. And if number 5 looks familiar, that is the schematic symbol for an LED, the two arrows indicate the light it gives off.
