8.5 Semiconductor devices (see also textbook sections 14.5 and 14.6) The need for devices is driven by applications such as computer memory storage, fast switching at low power levels, etc. It has led to development of nanometer size devices in some cases, e.g. as integrated circuits on a chip. Three basic devices will be described here:• p-n junctions — these can rectify signals (allow current flow in just one direction) • junction transistors — these can amplify signals and provide switching • MOSFET (transistors) — these can amplify signals and provide switching (a) p-n junction This acts as a diode — it is a 2-terminal device where the current flow is easier in one direction compared with the other. So it can be used to rectify ac signals. Basically it consists of a single piece of a semiconductor material where one half is doped p-type and the other half is doped n-type. A voltage supply can be connected in an external circuit in two ways, either forward bias — (+ve battery terminal to p side, −ve battery terminal to n-side) or reverse bias — (the opposite to the above) In forward bias, In reverse bias, electrons and holes move towards each other and mostly recombine (annihilating each other): el + hole → energy. A current can flow in the external circuit. electrons and holes move apart, leaving a region with almost no charge carriers, so it acts like an intrinsic semiconductor or like an insulator (so very little current flows). This effect (and the rectification process) can be illustrated by the following diagrams. For a p-n rectifying junction in: ─ forward bias, and ─ reverse bias. 30 The current-voltage characteristics of a p-n junction for forward and reverse biases. The effect of “breakdown” under large magnitude voltage is also shown. (a) Voltage vs. time for the input to a p-n rectifying junction. (b) Current vs. time showing rectification of the above voltage signal by a p-n junction with the I-V characteristics shown previously. (b) junction transistor If we form more than two doping regions in a semiconductor sample, we can achieve other more complicated functions. In a transistor there are 3 terminals (3 regions) — it can be used (e.g.) as a switch or as an amplifier. In a junction transistor we sandwich an n-type layer between two p-type layers (or vice-versa) to form: p–n–p (or n – p – n) Schematic diagram of a p-n-p junction transistor and its associated circuits, including input and output voltage-time characteristics for voltage amplification. 31 The mode of operation is to make one p-n (or n-p) junction (labeled 1) forward biased and the other (labeled 2) reverse biased. This makes holes from p-region on the left (called the emitter) move into the centre n-region (called the base). The base region is designed to be sufficiently thin (and with just the right doping) that most of the hole charges are swept right through to the pregion on the right (called the collector). Lots of charges are accumulated in the collector and this produces an amplification effect. For a p-n-p junction transistor, the distribution and movement of electrons and holes (a) when no potential is applied and (b) with appropriate bias for voltage amplification. (c) MOSFET This is an example of a Field Effect Transistor. The MOS stands for Metal-Oxide Semiconductor. In a p-type device there are two p-type regions formed near a surface and surrounded by n-type material. A thin connecting channel of p-type material is formed between the p-type regions. An oxide insulating film is deposited on the surface and metal contacts are formed (using etching) to give contacts to the p-type regions. Another metal contact (called the gate) on top of the insulating film, but adjacent to the channel, can have a bias voltage applied either to attract or repel holes. In the one case the channel will be conducting (have lots of charges) and in the other it will intrinsic-like (depleted of charges). Schematic cross-section view of a p-type Si MOSFET transistor 8.6 Other electrical properties (see also textbook section 14.8) Two other electrical effects that can occur in a relatively few compound materials, that have low crystalline symmetry or are distorted, are ferroelectricity and piezoelectricity. 32 Ferroelectricity This is somewhat analogous to ferromagnetism (where a magnetic moment or magnetization occurs spontaneously below a critical temperature TC but is zero at higher temperature). The difference is that, in ferroelectric materials, there is a spontaneous electric-dipole moment below some temperature TC. It vanishes for T > TC. The effect is associated with a distortion of the crystal lattice in an ionic material (so oppositely charged ions move relative to one another) — it sets in below some critical temperature and is absent (destroyed by the thermal energy) above that temperature. Common examples are: BaTiO3 with TC ~ 120 C and KH2PO4 (known as KDP) These materials (typically insulators) can have extremely high dielectric constants and are useful (e.g.) in capacitors. A unit cell of BaTiO3 in two views: (a) in perspective view and (b) looking at one face and showing the displacements of the Ti and O charged ions. Piezoelectricity In these materials, if a mechanical force is applied (in order to compress or extend the lattice), then an electric polarization and an electric field are produced, or vice-versa (i.e., applying an electric field will cause a mechanical strain in the material). (a) Electric dipoles within a piezoelectric material. (b) A voltage is generated when the material is subjected to a compressive stress σ. They have device applications as basic transducers (converters of electrical energy to mechanical strain, or vice versa), e.g., as in microphones, loudspeakers, ultrasonic imagers, etc. A common example of a piezoelectric material is quartz. 33