Pn-junction

The growth of solid films onto solid substrates allows for the production of artificial stmctures that can be used for many purposes. For example, film growth is used to create pn junctions and metal-semiconductor contacts during semiconductor manufacture, and to produce catalytic surfaces with properties that are not found in any single material. Lubrication can be applied to solid surfaces by the appropriate growth of a solid lubricating film. Film growth is also... 

The great energy consumption, limited recources of traditional fuels and environmental problems have lead to intensive research on the conversion of solar energy during the last fifteen years. Conversion into electrical energy has been realized in technical devices consisting of pn-junction photovoltaic cells. Efficiencies of up to 20 % have been obtained with single crystal devices and around 9 % with polycrystalline or amorphous layers. 

In principle there are two types of solid state devices (i) pn-photocells and (ii) Schottky type cells. The first one consists simply of a pn-junction whereas the other of a semi-conductor-metal junction. The energy schemes of these cells are given in Fig. 1 a and b. The current-potential dependence of both types of cells is given by (see e.g. ) ... 

Fi.g 1 a—e. Charge transfer processes at pn-junctions (left side) and semiconductor-metal Schottky junctions (right side). 

The photovoltage is esentially determined by the ratio of the photo- and saturation current. Since io oomrs as a pre-exponential factor in Eq. 1 it determines also the dark current. Actually this is the main reason that it limits the photovoltage via Eq. 2, The value of io depends on the mechanism of charge transfer at the interface under forward bias and is normally different for a pn-junction and a metal-semiconductor contact. In the first case electrons are injected into the p-region and holes into the n-region. These minority carriers recombine somewhere in the bulk as illustrated in Fig. 1 c. In such a minority carrier device the forward current is essentially determined... 

The Aviram-Ratner D-ct-A molecule is analogous to a pn junction rectifier the electron-rich donor region D would be similar to the electron-rich semiconducting n region, while the electron-poor A region would be similar to a semiconductor s p region [79]. However, note that under forward bias the preferred direction of Aviram-Ratner electron flow is from A to D, while in a pn junction rectifier the preferred direction is from n to p. 

The fundamental figure of merit for rectification, the rectification ratio, RR, is defined as the current at a positive bias V divided by the absolute value of the current at the corresponding negative bias —V RR = I(V)/ /(—V) I. Commercial doped Si, Ge, or GaAs pn junction rectifiers have RR between 10 and 100. 

Finally, an interesting concept, recently advanced, is the implementation of active materials as nanotube arrays. These systems have high surface area to optimize contact between semiconductor and electrolyte, and good light trapping properties. Their inner space could also be filled with catalysts or sensitizers and/ or pn junctions to obtain charge separation and facilitate electron transport [136]. 

Photodiodes are based on a pn junction operated at a reverse bias voltage (i.e., the opposite bias to the LED and diode laser). The reverse rather than forward bias voltage has the effect of increasing the voltage across the depletion region such that any photoinduced electron-hole pairs are rapidly swept across the junction, generating a current pulse in the external circuitry (Figure 12.24). 

Fig. 14 (a) Equilibrium energy diagram for a pn junction in an inorganic semiconductor material with intrinsic Fermi energy Ep , conduction band energy E, valence band energy The quantity Vu... 

Figure 6.4 Carrier flow in implanted emitter 4H-SiC BJTs. Most of the electrons injected from the emitter are recombined at or near the SCR of the emitter-base pn junction.

Debiasing of emitter-base pn junction can be minimized by using a double metal process. A simplified cross section of a 4H-SiC power BJT with double metal process is shown in Figure 6.11. In this structure, the emitter electrode covers most of the active area and is connected to emitter fingers through vias, whereas the base electrode is placed outside of the active area. Use of this structure eliminates most of the resistive voltage drop in the emitter fingers at an increased cost of the fabrication process. 

Figure 13 shows typical results of the degradation of crystalline Si solar cells having a back surface field and reflector structure (Si-BSFR), which were qualified by National Space Development Agency of Japan (NASDA) for space usage, when irradiated by 10-MeV protons and 1 -MeV electrons. The pn junction of the cell samples, with a size of 2 cm x 2 cm X 50 pm, was fabricated by phosphorus (P) doping to a depth of 0.15 pm into boron... 

For the development of SEU-hardened memory devices, it is expedient to reduce charge collected in a memory cell. For this purpose, the formation of buried oxide in device structures, i.e., the fabrication of SOI structure, is considered a useful method because such a buried oxide layer can be expected to suppress the charge collection due to the drift and funneling processes. However, no experimental approach had been made for the charge collection in SOI devices. To investigate the charge collection in the SOI structure, transient currents induced in SOI pn junctions by heavy ions such as 15-MeV carbon (C) or oxygen (O) ions have been measured. 

The samples were pn junction diodes formed on an SOI wafer fabricated by wafer bonding technique. The thicknesses of the n-type top Si layer with resistivity of 2-4 fl cm, the oxide layer, and the n-type Si substrate with resistivity of 1-50 H cm are 5.7, 0.48, and 630 pm, respectively. The p region, which is 50 pm in diameter and 0.5 pm in depth, was... 

Figure 17 Transient current induced in SOI pn junction diode by 15-MeV C-ion irradiation (solid line). The result obtained for bulk Si pn diode is also shown as a dotted line in the figure for comparison. The reverse bias of 10 V was applied to pn diodes during measurements.

A diode is a pn junction (Figure 15-27a). If n-Si is made negative with respect to p-Si, electrons flow from the external circuit into the n-Si. At the pn junction, electrons and holes combine. As electrons move from the p-Si into the circuit, a fresh supply of holes is created in the p-Si. The net result is that current flows when n-Si is negative with respect to p-Si. The diode is said to be forward biased. 

If the polarity is reversed (Figure 15-27b), electrons are drawn out of >i-Si and holes are drawn out of p-Si, leaving a thin depletion region devoid of charge carriers near the pn junction. The diode is reverse biased and does not conduct current in the reverse direction. 

Figure 15-27 Behavior of a pn junction, showing that current (a) can flow under forward bias conditions, but (i>) is prevented from flowing under reverse bias.

In a laser diode, population inversion of charge carriers in a semiconductor is achieved by a very high electric field across a pn junction in gallium arsenide. Most laser diodes operate at red and near-infrared wavelengths (680-1 550 nm). 

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