It is difficult to have a clear quantitative definition of semiconductor. Based on conductivity, materials can be classified into three groups: (1) metal (conductor), (2) semiconductor, and (3) insulator (non-conductor). A general guideline indicates their ranges of conductivity. Note that one important feature of a semiconductor is that it can be doped with impurities to different concentration levels, so every semiconductor material can cover a range of conductivity. The total range of conductivity for semiconductors is from 10~8 S/cm to 103 S/cm (resistivity from 10-3 П-cm to 108 fl-cm).
The conductivity of materials is ultimately related to the energy-band structure. For an insulator, the energy gap Eg is large. Consequently, the valence band is completely filled with electrons, and the conduction band is completely empty. Since current is a movement of electrons and electrons need available states to move to, current cannot be generated from a completely filled band and a completely empty band. A semiconductor has a smaller Eg. Even when the Fermi level is within the energy gap, thermal energy excites electrons into the conduction band, and some empty states are left behind in the valence band. These partially filled bands make electron movement possible. In a metal, the energy gap is even smaller, and the Fermi level resides within either the conduction band or the valence band. Another possibility for a metal is that the £(is above the Ec so that the two bands overlap, and there is no energy gap. In such a system, the Fermi level can be in any position. Since for the semiconductor, the Fermi-Dirac statistics are necessary to determine the electron populations, temperature is also a crucial factor. At a temperature of absolute-zero, all semiconductors would become insulators. For practical consideration, at room temperature, semiconductors have energy gaps ranging from к 0.1 eV to « 4 eV.
A semiconductor is distinguished from the insulator and the metal by the range of resistivity (or conductivity) it spans. Note that, unlike the metal and insulator, each semiconductor can be doped to vary resitivity.
For a historic perspective, some common electronic devices with the years were developed. The earliest device, not necessarily made of semiconductor material, is probably the resistor, implied by Ohm’s law back in 1826. Vacuum tubes started around 1904, and were the major electronic components in the early radio era through World War II. The real birth of the semiconductor industry was in 1947 with the invention of the bipolar transistor. Ever since, new semiconductor devices have been invented at a steady pace, although some are more commercially significant than others. There are some devices whose development is too gradual to assign a milestone. An example is the solar cell. Starting from the mid-1970s, with the advent of MBE and MOCVD technologies, there are numerous heterojunction devices that are also omitted because it is too early for them to have an impact commercially. Currently, there are more than 100 semiconductor devices. To include such a large collection, the hierarchy of semiconductor devices used in this guide needs to be clarified. This also explains why certain devices are put in separate chapters. For example, the LED, laser, solar cell, and tunnel diode are all variations of a p-n junction. But since each of these is made for a special purpose, their designs consider different device physics, and their structures are very different. As a guide, this book presents only the key background, principles, and applications.
To help gain a better perspective on this large variety of devices, chapters are ordered according to their functions or structures, with group names assigned to describe them. This also provides a means for comparison among devices in the same group. These groups are:
1. Diodes: I-rectifiers
2. Diodes: II-negative resistance
3. Resistive devices
4. Capacitive devices
5. Two-terminal switches
6. Transistors: I-field-effect
7. Transistors: II-potential-effect
8. Transistors: III-hot-electron
9. Nonvolatile memories
10. Thyristors
11. Light sources
12. Photodetectors
13. Bistable optical devices
14. Other photonic devices
15. Sensors
While most of these group names are self-explanatory, a few need clarification. The name diode comes from vacuum tubes, and refers to a 2-element diode tube. Other vacuum tubes are the triode tube, tetrode tube, and pentode tube, with the number of active elements being 3, 4, and 5, respectively. Since in the diode tube, the cathode emits only one kind of carriers-electrons-the diode tube has asymmetric I-V behavior and is a rectifier. Although semiconductor diodes inherited the name, some of them actually do not have rectifying characteristics. Examples are the tunnel diode and the Gunn diode. A more proper definition for a diode now is simply a two-terminal device having nonlinear DC characteristics. Rectifiers are therefore only a subgroup of diodes. Another subgroup of diodes that are distinctively different from rectifiers is those having negative differential resistance. Within this group of negative-resistance devices, there are two types: one that has a negative region, and the transit-time devices where the negative resistance is due to the small-signal current and voltage that are out of phase.
A switch, in semiconductor terms, is a device that has two states-a low-impedance state (on) and a high-impedance state (off). Switching between these two states can be controlled by voltage, current, temperature, or by a third terminal. A transistor, for example, is considered a three-terminal switch in digital circuits. Thyristors are also a special case of switch. They are included in a separate group from switches because they usually contain p-n-p-n layers, have more than two terminals, and are used mainly as power devices.
Unlike diode, transistor (transfer-resistor) was a new name coined at the beginning of the semiconductor era for the bipolar transistor, instead of keeping the old equivalent of triode. In the classification of devices, this book does not follow the common approach in literature to divide devices into bipolar and unipolar types. For transistors, the bipolar transistor has been used as a representative of the first type, and MOSFET and JFET of the second type. The reason behind that classification is for a bipolar transistor, the base current is due to one type of carrier while the emitter-collector current is of the opposite type; thus, both types of carriers are involved. For a MOSFET, the gate current is negligible, and the carriers in the channel are the only kind responsible for the current flow. However, the classification based on this bipolar-unipolar terminology is not clear, or maybe even incorrect. For example, in a bipolar transistor, the base current is a sort of leakage current. It is only a by-product of a base potential needed to modulate the emitter-collector current. If this base current is somehow made zero, the bipolar transistor would still work, and work even better. In fact, the main purpose of a heterojunction bipolar transistor is to suppress this base current, without affecting the main current. Next, let us consider an enhancement JFET. To turn the transistor on, the p-n junction gate is forward biased. This injects minority carriers into the channel. The JFET is therefore as “bipolar” as the bipolar transistor. This argument can also be extended to diodes. A p-n junction has been referred to as a bipolar device while a Schottky barrier as a unipolar device. For practical p-n junctions, they are usually one-sided in that one side is much more heavily doped than the other. A typical Si p-n junction has doping levels of 1020 cm-3 and 1016 cm-3, and the ratio of the two types of current is « 10-4. For a practical Schottky-barrier diode, even though the current is dominated by majority carriers, the minority-carrier current is not zero. It is a factor of « 10~4-10~6 (injection efficiency) smaller. As seen from these diodes, the transition from a bipolar device to a unipolar device is not clear.
In this book, transistors are divided into three groups.These are (1) field-effect transistor (FET), (2) potential-effect transistor (PET), and (3) hot-electron transistor (НЕТ). The field effect is defined, originally by Shockley when the first field-effect transistor (JFET) was envisaged, as “modulation of a conducting channel by electric fields.” An FET differs from a PET in that its channel is coupled capacitively by transverse electric fields while in a PET, the channel’s potential is accessed by a direct contact. The capacitive coupling in an FET is via an insulator or a space-charge layer. A hot-electron transistor is a special case of PET, whose emitter-base junction is a heterostructure such that the emitted carriers in the base have high potential or kinetic energy. Since a hot carrier has high velocity, HETs are expected to have higher intrinsic speed, higher current, and higher transconductance. One also notes that the energy-band diagrams of the FET and the PET (excluding HETs) are similar. This is because the way the channel is influenced, either capacitively for FET or directly for PET, is not indicated in these diagrams. One observation on FETs is that almost all have channel conduction by the drift process, and have a well-defined threshold voltage.
The essential information about each device is given: When was it invented and by whom? (History) How is it made? (Structure) How does it work? (Characteristics) What is it for? (Applications) For more than half of the chapters, there is another section “Related Devices”, to cover slightly different structures. This book is intended to be an engineering approach to understand semiconductor devices, giving a pragmatic overview. Because of its complete coverage, readers can also pick up the subtle differences that sometimes exist between devices.
In spite of the large number of devices, there are only a few building blocks, which are: interfaces of two materials or doping types. These fundamental interfaces are: a metal- semiconductor interface, doping interface, heterojunction, semiconductor - insulator interface, and an insulator-metal interface. The metal-semiconductor interface, known as the Schottky barrier, also includes the ohmic contact which is inevitable in every semiconductor device. The doping interface also includes the planar-doped barrier. The heterojunction is also the basis for quantum-well devices. A bipolar transistor, for example, is built of two p-n junctions. A MOSFET has two p-n junctions, one semiconductor-insulator interface, and one insulator-metal interface.
Since the compositions vary among different semiconductor devices, their current-conduction mechanisms also vary accordingly. These currents are due to drift, diffusion, thermionic emission, tunneling, recombination, generation, and avalanche.
Finally, we discuss what is meant by the recent, commonly used term high-speed device. Is it a device that has intrinsically fast response, or is it one that enables a high-speed circuit? This is important to clarify since different criterion calls for a different device design. The fundamental parameter is the transit time, the time it takes for the carriers to travel between the source-drain or emitter-collector. Direct measurement of this parameter is extremely difficult. The next level is parameters deduced from two-port, small-signal S-parameter microwave measurement. This is done with a single device, and thus not as a circuit. It is the highest frequency that can be measured on the device, but certain parasitics are ignored. The cutoff frequency, for example, is a current-gain measurement. The output is shorted so that the output capacitance is not included. fmax includes the output capacitance but the load is matched to optimize the power transfer. The simplest circuit measurement is a ring oscillator. It is usually designed with a minimum fan-out of one, and minimum interconnect distance. A real circuit has much larger load capacitance as well as larger interconnect capacitance. From this viewpoint, if the circuit speed is to be optimized, the current drive or transconductance of a transistor is more important than the intrinsic response. It is possible to predict the ultimate circuit speed based on the transit time, microwave measurements, or ring-oscillator speed, but care has to be taken to account for realistic parasitics. For PETs, the parasitic resistance is also critical since the input current is much higher than that in FETs.
