Forward-bias

Figure C2.16.7. A schematic energy band diagram of a p-n junction witliout external bias (a) and under forward bias (b). Electrons and holes are indicated witli - and + signs, respectively. It should be remembered tliat tlie energy of electrons increases by moving up, holes by moving down. Electrons injected into tlie p side of tlie junction become minority carriers. Approximate positions of donor and acceptor levels and tlie Feniii level, are indicated.

A band diagram of a biased n-p-n BIT is shown in figure C2.16.8. Under forward bias, electrons are injected from tlie n type emitter, giving rise to tlie current 7. flowing into tlie p type base. Some of tlie carriers injected into tlie base recombine in tlie base or at tlie surface. This results in a reduction of tlie base current by 7, tlie lost recombination current, and tlie base current becomes 7g = At tlie same time, holes are injected from tlie... 

A more effective carrier confinement is offered by a double heterostructure in which a thin layer of a low-gap material is sandwiched between larger-gap layers. The physical junction between two materials of different gaps is called a heterointerface. A schematic representation of the band diagram of such a stmcture is shown in figure C2.l6.l0. The electrons, injected under forward bias across the p-n junction into the lower-bandgap material, encounter a potential barrier AE at the p-p junction which inliibits their motion away from the junction. The holes see a potential barrier of... 

Fig. 2. Schematic diagram of active layer stmctures employed in LEDs under forward bias showing the conduction band (CB) and valence band (VB). The simplest devices employ (a) a homostmcture active layer wherein the bandgap is constant throughout the device. More advanced stmctures consist of (b) single and (c) double heterostmctures. Heterostmctures faciUtate the confinement and injection of carriers in the active region where the carriers may...

Fig. 4. Schematic cross section and the band diagram of a double heterostmcture showing the band-edge discontinuities, AE and AE used to confine carriers to the smaller band gap active layer, (a) Without and (b) with forward bias. See text.

When a load and a power source is connected across the anode and the cathode of the SCR. there will be no conduction and no current will flow, even when the tinodc is made positive with respect to the cathode unless the gate is also made forward bia.scd with the application of a positive potential at the gate. After the conduction commences, the gate potential can be removed and the... 

Figure 15-10. Schematic band diagrams for single-layer conjugated polymer devices at various values of forward bias. Forward bias is defined with respect lo ITO.

The simplest and most widely used model to explain the response of organic photovoltaic devices under illumination is a metal-insulaior-metal (MIM) tunnel diode [55] with asymmetrical work-function metal electrodes (see Fig. 15-10). In forward bias, holes from the high work-function metal and electrons from the low work-function metal are injected into the organic semiconductor thin film. Because of the asymmetry of the work-functions for the two different metals, forward bias currents are orders of magnitude larger than reverse bias currents at low voltages. The expansion of the current transport model described above to a carrier generation term was not taken into account until now. 

Figure 15-13. (a) Pholocurrenl action spectra at room temperature for a thick photodiode (ITO/780 nm MEH-PPV/A1) under illumination through the ITO electrode, under forward bias (dash dot) and reverse bias (solid line), with the room temperature absorption spectrum (dashed line) shown for comparison. (b) Pholocurrenl action spectra at room temperature for a thin photodiode (ITO/120 nm MEH-PPV/A1) under illumination through the ITO electrode, under forward bias (dash dot line) and reverse bias (solid line), with the room temperature absorption spectrum (dashed line) shuwn for comparison (reproduced by permission of the American Physical Society from Ref. 176)). 

Table 16-6. Electrical properties and efficiencies of single-layer and double-layer OPVS-LEDs with 1TO hole-injecting contacts in forward-bias operation.

Figure 13-1. Encigy level diagrams under forward bias, (a) Single-layer device Iransports both holes and clccu ons and emits (b) iwo-layer device with hole and electron transport layers, one or both of which may emit (c) three-layer device with emitting dye doped (here) into a thin region of the electron transport layer.

The first realization of a conjugated polymer/fullerene diode [89] was achieved only recently after the detection of the ultrafasl phoioinduced electron transfer for an lTO/MEH-PPV/CW)/Au system. The device is shown in Figure 15-18. Figure 15-19 shows the current-voltage characteristics of such a bilayer in the dark at room temperature. The devices discussed in the following section typically had a thickness of 100 nm for the MEH-PPV as well as the fullerene layer. Positive bias is defined as positive voltage applied to the 1TO contact. The exponential current tum-on at 0.5 V in forward bias is clearly observable. The rectification ratio at 2 V is approximately l()4. 

Figure 27. Minority charge carrier profiles near the semiconductor/electrolyte junction. calculated for a silicon interface at two different electrode potentials. Uf- -0.25 V and Uf= 5.0 V10 ((//= forward bias = t/ - Ufl>).

FIGURE 3.47 The structure of a p-n junction allows an electric current to flow in only one direction, (a) Reverse bias the negative electrode is attached to the p-type semiconductor and current does not flow, (b) Forward bias the electrodes are reversed to allow charge carriers to be regenerated. 

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... 

It reaches a value of about 0.75 for an ideal junction behavior. In practice, however, always lower values have been obtained. This is mainly caused by a deviation of the slope of the current-potential dependence under forward bias, i.e. the i — U dependence is not given by Eq. 1 but is mostly described by... 

Fig. 3a—c. Charge transfer processes at semiconductor-electrolyte interface a) and b) under forward bias. 

Electroluminescent devices were made to demonstrate the possible application of these a-Si H materials. Er-doped p-n diodes in c-Si show electroluminescence, both in forward and reverse bias [670-672]. Under forward bias the electrolu-... 

Incorporated in a device, the LPCVD -Si H material shows electroluminescence only in reverse bias [673]. The mechanism is similar to the one described for c-Si. The PECVD a-Si H material was incorporated in a p-i-n solar cell structure, with a thickness of the intrinsic layer of 500 nm (see Section 1.11.1). Oxygen was coimplanted at 80 keV (3.2 x 10 O/em-) and at 120 keV (5.5 x lO 0/cm ), which resulted in a roughly constant oxygen concentration of 1.0% in the Er projected range in the middle of the intrinsic a-Si H layer. Electroluminescence is observed under forward bias [674]. 

A rectifier, or diode, passes electrical current in one direction (the forward bias direction), but blocks it in the other direction (reverse bias). For a molecule between two electrodes in a metal I molecule I metal sandwich, there are three distinct processes that can give rise to such an asymmetrical conduction. 

Fig. 10 Aviram-Ratner rectification via HOMO and LUMO. (a) A D-o-A molecule is sandwiched between two metal electrodes. MD is the electrode proximal to the donor, MA is the electrode proximal to the acceptor, is the electrode metal work function, IPD is the ionization potential of the donor, EAa is the electron affinity of the acceptor, (b) No pathway for current exists when a voltage is applied in the reverse bias direction, (c) Under a comparable voltage to (b) but in the forward bias direction, rectification results from electrons flowing from MA to LUMO to HOMO to MD...

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