What are Transistors? – Categories and Features of Si Transistors

From this section, we will discuss “Si transistors”, which however include a number of categories, such as bipolar transistors and MOSFETs, depending on the manufacturing process and device structure. Si transistors can also be categorized according to the currents and voltages they can handle and their application areas. Here we are focusing on power devices, and so among the wide variety of transistors in use, we consider only power transistors. Among these, we intend to primarily address MOSFETs, which in recent years have been widely used in applications to control high power levels.

Let’s begin with the basics, reviewing transistor types and features.

Si Transistor Categories

When considering Si transistor categories, there are a number of methods of categorization depending on the criteria used. Here, although somewhat imprecise, we have categorized devices in terms of device structures and processes, as shown below. Of these, devices which are relevant to our current theme of power devices are indicated in a bold font and filled.

With bipolar transistors and MOSFETs, there are power type and small signal type. IGBTs are transistors that were originally developed to handle large amount of power, however, and so are essentially power devices.

In passing we note that MOSFET is an acronym for Metal Oxide Semiconductor Field Effect Transistor, and so these are one type of FET (field effect transistor). IGBT is an acronym for Insulated Gate Bipolar Transistor.

Si Transistor Features

Below we summarize the features, with respect to the major performance areas of transistors, for bipolar transistors, MOSFETs, and IGBTs.

Evaluations of various items are based on representative characteristics, and so there may not be consistency among separate devices. These should be regarded as overall tendencies and features. The structures, principles of operation, and representative parameters of these transistor types are described below.

A bipolar transistor (the example in the diagram is an NPN device) is configured from PN junctions; by passing a current through the base, a current flows between the collector and emitter. As indicated in the table summarizing features, where driving is concerned, it may be necessary to adjust the base current depending on the amplification and the collector current. A major difference from MOSFETs is the bias current that flows in the transistor (base) in order to perform amplification or turn the device on and off.

MOSFETs have a parameter called the on-resistance, which is particularly important when handling large amounts of power. Bipolar transistors, however, do not have an on-resistance. The world’s first transistors were bipolar devices, and so the order may appear to be reversed, but in recent years MOSFETs have been the mainstream for power supply circuits in particular, and I think many people will have first used MOSFETs, and so we will concentrate mainly on these devices. To return from our digression, the VCE (sat), called the collector-emitter saturation voltage, corresponds to the on-resistance of a bipolar transistor. When a predetermined collector current is passed, that is, when the transistor is turned on, there is a voltage drop, and the resistance while turned on can be determined from this voltage drop.

In MOSFETs (in the diagram, an Nch example is shown), by applying a voltage to the gate, a channel is formed between the source and the drain, and the device conducts. The gate is insulated from the source and the drain by an oxide film, and so a current in the normal sense does not flow. However, a charge, denoted by Qg, is necessary.

MOSFETs are later explained in further detail.

IGBTs have a structure that is a hybrid of bipolar transistors and MOSFETs. They were developed in order to exploit the advantages of both MOSFETs and bipolar transistors. Like MOSFETs, under gate voltage control they are capable of fast operation, but they combine this with the low on-resistance even for high voltages of bipolar transistors.

Operation is similar to that of MOSFETs: by applying a voltage to the gate, a channel is formed, and current flows. Whereas in a MOSFET (an Nch example) current flows between the source and drain, which are the same N type, an IGBT has a structure in which current flows from a P-type collector to an N-type emitter, similar to the structure of a bipolar transistor. Hence an IGBT has the gate-related parameters of a MOSFET, and the collector-emitter related parameters of a bipolar transistor.

Comparison of Basic Operation Characteristics

These three types of transistors have different operation characteristics. Below we describe the fundamental characteristic of Vce/Vds versus Ic/Id. Power devices are essentially used as switches, and so are used where possible under conditions such that Vce/Vds is low, This is one representative characteristic that is used when determining which transistor is optimal for the circuit conditions of the application.

• Si transistor
• Bipolar transistor
• MOSFET
• IGBT