Capacitance depletion layer

In one of the most common types of photodiodes used for time-resolved work, the p-i-n photodiode (see Figure 12.24), the depletion layer thickness (i for intrinsic) is fabricated to obtain this optimum performance. Manufacturers usually give full specification sheets detailing, active area, time/frequency response, responsivity amps/watt (AAV) at a given wavelength, dark current, depletion layer capacitance, and bias volts such that with minimal external electronics devices can be made operative. 

We have seen that, provided the Fermi level lies at least 3 kT from either the valence or conduction bands, the depletion-layer capacitance will take the form [7]... 

There are several complications in using this technique for a-Si H, first of which is the frequency dependence. The capacitance is measured by the response to a small alternating applied electric field. The depletion layer capacitance is obtained only when the free carriers within the bulk of the semiconductor can respond at the frequency of the applied field, dielectric relaxation time. 

The optical absorption arising from the defect transitions is weak because of the low defect densities and in a thin film cannot be measured by optical transmission. The techniques of PDS, CPM and photoemission yield, described in Section 3.3, have sufficient sensitivity. Photocapacitance, which measures the light-induced change in the depletion layer capacitance, is similarly sensitive to weak absorption (Johnson and Biegelsen 1985). PDS measures the heat absorbed in the sample and detects all of the possible optical transitions. At room temperature virtually all the recombination is non-radiative and generates heat by phonon emission. CPM detects photocarriers and so is primarily sensitive to the optical transitions which excite electrons to... 

In ideal semiconductor/electrolyte junctions, the presence of an energetic barrier in the semiconductor phase, originated from the equilibrium of the Fermi level in the solid and liquid phases, can be approached by the semiconductor depletion layer capacitance or space charge capacitance, Cgc- Measurement of capacitance versus... 

In the absence of electronic equilibrium established by a redox electrolyte, the potential difference across the semiconductor/electrolyte jimction can be controlled in a three electrode cell with a reference electrode. The variation of the depletion layer capacitance with potential, U, is described by the Mott-Schottky equation [1] ... 

This section presents results that show how the rates of photoelectrochemical processes can be derived from time resolved measurement of the photoinduced current or potential in the external circuit of a photoelectrochemical cell. The capacitance of the Helmholtz-double layer is of the order of lO Fcm , the depletion layer capacitance of an extrinsic semiconductor junction is typically 10 -10 Fcm , while the capacitance of an insulator is orders of magnitude lower. With a value of 100 Ohm for the resistance Rd + R of the cell, the time constant of photoelectrochemical cells is 10 s for metallic electrodes, 10 -10" s for semiconductor electrodes and much lower for insulator electrodes. The rates of photoelectrochemical processes also span a wide range. This makes photoelectrochemical kinetics a rich, albeit demanding, area for research. 

Figure 1. A schematic diagram (a) and a partial equivalent circuit (b) are given for the LAPS. The components 4, Ci, Cdf Re, Vref and Vchem, respectively represent the applied bias potential, the insulator and depletion layer capacitances, the electrolyte resistance, the potential across the reference electrode, and a chemically sensitive surface potential. Ip represents the photogeneration of hole-electron pairs, and I the measured alternating photocurrent. Solution potential is maintained by a potentiostat using a Pt controlling electrode (CTL) and Ag/AgCl reference electrode (REF). The potential is defined as the potential from the output of the reference electrode to ground.

In the simplest case as more fully discussed elsewhere [14, 15, 29], one obtains the Mott-Schottky relation (for the specific instance of a n-type semiconductor) of the semiconductor depletion layer capacitance (Csc), again by invoking the Poisson equation... 

Cq thus increases as the well fills up and W decreases. Before the well fills up, if (2ee5N ) /Co <(KG), then the oxide capacitance will be much larger than the depletion layer capacitance and (f) will track Vq. For this reason lightly doped material and thin gate oxide layers ( 1000 A) are needed. Examination of (6.1) also shows that barriers can be permanently built into the device by varying and/or by changing the oxide thickness (which changes C ). These techniques are used to form channel stops to define the CCD channel and can also be used to provide the built-in barriers necessary for two-phase CCD operation. 

The depletion layer capacitance is usually smaller than both the gate oxide capacitance and the external capacitance so that approximately... 

The depletion layer capacitance C, is usually less than so that... 

---