Selecting Semiconductor Components For Prototype Design

Whether you design circuits as a hobby or high-tech computer devices for a living, your biggest concern is finding the best components for your project.

Diodes – SCR

The simplest solid state device, the diode, serves only one purpose: conducting the flow of electricity in one direction. A Silicon-Controlled Rectifier, or SCR, is a device that supplies power to a load when triggered by a pulse at the gate input. Once triggered, the voltage remains on until the load is removed. SCRs are used in alarm systems.

diodes scr

Bipolar Transistors

The bipolar transistor is well suited for the majority of applications. There are two types of bipolar transistors – NPN and PNP – that differ only by their polarity. An NPN transistor requires a positive bias for audio projects to provide the threshold voltage for audio signals; however, for digital signals, neither positive nor negative biasing is necessary. Bipolar transistors are linear devices, suitable for analog or digital designs. The only drawback is their signal-to-noise ratio (the amount of inherent noise the device generates on its own).

bipolar transistors


Transistors come in several types of cases, as illustrated below. The metal, can, and power cases are made out of metal, while the plastic, round head, and bullet cases are made out of plastic.

case types

The power transistor has only two leads because the case itself serves as the third lead. Some older metal devices may have a fourth lead actually connected to the metal case that acts as a ground shield. One of the most popular transistors is the 2N2222, a very reliable plastic NPN transistor. I have used it in many circuits, including audio and switching circuits, and it consistently performs well. However, hundreds of transistor types are available from most distributors that will serve the same purpose.

Darlington Transistors

The Darlington transistor, developed by Sidney Darlington, combines two transistors in a single package. This type of transistor is more sensitive and can handle a greater current load. One application is supplying power to small DC motors in robots. Some transistors can supply enough current to require a heat sink connected through a mounting hole.

darlington transistors

darlington case types

Their quick reaction time also suits them to solenoid and relay control.

Field-Effect Transistors

Field Effect Transistors (FETs) are used in computers because of their low noise and bi-directional signal passage. It does not matter which terminal – B1 or B2 – is the input or output. This makes the junction FET, or JFET, one of the most versatile devices.

fet transistors

The three pins are labeled B1 for base 1, G for gate, and B2 for base 2. The N channel is like the NPN bipolar transistor and the P channel is like the PNP transistor. The main difference is that the bipolar transistors are linear, while the FETs are exponential.

Unijunction Transistors

If you need to generate a signal, use a unijunction transistor. Since their sole function is signal generation, they are very stable over a wide range of frequencies, making them well suited for tone generators in electronic music synthesizers. The unijunction transistor is an offshoot of the FET design; it has B1 and B2 terminals, but instead of a gate, an emitter passes voltage through the transistor to ground through B2.

The programmable unijunction transistor is somewhat different, with anode and cathode leads and a gate trigger of the SCR. Their purpose is strictly to generate a signal output of two wave shapes — a pulse and a reverse exponential signal.

programmable unijunction transistor


Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are voltage-controlled power devices and very much like JFETs, but are more suited to power supply circuits. The B1 and B2 pins have been replaced by Drain and Source labels.

mosfet transistor

While the components that have been manufactured in the last 20 years can operate in today’s computers in the Gigabyte range, we are entering the Terabyte age of supercomputers. Thus, it is getting harder to design components for use in modern computer circuits.


One of the most significant devices in recent solid state development is the optoisolator, which was primarily introduced with the advent of digital electronic music. The common ground connection that existed between music components produced hum and noise that did not interface well with the computer modules of digital synthesizers. A method of transferring signals without sharing a common ground was developed with optoisolators.

An LED that is fed a digital signal can transfer its information to a light-sensitive device and operate another unit with no electrical connection. This means that no distortion would be passed from one unit to another. This resulted in the Musical Instrument Digital Interface (MIDI) system, which worked so well that all digital music synthesizers now communicate through optoisolators. Other audio systems have also adopted this new approach — optical devices are now used in telephone circuits and televisions. With optical manipulation, problems with impedance matching, hum and noise transfer, and circuit loading are all a thing of the past. Of course, more research and other devices will make these circuit designs more plentiful.

Most optoisolators — or optocouplers, as they are sometimes called — can be found in packages like the one shown below. The most common is the six-pin device. Pins 1 and 2 connect to the LED. Pins 4 and 5 provide the output from the phototransistor. Pin 6 serves to disengage the device. A smaller four-pin version of the device is available but does not have the override control pin.


Other optical devices are also available, including photodarlington, photo-SCR, and phototriac.

Optics has so established itself as a necessary part of future designs, that even home-brew devices are becoming popular. An LED and phototransistor sealed together in a spacer tube or shrink tubing can provide excellent results. Don’t forget to paint the ends of the two devices black to prevent light from entering the backside.

homemade optoisolator

It is very important that you are aware of the specifications for your device. It is best that any component be rated higher than the load it must bear, especially if motors or heavy current-drawing devices are used. Semiconductors come with spec sheets or the catalog will give a description that should suffice when choosing an appropriate application.


Many manufacturers will combine components into arrays, or a number of devices, in a single case. This may come in handy when you need several devices working together.


There is a plethora of electronic parts distributors on the internet, such as Allied Electronics, Mouser Electronics, MCM Electronics, HobbyTron, and Jameco Electronics. I have had personal experience with Jameco and have found them to be an excellent source for all kinds of parts.