We have developed several kinds of HV MOS devices whose device structures and doping levels in the offset regions differ depending on the specifications of the devices. We have developed the bi-directional HV MOS device (e.g., it can be used as a bi-directional MOS switch) with both drain and source offset regions, and we previously described its SPICE model [4] - [5] based on BSIM3v3 [1] - [3]. On the other hand, the uni-directional HV MOS device has only a drain offset region. That is, it does not have a source offset region with corresponding source resistance.

In this paper, I have used the same technique to model uni-directional HV MOS devices as previously reported for bi-directional HV MOS devices [4] -[5] while adopting a new parameter extraction technique. With the new uni-directional HV MOS modeling technique, the simulated I-V characteristics of the uni-directional n-channel HV MOS device match the measured characteristics well, which confirms its effectiveness.

Figure 1. Schematic structure of the uni-directional n-channel HV MOS device.

2. Uni-directional HV MOS Device Technology

Figure 1 shows the structure of our 45 V n-channel HV MOS device, with a gate-oxide film thickness (Tox) of 1350, channel length (L) of 3.2µm and offset of 3.2µm . A n-offset region of low doping concentration is used in the drain region, in order to realize high uni-directional drain-source breakdown voltage. Based on our device simulations, we consider that the basic operation of the bi-directional HV MOS device is as explained below , and this is also valid for the uni-directional HV MOS device. When drain-source voltage (V_ds) is applied to the HV MOS device, a depletion region grows in the n- offset. The effect of the electric field from the gate electrode on the depletion region causes the drain-side channel terminal voltage to saturate at a low voltage. This reduces the voltage across the channel, and hence increases the breakdown voltage between the drain and source. Moreover, as the drain-side channel terminal voltage saturates in the triode region, g_m is reduced.

3. Application of Bi-directional HV MOS Model to Uni-directional HV MOS Device

In this section, I directly apply the bi-directional HV MOS model and its parameter extraction technique to the uni-directional HV MOS device, and investigate the results. The bi-directional HV MOS model parameter extraction technique is outlined below [4] - [5]:

First, extract all BSIM3v3 parameters by the standard method.
Then set the value of VSAT to a large value (e.g., 1x10⁹ m/sec) assuming a long-channel device.
Optimize AGS and DELTA at the same time. This is to optimize the saturation voltage of the drain-side channel terminal Vdsat and the source resistance due to the offset region.

Using the above procedure for the bi-directional HV MOS device, the parameter AGS optimizes V_dsat and source resistance in a well-balanced manner, and the measured and simulated I-V characteristics match well. Furthermore, this method can precisely reproduce the g_m-reduction phenomenon, which is inherent to HV MOS devices. On the other hand, step II in Table 1 shows the parameters obtained by directly applying the bi-directional HV MOS device model and parameter extraction technique to the uni-directional HV MOS device, and Figure 2 shows a comparison of the measured and simulated I-V characteristics. In Figure 2 large discrepancies are observed between the measured and simulated I-V characteristics.

Figure 2. Comparison between measured and simulated.
(with the bi-directional HV MOS model)
I-V characteristics of the 45V HV MOS device.

4. Proposed Uni-directional HV MOS Device Modeling Technique

In this section I propose a uni-directional HV MOS modeling technique: I use the bi-directional HV MOS model in [4] - [5] as a basis, and the main equations are as follows:

Here, W is the channel width, ?_eff is the effective mobility, _ox is the permitivity of the silicon-oxide film, V_th is the threshold voltage, AGS is the gate bias coefficient of the A_bulk, V_dsat is the saturation value of the drain-side channel terminal, V_dseff is the effective drain-source voltage and DELTA is the effective drain voltage smoothing parameter of V_dseff. Where, V?d_s is the drain-side channel terminal voltage, which is equivalent to the V_dseff of BSIM3v3.

V_rs is voltage drop across the source resistance in the offset region. It cannot be eliminated from equation (1) as V_rs is inherent for the bi-directional HV MOS model with BSIM3v3. Hence, it is necessary to compensate for part of the drain current component by V_rs in equation (1) for the uni-directional HV MOS model. Here, I define the drain current I_drs0 in which the voltage V_rs in equation (1) is eliminated as shown below. I_drs0 is a model equation for the uni-directional HV MOS device model.

Further, by defining I_vrs as the variable component of drain current due to V_rs, I obtain the following expression:

The above expression indicates that the way to compensate for I_vrs in the uni-directional HV MOS model is to optimize I_drs0 by making it larger than Ids by the value of I_vrs. Here it can be assumed that the value of I_ds is equivalent to the measured data. Suppose the parameter AGS is set to a large negative value, the value of A_bulk in equation (4) increases, which in turn decreases the value of Vdsat in equation (5) and results in decreasing the value of I_drs0. At the same time, A_bulk increases the value of I_vrs larger. So, AGS cannot optimize V_dsat correctly, and I_ds cannot represent the g_m-reduction (refer to section 7.). In addition, the value of I_vrs cannot be obtained directly by calculation; instead use the following equation:

I propose the following model which can express the g_m-reduction.

BSIM3v3 SPICE model combines the source and drain resistance with R_ds (whose value depends on V_gs) as follows:

where RDSW is the source and drain resistance per unit channel width and PRWG is a V_gs coefficient. R_ds is incorporated in the drain current expression of the simplified model as an explicit function of G_ds0 shown in equation (3), and R_ds hardly affects the saturation voltage (V_dsat) of BSIM3v3. Note that G_ds0 has R_ds in its denominator, and by making R_ds a function of V_gs, it is possible to accurately express the g_m-reduction for I_dsr0 and I_ds mathematically.
R_ds provides the g_m-reduction for I_drs0 and I_ds, but they are not the same. I_drs0 with a larger absolute value has larger g_m-reduction, so that it can give most appropriate g_m-reduction in order to compensate the value of I_vrs, which is I_drs0 - I_ds.
R_ds has little influence on V_dsat, and it can be utilized independently of AGS.
AGS can be used to optimize the absolute value of I_ds, through V_dsat.
For HV MOS devices, both V_gs-caused mobility degradation effect and g_m-reduction occur at the same time and influence of the latter is more predominant. So in the HV MOS model, it is possible to ignore the mobility degradation effect. By defining ?_eff as a constant regarding to V_gs, we can obtain about five times faster optimization of the parameters for the parameter extraction tool.

5. Proposed Parameter Extraction Technique for Uni-Directional HV MOS Device Modeling

In this section I describe the proposed parameter extraction technique for the uni-directional HV MOS model which can be implemented by using a SPICE model parameter extraction system, such as UTMOST [6].

First, extract all BSIM3v3 parameters by the standard method (step I in Table 1).
Then set the value of VSAT to a large value (e.g., 1x10) assuming the long channel model.
Optimize the parameters of U0, RDSW, PRWG, AGS, and DELTA together in all the triode region (up to close to the saturation region) i.e., from (V_ds=0V, V_gs=1.93V) to (V_ds=20V, V_gs=45V). RDSW and PRWG have to be optimized such that R_ds expresses g_m-reduction of I_drs0 while AGS has to be optimized such that A_bulk and V_dsat compensate for I_vrs.
Optimize PDIBLC1 and PDIBLC2

	AGS	U0	UA	RDSW	PRWG
Step I	0.084	616	0.7x10	3.8x10	0.0
Step II	-0.049	520	0.4x10	3.8x10	0.0
Step III	-0.076	463	0.0	1.9x10	0.092
Step IV	-0.077	588	0.0	5.5x10	0.018

Table 1. Extracted BSIM3v3 Parameter Values for each step.

Since optimization of the absolute value of I_ds and g_m interact with each other, it is necessary to optimize them simultaneously. Step III in Table 1 shows extracted parameter values based on the above procedure, and Figure 3 shows a comparison between the measured and simulated I-V characteristics. I see that good agreement is obtained between the measured and simulated results (except for the region where negative g_ds appears.).

In Figure 3, accuracy of simulation decreases in the linear region with higher value of V_gs. This is because R_ds provides the g_m-reduction in all the region. However, g_m-reduction due to the drain-side offset region appears only in the saturation region. To compensate for the worsening accuracy, it is effective to implement Step III again by setting the channel length modulation parameter PCLM=0 (or optimize). The 0 value of PCLM amplifies the g_m-reduction in the saturation region, so that V_gs dependence of Rds is optimized smaller (Step IV in table 1). As a result of this, g_m-reduction caused by R_ds in the linear region gets smaller. Step IV in Table 1 shows the parameters with PCLM=0 and Figure 4 shows the comparison between the measured and simulated I-V characteristics in the triode region. Figure 5 shows I_ds-V_gs characteristics in triode region. Note that I_ds monotonically increases under the condition of V_gs 40V, however, Ids slightly decreases when V_gs 40V, and this phenomenon is more predominant when V_ds = 1V than V_ds = 2V. This is due to the decrease of the channel voltage of the drain terminal by the influence of the gate electric field.

Figure 3. Comparison between the measured and simulated
(with the uni-directional model) I-V characteristics
of the 45V n-channel HV MOS device.

Figure 4. Comparison between the measured and simulated
(with the uni-directional HV MOS model and PCLM=0)
I-V characteristics in the triode region.

Figure 5. Comparison between the measured and simulated
(with the uni-directional HV MOS model)
Ids-Vgs characteristics in triode region.

References

Y. Cheng, M.-C. Jeng, Z. Liu, J. Hubg, M. Chen, P. K. Ko and C. Hu, "A Physical and Scalable I-V Model in BSIM3v3 for Analog/Digital Circuit Simulation, " IEEE Electron Devices, vol.44, no.2, pp.277-284 (Feb. 1997).
J. H. Huang, Z. H. Liu, M. C. Jeng, K. Hui, M. Chan, P. K. Ko and C. Hu, BSIM3 Manual (version 2.0), University of California, Berkeley, (1994).
BSIM3v3 Manual (final version), University of California, Berkeley,(1995).
T. Myono, S. Kikuchi, K. Iwatsu, E. Nishibe, T. Suzuki, Y. Sasaki, K. Itoh and H. Kobayashi, "High-Voltage MOS Device Modeling with BSIM3v3 SPICE Model, "IEICE TRANS. ELECTRON, vol.E82-C, No.4, pp630-637, (April 1999).
T. Myono, E. Nishibe, S. Kikuchi, K. Iwatsu, T. Suzuki, Y. Sasaki, K. Itoh and H. Kobayashi, " Modeling and Parameter Extraction Technique for High-Voltage MOS Device", Proc. of International Symposium on Circuits and Systems, pp.230-233, Orlando Florida (June 1999).
UTMOST III Extraction Manual,Vol.1, ver. 12.03, SILVACO International, Santa Clara, CA