The P-N Junction Diode

Nakshima et al. have fabricated p-n junction devices by employing A1 implantation to yield a p-doped layer in n-type 6H-SiC [66]. A Pt layer on top of the p-type ohmic contact (PtSi) provided both protection and a catalytic metal contact to create a chemical gas sensor device. A response (30 and 60 mV, respectively) was obtained to both 50 ppm and 100 ppm of ammonia in nitrogen at 500°C. 

Another p-n junction-based hydrogen sensor has been produced by implanting palladium ions into 6H n-type SiC material [67]. Gold-plated copper contacted the p-n junction device. The gas response was measured as (small) changes in current as the gas ambient was varied between air and 4% in argon in the temperature range 23-240°C. For an absolute voltage above 1.2V, the p-n junction broke down. 

The electrical behaviour that emerges when a region of p-type semiconductor is adjacent to a region of n-type semiconductor, a p-n junction diode, often just 

The transition region has a width of about 1 pm. The density of mobile charge carriers in the transition region is low, and for this reason the transition region is also called the depletion region. At equilibrium (thermal and electrical) there will still be an exchange of carriers at the junction, but the current in each direction will be the same. Dynamic equilibrium holds. 

The change of current with applied voltage is given by the Shockley equation. In a simplified form this is  

The total current across the device, which is constant for any applied voltage, is made up of 

This is quite different than in a metal, in which an applied voltage allows the mobile electrons to acquire a drift voltage. In a p-n junction, minority carriers are injected into both regions. They were not there originally and arise as a consequence of the applied voltage. 

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The p-n junction diodes employing catalytic metal contacts have recently been tested for their gas-sensing properties [66, 67]. The catalytic metal contact was placed directly on the semiconductor in this device, as shown in Figure 2.6. In general the response appears to be lower than for the traditionally used devices described earlier in this section. However, this means that for any catalytic metal used as an ohmic contact to a p-n junction, it can be expected that the /-Vcharacteristics will be influenced, for example, in a hydrogen-containing atmosphere. 

A thin slice of the semiconductor serves as the electrode, one side of which is contacted with another material to form a p-n-junction. If the distance between the surface of the slice used as electrode and the p-n-junction on the other side is in the order of the mean free path of the minority carriers, then injection or extraction of minority carriers through the p-n-junction diode will increase or reduce the supply of minority carriers for the electrode surface. This can clearly be seen in the influence of the diode voltage on the electrolytic current as shown in Fig. 23 for the case of a Germanium-electrode. Such a tool is very useful for quantitative measurements because the number of minority carriers reaching the electrode surface can be calculated and compared with the change of the electrolysis current. 

The silicon diode (photodiode) detector consists of a strip of p-type silicon on the surface of a silicon chip (n-type silicon). By application of a biasing potential with the silicon chip connected to the positive pole of the biasing source, electrons and holes are caused to move away from the p-n junction. This creates a depletion region in the neighbourhood of the junction which in effect becomes a capacitor. When light strikes the surface of the chip, free... 

Typical photodiode detectors consist of a p layer which is made of an electron deficient material an n layer which is electron abundant and a depletion region, the p-n junction, located between the two layers. At equilibrium, when no light or current is applied to the system, the p-n junction is in electrostatic equilibrium and the alignment of electrons and electron holes on the two sides of thejunction region creates a contact potential voltage. As incident light strikes the surface of the diode, the... 

Figures 2.13(a) and 2.13(b) illustrate the basis of a semiconductor diode laser. The laser action is produced by electronic transitions between the conduction and the valence bands at the p-n junction of a diode. When an electric current is sent in the forward direction through a p-n semiconductor diode, the electrons and holes can recombine within the p-n junction and may emit the recombination energy as electromagnetic radiation. Above a certain threshold current, the radiation field in the junction becomes sufficiently intense to make the stimulated emission rate exceed the spontaneous processes.

Figure 6 Schematic diagram of the double-heterostructure p-n junction diode used for chemical sensing experiments. Electrical contact is made to the top and bottom surfaces with metal films. (Adapted from Ref. 3.)...

A simplified example will illustrate the process of microstructure fabrication. With reference to Fig. 1, an //-type region has been created by diffusion of a donor impurity into a surface of p-type silicon, forming a p — n junction diode. There is a metal contact to the /(-region, and the contact line is insulated from the p-type surface by a layer of silicon dioxide. The diameter of the diode is on the order of 10 micrometers. 

An interesting variant on this theme is the polypyrrole-polythiophene p-n junction diode, prepared by forming the two anion-doped polymers sequentially and then redoping the thiophene layer with cations. At 10 V the diode had a forward current of 15 mA and a current of 1 mA in reverse bias3l6). 

Sullivan and Eigler (42) developed a technique for removing the damaged surface layer from the p-n junction area of germanium diodes without masking. The germanium diode is placed inside a hairpin-shaped platinum cathode and a stream of 0.1% KOH is allowed to flow down between the cathode and the germanium. Surface tension confines the electrolyte to the junction area. A polished surface is usually completed in 1 to 2 min at 1. 5 amp/cm. ... 

Since the cross section of planar surface cell LECs can be imaged by a microscope, a number of interesting experiments can be done to investigate the operation mechanisms. In a p-n diode, there is a built-in electric field at the p-n junction. This field can be measured by optical beam induced current (OBIC) microscopy, a technique in which a focused laser beam is scanned across the device while the photocurrent is monitored. When the beam excites a region that... 

A reverse voltage is applied to a silicon p-n junction diode to make a thick depletion layer. Most of the light is absorbed in the depletion layer. The resulting electrons and holes are separated and caused to drift in the electric field within the depletion layer, producing a photocurrent. 

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