Biased junctions

The current density given in Equation 3.7 must be rewritten to include both carrier types to completely describe the situation in a p-n junction  

The drift current is relatively insensitive to the junction electric field. As noted above, this field sweeps away mobile electrons and holes. However, this sweeping effect does not depend upon how large the field is as long as it is not too small. 

When a forward bias VappUed is greater than the built-in voltage, the drift current changes direction as the field at the junction is reversed. This means that the carriers change direction, and that majority carriers would then drift across the jrmction in addition to diffusing. However, the drift current is so small by comparison with the diffusion current under these conditions that it is undetectable. 

Reverse current density is important in deterrnining the power that a device consumes when in the off state as it determines the effective resistance of the device in reverse bias. Hence reverse current is also referred to as leakage current. From Equation 3.24, the reverse current is carried by drift of minority carriers from the lightly doped side of the junction (that is the side having the higher minority carrier density) toward the heavily-doped side. If one assumes a p -n diode (the p-type side is very heavily doped) then Equation 3.24 becomes  

The hole current flowing from the lightly-doped n-type side of the junction can then be related to the doping level on the n-type side and the intrinsic carrier concentration through the relation nnPn=ni as  

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Figure 11.12. Si electrode and NW hybrid device, (a) Schematic of a single LED fabricated by the method outlined in Fig. 11.11. (b) I-V behavior for a crossed p-n junction formed between a fabricated p+-Si electrode and an n-CdS NW. (c) EL spectrum from the forward biased junction, (d) SEM image of a CdS NW assembled over seven p+-silicon electrodes on a SOI wafer (e) EL image recorded from an array consisting of a CdS NW crossing seven p+-Si electrodes. The image was acquired with +5V applied to each silicon electrode while the CdS NW was grounded. [Reprinted with permission from Ref. 59. Copyright 2005 Wiley-VCH Verlag.]...

In order to produce significant currents across moderately doped wafers the reverse biased junction has to be illuminated. Hence the anode (for the case of p-type substrates) or the cathode (for the case of n-type substrates) should be made of a platinum mesh to be sufficiently transparent. 

For a reverse-biased junction, that is, negative voltage to p and positive to n, a depletion layer will form whose width is given by... 

The leakage current IR of a reverse-biased junction can be expressed as a sum of several components that have been discussed in detail by Tamg and Pankove (1979). Under the present experimental conditions, the major components are the surface generation-recombination current Iv and the tunneling current /,. 

The equivalent circuit of a semiconductor detector operated as a spectrometer is shown in Fig. 5.30. In most cases, effects of high resistance of the reverse-biased junction are negligible. If a zero-electric-field radiation-insensitive region is present in the detector, its impedance (a parallel RC combination) appears in series with the circuit and is indicated in Fig. 5.30 by the impedance Z. The impedance also accounts for any resistance (or resistance-capacitance combination) appearing in series with the contacts. 

A microcircuit may be described as a collection of devices each consisting of an assembly of active and passive components, interconnected within a monolithic block of semiconducting material [4]. Each device is required to be isolated from adjacent devices in order to allow for maximum efficiency of the overall circuit. Furthermore within a device, contacts must also be electrically isolated. While there are a number of methods for isolating devices in a circuit (reverse-biased junctions, mesa isolation, use of semi-insulating substrates, and oxide isolation), the isolation of active components of a single device is almost exclusively accomplished by the deposition of an insulator. 

In Figure 5-la is shown a schematic representation of a silicon MOSFET (metal-oxide-semiconductor field effect transistor). The MOSFET is the basic component of silicon-CMOS (complimentary metal-oxide-semiconductor) circuits which, in turn, form the basis for logic circuits, such as those used in the CPU (central processing unit) of a modern personal computer [5]. It can be seen that the MOSFET is isolated from adjacent devices by a reverse-biased junction (p -channel stop) and a thick oxide layer. The gate, source and drain contact are electrically isolated from each other by a thin insulating oxide. A similar scheme is used for the isolation of the collector from both the base and the emitter in bipolar transistor devices [6],... 

A semiconductor junction device in the non-equilibrium state is either forward or reverse-biased. During forward-bias there are excess carriers in the device. The source of the excess carriers is either a forward-biased junction or light incident on, and absorbed by, the device. A semiconductor without a pn junction but with ohmic conctacts or one without any contacts can only be excited by light or some other form of radiation such as X-rays, electrcMi beams etc. 

Potential Step Transients This refers to the current transient in response to a potential step. In solid-state devices a technique known as junction recovery is applied to barrier junctions as a probe of the charge density in localized states, particularly in amorphous materials [104]. A large negative potential step is applied to a forward biased junction and... 

This structure employs two reversed-biased junctions so that the current also becomes laterally confined to the lasing region. With this type of optical and current confinement, threshold currents of less than 1 mA have been achieved. 

For lightly or moderately doped semiconductors, Nj lO cm , the depletion region is relatively wide. It is, therefore, nearly impossible for electrons to tunnel through the barrier unless aided by defects, which are considered not to exist in this ideal picture. In a forward-biased junction, however, the electrons can surmount the top of the barrier, which is lowered with respect to the Fermi level in the semiconductor by an amount equal to the applied bias. This thermally activated process is called the thermionic emission as shown in Figure 8.4a and has been treated in many studies and early literature such as the one by Henish [11]. In reverse bias, the barrier for electrons from the semiconductor to the metal is made even larger, and the electron flow from the semiconductor to the metal in this ideal picture is cut off On the metal side, if the electrons in the metal gain sufficient energy by the applied bias, they too can overcome the barrier, which is the dominant mechanism for the reserve-bias current in an ideal picture. Naturally, an ohmic behavior is not observed. The electron flow from the metal to the semiconductor and from the semiconductor to the... 

Ei is the intrinsic energy, near midgap, given by Equation 2.31. The quasi-Fermi levels for a forward biased junction are shown schematically in Figure 3.6. The excess carriers that diffuse across the junction eventually recombine with the local majority carriers and the quasi-Fermi levels return to the equilibrium value, Ep. 

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