Choosing a transistor can be a daunting task, not because it is necessarily complicated (but it can be!) but because there are so many to choose from. Digi-Key lists over 40,000 MOSFETs for example. Even before you start your search you need to decide on which type of transistor. For a simple, microcontroller driven switch a MOSFET would be the simplest but for analog design you might be considering other parameters such as noise. You may be looking for a specialized device such as an avalanche transistor or IGBT (Insulated Gate Bipolar Transistor).

A low noise application may be best served by a bipolar transistor although if you have a very high input impedance amplifier to design, e.g. a charge amplifier, a JFET might be more suitable. For RF (radio frequency) amplifiers you would also be looking at bipolar transistors, but also GaAsFets and maybe SiGe or SiGe:C transistors. For high power RF design, MOSFETs are a possibility.

For simple switches, MOSFETs are often favored although “digital” bipolar transistors which include a couple of resistors are also useful for simple switches driven by a microcontroller. On Semiconductor call them BRTs – Bias Resistor Transistors and NXP call them RETs – Resistor Equipped Transistors. There are also devices which include two complementary transistors and resistors so you can easily create a high side switch for say 12V from a microcontroller powered by 3.3V without any extra discrete devices e.g. the On Semiconductor NSBC114YPDP6. Given that the placement cost of devices can exceed their actual cost, keeping the actual number of components in a design low is important in a cost critical design. So, resistor arrays can be used for pull-ups rather than discrete resistors and transistor arrays when you have multiple outputs to drive. In both cases the higher cost of the array compared to buying several discrete devices can be offset by lower production placement cost, particularly in low to medium volume products.

The IGBT is an interesting device to consider if you have a high voltage, high current application. IGBTs are high voltage devices (250V or more) which try to combine the low saturation voltage of a bipolar transistor with the easy driving of a MOSFET. While you may think of MOSFETs being the best for switching high currents with ON resistances a low as a few milli-ohms, that is not true of high voltage devices. For example, a typical 20A, 600V MOSFET may have 0.26 ohms of ON resistance at 20A which equates to 5.2V of voltage drop. An IGBT may have 1.45V of saturation voltage. The difference in power dissipation (i.e. heat) is 104W compared to 29W. So, if you have a high voltage, high current application, IGBTs are a good place to start. The following schematic illustrates the difference.

The results of switching a 20A load are shown below.

 

This illustrates one of the disadvantages of the IGBT – the switching times are slower, particularly the switch-off. However, zooming in to show the huge difference in voltage drop when turned on is where the IGBT wins.

If you are looking for a bipolar transistor, you presumably have some reason for preferring them over MOSFETs. Bipolar transistors are the better option for analog amplifiers. You do have a lower input impedance with a bipolar transistor (but don’t ignore the gate capacitance of a MOSFET) but the transconductance of a bipolar transistor will usually be higher and the noise will be lower than a MOSFET. Most MOSFETs are not aimed at linear amplification – they are designed for switching applications. For low voltage, high current switching applications you are likely to find the MOSFETs are a better match. Unlike high voltage MOSFETs, low voltage MOSFETs have a considerably lower on resistance. A 60V 20A MOSFET can have an on resistance as low as 5 milli-ohms compared to 260 milli-ohms for a 600V MOSFET. You won’t really find general purpose MOSFETs aimed at analog small signal amplification though – even small MOSFETs are still targeted at switching applications.

So, for a low voltage switching application you are likely to be looking at MOSFETs. Breakdown voltage, on resistance and current capability are likely to be the key parameters you will be starting with, maybe followed by the package. Don’t bother including high voltage devices in your search if you only need a low voltage – the on resistance will be considerably higher for high voltage devices. On resistance is usually a selectable parameter in product selection tables which will eliminate the high voltage devices anyway. You may also want high speed but that often isn’t something that you can search on with either manufacturers’ or distributors’ parametric tables. Smaller, lower gate capacitance devices are usually faster and will also be easier to drive quickly due to the lower gate capacitance. So, don’t over specify the current capability and try setting a maximum gate capacitance in your search. Gate capacitance is usually available in manufacturers’ parametric tables. Some tables also include charge storage but then have lots of blank spaces in the tables which doesn’t help much.

For bipolar transistors you will most likely be using parameters such as hFE (current gain) and fT (transition frequency) to narrow your search after selecting Vceo (open circuit collector-emitter voltage) and maybe maximum collector current. If you are looking for a low noise transistor it is trickier because you are unlikely to find noise in a parametric selector. There are a few low noise transistors around which are advertised as such. NXP list the BC849 and On Semiconductor list the 2N5088 but the BC849 is really just a general purpose transistor and the 2N5088 is an old transistor in a TO-92 case so is not very useful for modern designs where surface mount is a must. You will find noise as a parameter for RF transistors which is fine if you are designing an RF amplifier but otherwise you will need to select based on other parameters and then look at the detail of the datasheet to find out what the noise is like (if given), and then do some simulations to compare. Bear in mind that a “low noise” transistor will be noisy depending on how it is biased. One advantage of transistors aimed at low noise applications, is that they will have information which will help you design for low noise such as this graph from the 2N5088 datasheet:

The graph will help you choose the optimum collector current for your low noise application. At mid to high frequencies anywhere from 0.1mA to 10mA is OK but for low frequencies you will be looking around 0.1mA to 0.5mA of collector current for the lowest 1/f noise. In fact the 2N5088 has a whole page of graphs related to noise. If the optimum collector current is too high then you will need to continue searching. While RF transistors will be specified for noise, the collector current at which the optimum noise is achieved will probably be quite high compared to a lower fT transistor.