FOXFET structure — device modelling and analysis

Nuclear Instruments and Methods in Physics Research A 384 (1997) 482-490

NUCUIR INVNTS &MtTNoDs IN PNVSICS “USZY

ELSEVIER

FOXFET structure - device modelling and analysis Dejan Kriiaja’*, Walter Bonvicinib,

Slavko Amona

“Laboratory for Electron Devices, Faculv of Electrical Engineering, University of Ljubljana, TriaSka 25, IO00 Ljubljana, Slovenia blstituto Nazionale di Fisica Nucleare (INFN), Sezione Trieste. Padriciano 99, I-34012, Trieste, Italy

Received 23 May 1996 Abstract A FOXFET structure for biasing AC coupled detector structures has been analyzed by a two-dimensional device simulation. For this purpose, a floating strip junction with zero current boundary condition has been applied. The floating strip voltage increase is analyzed from depletion layer spreading through three charge regions: electron accumulation surface region, hole current flow region and depleted bulk region. As a result, the floating strip potential increases approximately as the square root of the drain/backside reverse voltage. Strip potential saturation is observed for oxide charge densities larger than 5 X 10” cm-’ and results in a weaker gate control and oxide thickness influence. Current conduction mechanisms are critically discussed and drift-diffusion injection from the floating strip junction is proposed instead of the thermionic emission model. Strip potential increase by an additionally injected strip current is due to the effect of space-charge-limitedcurrent (SCLC) for an injected strip current larger than approximately 10e9 A/pm. The dynamic resistance calculated from numerically obtained strip current/voltage curves has a slope of 0.85 for a lower injected strip current (Z, < lo-” A/Fm) and decreases to 0.6 for a larger one.

1. Introduction In AC coupled silicon microstrip detectors, the capacitive coupling of the readout electrodes from the diode strips requires a suitable biasing of the strips. One possible solution, proven to be effective, is to bias the p+ (and, for double-sided detectors, also the n’) strips via integrated polysilicon resistors of appropriate value [ 11. This method implies extra processing steps in detector fabrication together with difficulties in obtaining sufficiently high and uniform values of the resistors over the whole detector sensitive area. To avoid these problems, other biasing solutions have been investigated and tested. These include the electron accumulation channel resistors for the n+strips [2] and the use of the so-called “punch-through” effect for the p+ strips. A biasing structure based on the punch-through phenomenon can be realized by a MOSFET-like device by adding a gate electrode on top of the oxide which covers the punch-through gap (Fig. 1). This gated punch-through structure is often referred to as a FOXFET (Field Oxide Field-Effect Transistor). In recent years, much work has been devoted to the analysis and understanding of the operation of the FOXFET structure [3-133. Most of this work has been performed through

*Corresponding author. Tel.: +386 61 1768 303, fax: +386 61 126 43 30, e-mail: [email protected]

measurements on test structures and complete detectors, showing that very high dynamic resistances can be obtained by the FOXFET structures, but that there are also strong influences of imperfections, deriving from the processing steps as well as from the device operation. Beside the analysis of the measurement results, analytical models for predicting the basic operation of the FOXFET structure could also significantly facilitate the design of these structures. Since appropriate analytical models are still not available, device modelling would seem to be the only reasonable alternative. The first steps in device modelling of the FOXFET structure have already been performed by Ellison et al. (41, who solved only the Poisson equation, disregarding the current continuity equations. The “threshold voltage” of FOXFET structures was determined when the potential barrier of the strip/substrate junction had almost disappeared. A more sophisticated 2D simulator HFIELDS [14] has been used by Bacchetta et al. [10,13] to take into account also the current equations. The strip potential was imposed as a boundary condition and a “floating” strip potential was determined at a positive source current of is = 1 nA by varying the strip potential. This work presents an attempt at modelling a FOXFET structure with a floating strip voltage, thus enabling an exact study of the depletion layer spreading from the drain to the strip junction. From these results, the problems related to strip potential saturation at high depletion

0168.9002/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PII SO168-9002(96)00859-5

D. Kriiaj et al. I Nucl. Instr. and

Meth. in Phys. Res. A 384 (1997) 482-490

Front-end preamphfia

Readout electronics

Substrate

Back

II-

bias

I

electrode

Fig. 1. Detector structure with AC coupled floating strip junction

and FOXFET biasing structure.

voltages and analytical determination of strip potential are addressed. The current injection mechanism from the strip to the drain junction is crucial for determination of one of the most relevant parameters of the FOXFET structure, i.e. the dynamic resistance. This mechanism has been first modelled by a simple forward biased pn junction mode1 [8], resulting in inverse proportionality between dynamic resistance and current, and later enhanced by a thermionic injection model by Baccheta et al. [13]. In this work the validity conditions for the thermionic injection models are discussed and an alternative model, based on drift-diffusion injection, is also discussed. The device modelling approach is shown to be a valuable tool for the study of internal and external properties of the FOXFET structure.

2. Device modelling approach The most common numerical device modelling approach is based on solving continuity equations for electrons and holes coupled with the Poisson equation [15,16]. Except for modelling submicron devices, it is usually sufficient to apply a drift-diffusion model for the current flow, taking into account the flow of electrons and holes as a consequence of drift in the electric field or diffusion by the gradient of carrier concentrations. Generation-recombination mechanisms such as SRH, Auger, photo generation, impact ionization, etc., are modeled as a function of local variables such as electric field and carrier concentrations, For high-energy detector structures, some additional models are introduced, describing carrier generation due to high-energy particles crossing the detector structure (SEU - single event upset). Such models are included in

483

most popular device simulation programs such as MEDIC1 [ 151 and SPISCES [16]. Simulation results are typically given as a distribution of the potential and carrier concentrations inside the structure, enabling further extraction of secondary variables as electric field components, current densities, etc. In this article the drift-diffusion equations with basic physical models for SRH generation-recombination and concentration dependent mobility were applied to analyze the operation of the FOXFET structure. In contrast to the wok of Bacchetta [lo] and, formerly, Longoni et al. [ 171 and Ellison et al. [4], a floating strip potential was assumed. This is also a necessary condition for proper analysis of the FOXFET structure as the potential of the strip depends on the depletion layer spreading from the drain/substrate reverse biased junction. The floating strip potential was modelled by imposing a zero current boundary condition on the strip contact, so that the whole reverse current from the strip/substrate junction is transferred to the drain junction. A schematic representation of the simulated FOXFET structure with basic design parameters and dimensions as well as notations is shown in Fig. 2. It should be noted, that a strip length of only 200 pm has been used for simulation, while in reality, strip lengths can be as much as few centimeters [18]. This was necessary in order to attain a fine mesh in the FOXFET channel region and a reasonably coarse one in the rest of the structure, while still not exceeding the maximum number of grid points allowed. A longer strip can be (and has been) modeled by imposing additional current on the strip contact, as the strip is in principle operating as a current source by transferring the current of the reverse biased strip/substrate junction to the drain. The parameters used in modelling of the FOXFET structure are: n-type substrate doping concentration NIub = 3.8 X 10” cm-3 (10 kfi cm), p-type junction with Gaussian doping profile with surface doping concentration Naurf = 1 X 10” cmm3 and junction depth x, = 0.5 urn, oxide thickness xox = 0.4-l pm with fixed charges of N, = 1 X lo”-8 X 10” cm-‘, substrate thickness Wsub = 300 pm, channel length L = 13 pm and carrier lifetimes 7. = rp = 0.5 ms.

3. Carrier concentrations and current flow By applying a reverse bias between the drain and the backside contacts (the usual operation of the FOXFET structure), the “floating” potential of the strip has to establish a value in between the applied reverse voltage. If the strip junction is placed far from the drain junction, that is, if the channel length is larger than the depletion width of the reverse biased junction, the strip potential would be close to the potential of the backside contact. However, since the substrate doping is very low (3.8 X 10” crnm3 in

D. Kriiaj et al. 1 Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 482-490

484

Vs=float. Xox (FOX) xj=O.S pm

Wsub=300 )~rn

substrate (N,,,)

VbZO Fig. 2. Schematic

representation

of a simulated

FOXFET structure

our case), the depletion layer width for an abrupt junction is more than 40 p,m already at the built-in voltage and increases to more than 70 Km at 1 V reverse bias. For a typical channel length of 13 pm, the depletion layers from the strip/substrate and the dram/substrate junction already touch at no externally applied voltage (due to the built-in voltage), resulting in total depletion of holes between the source and the drain. The strip to drain voltage at which the depletion regions meet is usually defined as a reachthrough voltage, while any voltage higher than this is regarded as a punch-through state. The FOXFET structure is well known to operate in the punch-through state [3131. However, a typical punch-through effect in a pnp structure is resulting in sharply increased hole current flow through a completely depleted n region, which at high current densities is limited by the SCLC effect [19,20] similarly for npn structures. However, abrupt current increase is not observed when FOXFET is operating with a floating strip junction, as the strip current is further limited by the reverse current of the strip/substrate junction. The operation of the FOXFET structure is comparable to operation of the floating-ring structures used for decreasing high electric field peaks at the junction curvatures and consequently increasing the breakdown voltage of planar devices. The main difference between the two structures is that the floating ring structure completely surrounds the active device area and that the area occupied by the termination structure is usually smaller than the active device area, while the FOXFET structure connects the strip and the drain junction with a channel width of about IO km, which is usually a very small fraction of the strip length. For floating ring structures, several analytical models exist for determination of the floating strip potential [21,22]. However, for detector structures built on very high substrate resistivities, some specific phenomena have to be taken into account. First of all, the oxide charges, which are a consequence of structural and process imperfections [23] that can increase also during device

with basic design parameters

and notations.

operation, have a strong effect on the depletion layer spreading from the drain to the source (strip) junction, as already addressed by several authors [3-131. Figs. 3 and 4 present equipotential lines, concentration of holes and current density for a FOXFET structure with reverse bias of 30 volts, zero gate voltage, oxide charges of 5 X IO” cm-’ and additionally injected current (to simulate the effect of a longer strip) of 1 X 10e9 A/pm. The concentration of holes is minimal at the semiconductor/oxide interface due to the existence of an accumulation layer of electrons, compensating for the positive oxide charges. The surface concentration of electrons for a typical oxide charge density of 5 X IO” cm-* is as high as 1 X lOI cmm3, which is substantially higher than the substrate doping concentration. Under the electron accumulation layer, a layer of increased hole concentration is established as a result of hole current flow from the strip to the drain junction. This can be clearly seen from Fig. 4 as a ribbon of increased total current density. Thus, a hole current does not flow at the semiconductor/oxide interface but rather in the bulk of the device - a few microns below the surface. A similar result has been obtained by Bacchetta et al. [IO]. The area under the hole current flow region is depleted of holes as well as of electrons. Due to the very high electron concentration in the accumulation layer, the depletion layer spreads much slower at the interface than in the bulk. As shown by the path of the current flow in Fig. 4, the strip/substrate junction is due to the effect of positive oxide charges forward biased, not at the semiconductor/oxide interface but in the bulk region. The potential difference from the strip junction to approximately half of the channel length is very small as a result of the forward biased strip/substrate region, leaving almost complete source/drain voltage to be distributed in the other half of the channel. As a consequence, hole current flow is dominated by diffusion in the first part, while drift current flow prevails in the second part of the channel.

D. Krifaj et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 482-490

ciata

485

drain

30

210

220

Distance

230

240

[pm]

Fig. 3. Hole concentration and equipotential lines for simulated FOXFET structure with 1 p_rn oxide thickness, oxide charge density N, = 5 X 10” cm-2 and injected strip current I, = IO-’ A/pm.

4. “Floating” One proper

of the operation

strip potential most

important

of the FOXFET

parameters structure

back bias V, = 30 V, fixed

too high, the strip/backside voltage is not able to completely deplete the bulk under the strip junction, resulting in non-optimal detector operation. Measurements typically show that the strip voltage follows the increase of the drain/backside reverse bias for small voltages and then

determining is the potential

of the strip. If the voltage between the drain and the strip is

40 190

200

210

Distance Fig. 4. (Hole) Current density for simulated

FOXFET

structure

220

230

240

[pm] and same simulation

parameters

as in Fig. 3.

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D. Krifaj et al. / Nucl. Instr. und Meth. in Phys. Res. A 384 (1997) 482-490

“saturates”, or better, increases much more slowly. With some authors, the voltage at which a “saturation” is reached has been denoted as a punch-through voltage [8]. The depletion layer spreading from the drain to the strip junction is of strong two- or even three-dimensional nature and is governed by three completely different charge regions as shown in Fig. 5, which can be extracted from Figs. 3 and 4: a region of strong electron accumulation with total charge density p = -q(n - Nbub), a current flow region with p = q(p + N,,,) and a depleted bulk region with p = qN,,,,. The surface depletion layer spreading is thus influenced by the charge in the accumulation region, and the bulk potential by the substrate doping concentration, while the lateral depletion layer spreading is a function of charge in the current flow region, affected also by the charges in the upper accumulation and underlying bulk region. The magnitude of strip potential depends on depletion layer spreading from the reverse biased drain/substrate junction. Assuming the lateral depletion layer spreading to be governed by an effective substrate doping concentration as an influence of the three different charge regions, a simplified solution of a one-dimensional Poisson equation is approximately given as

while the effective doping concentration can represent the substrate doping concentration modulated by the non-zero change in the vertical electric field. Such simplifications are also used for analytical treatment of floating ring structures [21,22]. Following Eq. (I), the potential of the strip V, increases approximately as a square root of the drain/backside reverse bias V,. This equation does thus not anticipate any floating strip voltage saturation, however, it predicts slower strip potential increase at higher drain/ backside voltages. Such results have also been confirmed by device modelling results in Figs. 6 and 7. Fig. 6 shows

-1 la)

10

i

vg=ov

0 0

10

20

30

40

50

Back biis voltage M

(1)

12 1

where the channel length L has been replaced by an effective channel length L,,, and substrate doping concentration NIub by an effective channel doping concentration N,,,. Both of these can be understood as an influence of several charge regions (Fig. 5) and should be a function of the substrate doping concentration, oxide charge density, gate voltage, backside voltage and injected current density. The effective channel length Le,, can represent the length of the current flow region from the strip to the drain, which is larger than the channel length,

(W

-15v

o,....,....,..,.,....,.... 0

10

20

30

Back bias voltage

40

50

40

50

M

Vd=O

Nsuh

I

oy..~.,....,,.,,,,,,.,..., 0

10

20

30

Back bias voltage

Fig. 5. Charge regions in a biased FOXFET structure.

M

Fig. 6. (Floating) Strip voltage as a function of back bias V, for gate voltages from V, = 0 to V, = -20 V and fixed oxide charge densities of (a) NF = 2 X 10” cm-*, (b) N, = 3 X IO” cm-’ and (c) N, = 5 X 10” cm-‘.

487

D. Kriiaj et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 482-490 strip potentials for increasing backside voltage V, (V, = 0) as a function of gate voltage for fixed oxide charge and 5X densities N F = 2 X 10” cm-* , 3XlO”cm-* 10” cm-*. Negative gate voltage has a strong effect on the strip voltage as it compensates for the positive oxide charges and lowers the accumulated electron concentration. If the fixed oxide charges are assumed to be located at the semiconductor/oxide interface, an approximate relation between the fixed oxide charges and the gate voltage change is given by [23]

*vo=$,

(2)

ox

where C,, = .eO&,,,, Ix,, is the geometric capacitance and xox is the oxide thickness (co = 8.854 X lo-l4 F/cm, &,,x(SiO,) = 3.9). Following Rq. (2), the change of the gate voltage necessary to compensate for the effect of oxide charges for oxide charge densities N, = 2 X 10” cm-*, 3 X 10” cm-*, and 5 X 10” cm-*, and 1 pm oxide thickness approximately equals 9, 14 and 23 V, respectively. At these voltages an electron accumulation layer vanishes and the drain depletion layer is free to spread at the surface in the same manner as into the bulk. As a result, strip voltage value is close to the drain one (OV) as found also by the

simulation results presented in Fig. 6. For the same reason at higher reverse voltages the strip potential “saturates” for larger oxide charge densities and lower gate voltages. Fig. 7 presents the simulation results of floating strip potential increase with the drain/backside reverse voltage for FOXFET structures with fixed oxide charge of NF = 5 X 10” cm-* and oxide thicknesses from 0.4 to 1 Fm. With no gate voltage applied the V,lV, curves differ only slightly (Fig. 7(a)) while the difference is more pronounced at higher negative gate voltages (Fig. 7(b)). Strip potential at high reverse drain/backside voltage is thus reduced only slightly for zero gate voltage (Fig. 7(a)), and quite strongly for high negative gate voltages (Fig. 7(b)). Simulations as well as experiments [8,12] have shown, that this is due to the strip voltage saturation at high oxide charge densities that should, however, not be mixed with the strip potential “saturation” at high drain/backside reverse voltages.

5. Carrier transport models and dynamic resistance The next important parameter determining the operation of the detector is the dynamic resistance R, of the FOXFET structure [4-5,6,8,9,1 l-131. R, is calculated from the derivative of the strip voltage versus current according to the equation R, = dV,ldl,. If a simple forward biased diode is assumed to govern the conduction of a FOXFET structure, as shown by Laakso et al. [8], a well known inverse proportionality between dynamic resistance and current is obtained. This model relatively well describes the measured characteristics, that is, usually showing a 1-o.6 to Z-o’9 relationship [8,13]. 5.1. Transition between drift-diffusion emission conduction

00 0

10

20

30

BaCkbia8vonp(le

40

50

M

l2

and thermionic

Conduction mechanism in a pnp FOXFET structure which is based on hole current flow is rather different from conduction mechanisms in a classical pn diode. For this reason, a thermionic injection model of Chu et al. [24] for a BARRIT pnp diode in a reach-through conduction was proposed by Bacchetta et al. [ 131, resulting in strip current flow

and thus (4) 0

10

20

30

40

50

Fig. 7. (Floating) Strip voltage as a function of back bias V, for oxide thicknesses from 0.4 to 1 pm with fixed oxide charge density NF = 5 X 10” cm-* and gate voltages (a) V, = 0 V and (b) v, = -1ov.

where V&, is a strip (flat-band) voltage necessary for maximal punch-through conduction (strip voltage necessary to completely compensate the build-in potential of the forward biased part of the strip/substrate junction) and ZsT.= AA*T* is the saturation current of the thermionic

D. Krifaj et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 482-490

488

emission model where A denotes the carrier flow crosssection area and A* is the effective Richardson constant. If V& is assumed constant or it increases slower than V,, the dynamic resistance increases more slowly than I ’ due to increase of the strip voltage at higher currents. Nevertheless, the thermionic emission model does not seem to be appropriate for describing the carrier transport mechanisms in the FOXFET structure. The thermionic emission (injection) model assumes carriers to move at Richardson (thermal) velocity [27], which holds in the cases where potential energy changes rapidly over distances comparable to the carrier mean free path [25]. Such distances usually apply in Schottky barriers or bipolar epitaxial heterojunction transistors where a continuum assumption for the current equation fails. Persky [24] applied a diffusion model with a correction for diffusion currents approaching the thermionic emission limit for a floating base transistor in a punch-through, resulting in a breaking point between drift-diffusion transport and thermionic emission at the condition &A,

= Dl ure .

(5)

where A, is the extrinsic Debye length, D is the diffusion constant and I+, is the thermionic emission velocity. Using Einstein’s relation between the diffusion constant and carrier mobility (D/p = U/q) in Eq. (S), a condition for the necessary base (substrate) doping concentration for thermionic emission is Ncuh > E/(/J~~*). For a hole mobility in high-resistivity substrates of 5OOcm*/V s the thermionic emission starts at N,ub > lO”cm~‘. With this result, we can assume the transport mechanisms in the FOXFET structure to be completely and satisfactorily described by the drift-diffusion mode1 Nonetheless, the final equation governing the carrier transport by only drift-diffusion through the depleted base transistor (FOXFET) is similar to the thermionic emission model and is given by [26] (6)

is the saturation current of where I,, = qADN, I x&A,, the drift-diffusion model with N, the strip (drain) doping concentration. The basic difference between the two models is thus a less significant temperature dependence of the drift-diffusion than in the thermionic model [ 181. A more elaborate but less straightforward model for the transition between drift-diffusion and thermionic emission can be found in Grinberg and Luryi 1271.

F.“.,,

.

OWO

2e-a

.,I,

,I,.

4e-9

,,,

6e-9

Stnp Current

Be-9

,e-7

[A/pm]

Fig. 8. (Floating) Strip voltage as a function of injected strip current at V, = 30 V and N,: = 5 X IO” cm-* (lin/lin scale).

is obtained by simulation as shown in Fig. 8 for a structure with NF = 5 X 10” cm-’ at V, = 30 V. The strip voltage rapidly increases and then “saturates” at values depending on the gate voltage. If the same results are presented in the lin/log scale (Fig. 9), the strip voltage is independent of strip current for small currents, but gradually increases for higher ones. The low sensitivity at low current densities can be explained by the low hole concentration in the channel region. At higher injections, the hole concentration in the channel surmounts the substrate doping concentration and thus alters electric field and potential distribution. This results in an increased strip potential as the current is limited by the space charge effect [Is]. Such a situation starts at injected current densities of more than 1 X IO-’ A/pm, as can be deduced from the hole concentration in the channel in Fig. 3 which approaches the substrate doping concentration. Dynamic resistance is obtained by numerical derivation of the V,/I, results from Figs. 8 or 9 and is presented in Fig. IO for gate voltages from V, = 0 to - I5 V. The dynamic resistance is drawn in [Cl pm] and the current in [A/pm]. so for the comparison with the experiments, the dynamic resistance in Fig. 10 needs to be divided by the FOXFET channel width (probably even more than the

18

vg=o v /

5.2. Dynamic resistance calculation using numerical simulation results Measurements of strip voltage vs. strip current [KS] have shown that strip voltage increases by a few volts as the current increases, and then remains more or less constant for the further increasing current. A similar result

le-15

WI-14

lb13

lo-12

la-11

Stip Currant

le-10

b-9

l&l

,e-7

[P/pm]

Fig. 9. (Floating) Strip voltage as a function of injected strip current at V, = 30 V and NF = 5 X 10” cme2 (h/log scale).

D. Kriiaj et al. I Nucl. Instr. and Meth. in Phys. Res. A 384 (1997) 482-490

,e+11

E; ,e+,o -i r ;

1e+s

1e+B

1e+7

I ......q le.15

1s.14

I.....q . .....l I.313

la-12

IO-11

Ship Cummt

ICI-10

1.39

1~.8

1e-7

[A&m]

Fig. IO. Dynamic resistance, calculated from the V,ll, curves in Fig. 8. channel width, as the current flow is free to flow also from the sides of the channel) and the strip current multiplied by the strip width (or rather multiplied by the strip pitch, as each strip collects reverse current from the width of the strip pitch). As also obtained by measurements [8,11], almost no gate voltage influence on dynamic resistance is observed which is due to the punch-through current flow from the strip to the drain that cannot be stopped (closed) by the gate control. Furthermore, it is also not significantly influenced by the space charge effect. Very high and constant dynamic resistance is obtained for low injected currents (short strip lengths) that starts to decrease for injected currents larger than approx. lo-l3 A/ km with a slope of 0.85. For higher injected currents (longer strips) the slope of dynamic resistance decreases to approximately 0.6.

6. Conclusions In this work, an analysis of the floating strip potential and conduction mechanisms in the FOXFET structure was performed. SPISCES 2D device simulation program solving drift-diffusion equations together with necessary generation-recombination mechanisms was used. The “floating” potential of the strip was successfully modelled by imposing a zero current boundary condition on the strip contact. Depletion layer spreading from the drain to the strip junction has been shown to be influenced by three charge regions: the surface electron accumulation region resulting from the influence of positive oxide charges, the current flow region occupied by holes transversing the channel, and the depleted substrate region with positive charge concentration equal to substrate doping concentration. If the influence of all three regions is summed up in an effective doping concentration, the strip potential approximately increases as a square root of the drain/backside applied voltage. As a result, the strip voltage increases slower at high strip/backside reverse voltages, which may

489

explain the effect of “strip voltage saturation” at high reverse voltages. Device modelling has shown that the floating strip voltage is mostly influenced by the oxide charges. If no oxide charges are assumed, the strip potential is close to the drain one and the FOXFET structure loses its functionality. The same occurs if the gate voltage is high enough to completely compensate th effect of positive oxide charges. Furthermore, the strip potential saturates for high oxide charge densities (NF > 5 X 10” cm-‘), which results in weak control of the gate voltage for small changes in the gate voltage and independence of the strip potential on the oxide thickness. On the other hand, gate control of the floating strip potential is strong for lower oxide charge densities (NF < 3 X 10” cm-‘) as well as for thinner oxides (x,, < 0.6 pm). Due to unipolar (hole) conduction in the punch-through mode, the conduction mechanism has been regarded, through the literature, as a thermionic injection. However, due to the large distances compared to the mean free path of carriers and low substrate doping concentration, a more likely conduction mechanism is a drift-diffusion injection of carriers over the reduced strip/substrate potential barrier. The low doping concentrations and modest electric fields in the FOXFET structure have also been confirmed by the device simulations performed, resulting in floating strip potentials as well as dynamic resistances that are in general agreement with the experimental results. The two conduction mechanisms differ mostly in a weaker temperature dependence of the drift-diffusion model than of the thermionic one. The strip voltage shows a very weak dependence on the strip current at low current densities, denoting a low concentration of holes in the channel region. Another reason is the non-limited channel depth and width, as the current flow can extend into the bulk and to the side, which also results in low gate voltage control on the dynamic resistance of the FOXFET structure. At higher current densities, the concentration of holes overcomes the substrate doping concentration and results in redistribution of the electric field and thus in equipotential lines. The strip potential starts to increase with the current as a result of SCLC conduction for injected currents of approx. more than lO--’ A/pm.

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