DIODE PHYSICAL PARAMETERS FOR HEXFETS CHARACTERIZATION OF DOSE EFFECT

Modeling techniques of P-N junctions have been applied for studying the physical parameters in metal-oxide semiconductor field-effect transistor structures. A parameter extraction method provides a precise description of the changes in conduction processes due to radiation damages in the integral body-drain junction. A large increase of the ideality factor and series resistance is related to radiation-induced defects.


INTRODUCTION
Power metal-oxide-semiconductor field-effect transistors (MOSFETs) have emerged as the most suitable device for a variety of applications that require high current, high voltage devices.Up to the early 1980's, power bipolar transistors provided the best performance.Since then, power MOSFETs using double-diffused metal-oxide-semiconductor (D-MOS) technology have overtaken bipolar transistors in overall performance.Their electrical characteristics, higher switching speeds, negative temperature coefficient, and greatest ease of drive, make them interesting for use in space systems [1,2].*Corresponding author.Space applications of power MOS devices require knowledge of the behavior of these devices when exposed to ionizing radiation.Most works [3,4] characterizing the effects of ionizing radiation on MOS devices have focused on the determination of threshold voltage shifts, carrier mobility reductions and degradation in the transistor transconductance related to oxide-trapped charge and to the increase in the density of interface-traps.The threshold voltage has been considered to be the most radiation sensitive parameter.
This study shows that other parameters might be of importance to qualify commercial microelectronic devices for use in space applica- tions.These are the junction body-drain parameters which are shown to be monitors of radiation degradation.
When MOS structures are exposed to ionizing radiation, electron- hole pairs are created by the incident energy.A fraction of these electron-hole pairs recombines in the insulation oxide layers and a fraction of the deposited energy forms radiation defects in the body layer and radiation-induced leakage paths between the drain and the body which modifies the body-drain junction properties.
The aim of this work is to show modifications of the structural parameters of the body drain junction which are enhanced by the radiation dose.Dose dependent large increases of the series resistance and of the ideality factor are obtained and found to be related to radiation-induced defects.These results introduce a new method for micronic device characterization.

METHOD
Measurements on hexagonal field effect transistors (HEXFETs) are taken on the gate controlled body-drain diode.A representation of the structure of the device is shown in Figure 1.
The transistor is based i.e., the source terminal is made positive with respect to the drain (Fig. 2) so the diode is forward biased.The gate voltage is kept below the threshold voltage value, i.e., there is no current through the channel.
The experimental method has been commented and detailed elsewhere [5].Current-voltage characteristics of the diode are obtained by a computer drivers acquisition system.Current (I) and

FIGURE
The schematic structure of HEXFET showing direct transistor current (DTC) and direct diode (DDC) paths.voltage (V) values of one hundred points of I-V characteristics are used for the modelling analysis of the diode.Physical and electrical parameters are extracted from the theoretical description of the current-voltage characteristics for each body-drain diode of all transistors.
Models of the silicon p-n junctions are reasonably well developed.I-V characteristics of the jonction can be described by a first order single exponential model (SEM) or a second order double exponential model (DEM) with the following equations [6]: I --+IoV + RsI [e(q/kT)(V+Ri --1]/ Io2[e (q/nkT)(V+RI) 1] (1) Rsh These models introduce the series resistance R to take into account the power losses, the shunt resistance Rsh for the leakage currents and the diode ideality factor n. Currents I01 and 102 are the two components of the diode reverse currents.
The SEM model (where 101 0) gives a global description of the physical processes in the operating diode.For an ideal jonction n and the reverse diffusion-recombination current 102 is low (of the order of 10-4Acm-2) which implies a predominant diffusion process.
Values of the ideality factor n greater than unity [7] are related to carrier recombination via traps and the increase of n is correlated with an increase of the reverse 102 current.
The DEM model (I01 : 0 and 102 : 0) separates electronic diffusion- recombination phenomena (I01) in the quasi-neutral regions of the jonction from recombination in the space-charge region (I02).

RESULTS AND DISCUSSION
The results discussed below were obtained for Iternational Rectifier (IRF 530) hexagonal field-effect transistors (HEXFETs) before (NI: non-irradiated) and after (IR: irradiated) irradiation.They were irradiated at five different doses (1,2, 5, 10 and 20 krad(Si)) by a 103,8 rad.mn -1 Co-60 source.The gate was biased at 4.0V, the drain at 0.3 V while the source was grounded during irradiation.
Radiation effects are usually evaluated by the induced modification in oxide trapped-charge (/kNot) and interface trap (/kNit) densities.Figure 3 displays the variation of the (/kNot) and (/kNit) versus the total dose.For these devices, the density of oxide trapped charges is much larger than the density of interface-trapped charge [8].The study of the physical processes of the body-drain junction transport phenomenon was performed under low voltage operating conditions with gate bias lower than the threshold voltage value VT.This is to ensure the characterisation of recombination phenomena in the junction.The properties of structural parameters of the physical junction should be, quite evidently, related to VG.This is confirmed here by the large increases of the series resistance Rs (Fig. 4) with VG.
The physical parameter R is capital for the user in terms of high frequency losses.Rs is directly related to the internal structure.It would be ideal choice for hardness evaluation of components.
As the gate bias increases, a surface potential (V) appears and the large increase of R, after accumulation, may be attributed to the extension of the junction space charge region within the induced depleted layer (W) under the gate.A limit value of R is reached when the depletion layer width reach a maximum value [9] (Wmax (2s/ qNA) 1/2 (VD + 2)1/2).A similar effect has been previously observed with capacitance measurements [10].The dose rate is observed too.Furthermore, Figures 5 and 6 points out respectively an increase of series resistance R and the ideality factor n of the irradiated device, that reflects the radiation damages in the transient region of the junction and at the oxide-semiconductor interface near the drain junction.These results have been obtained with an applied gate voltage Va measured relative to the threshold voltage: Va-Vr 1V.This large increase of the R and n is of importance since it constitutes a directs evidence of radiation effects.
The increase of R and of n is consistent with the carrier mobility reduction produced by irradiation and my be related to the surface electrical field which depends on the surface potential (see Appendix I).The appearance of the interface charges due to the ionizing radiation modify the surface electrical field (E) (Fig. 7).This evolution of E may be confirmed the increase of the series resistance and the quality factor.The ideality factor has been discussed as a function of the densities of recombination states [11] and shown to be dependent on the  localisation in the depleted layer.This observed large increase of the (n) indicates high values of the recombination current.It is confirmed with the expected increase of the reverse current, to the total dose [5].

CONCLUSION
This study which concerns the degradation of structural parameters of the body-drain junction of irradiated D-MOS power HEXFETs.It is shown that, in weak inversion, the body-drain junction is altered by radiation and its conduction properties are affected by the interface states and trapped-oxide charges which participates in the degradation of series resistance and quality factor.
These results introduce a method for microelectronics devices characterization.This method is more efficient to quantify structural degradation than the usual measurements of the threshold voltage or of transconductance.

FIGURE 2
FIGURE 2 Experimental set-up with bias conditions for n-channel HEXFET mounted as a gate controlled diode.

FIGURE 3
FIGURE 3 Variation of the oxide trapped-charge (ANot) and interface (ANit) densities versus total dose.

FIGURE 4
FIGURE 4 Variation of the series resistance, R, against gate bias V calculated before (NI), and after (IR) irradiation.

FIGURE 5
FIGURE 5 The body-drain junction series resistance, Rs vs total dose (IRF530).