I'm looking for a practical method to simulate Schottky diode's operations in ADS rather than using its available libraries. The equivalent circuit of a Schottky diode is as attached (in the red box). CL, RL, Cj and Rs are constant. But the problem is about Rj. According to a datasheet (HSMS-285x Series for example), Rj = 8.33 x 10e-5 x n x T /(Ib + Is). The saturation current IS is also constant, but Ib is the externally applied bias current! How can I get Ib in that circuit?
Another equation is the relationship between iD and vD: iD = IS x [exp(vD/nVT) - 1], with VT is constant too. But I do not understand that relationship with Rj. So, I do not know how to input Rj value in the ADS model. Any advice would be highly appreciated.
Dear Minh,
The Schottky diode has the i-v relation as
I= Is( exp V/ nVt -1),
Is is the reverse saturation current , n is the ideality factor= 1 for the such diodes and Vt is = the thermal voltage = 25.5 mV at room temperature 300K.
The small signal diode resistance Rj= dV/dI , differentiating the above equation one gets : Rj = 8.33 x 10e-5 x n x T /(Ib+ Is). which is the above formula.
The junction capacitance Cj= dQ/dV , where Q is the charge in the diode and V is its voltage. In case of the Schottky diode, the capacitance is the jucnction capacitance and no diffusion capacitance Cj= Cj0/ sqroot ( 1 - VF/phi), Vf is the forward voltage and phi is contact difference of potential.
The most straight forward for obtaining these parameters to obtain them from device model library as these are the models built in the simulators.
For more information about the small signal parameters of the diode please refer to the book:Book Electronic Devices
Best wishes
Dear Minh
You are right, the equivalent circuit parameters are voltage and frequency dependent so that finding an accurate model is difficult. What I would suggest you to do is that in most cases, the series resistance and junction capacitance of the Schottky diode are almost constant over a range of frequencies and this can help to keep their values maintained in the simulation. However, this is not the case with the junction resistance, which can be modelled utilising SDD2P block available in ADS. Inside SDD2P block, you can use a fit equation that represents the behaviour of I-V characteristic of the diode (i.e. Rj will be included in the I-V curve ) and during the simulation, ADS will take the value of the diode current depending the bias voltage. You can even add any additional parasitic elements, making your simulated data close to actual results. If you need any help regarding this matter, please do not hesitate to contact me.
All the best
Dear Mr. Zerky and Mr. Muttlak,
Thank you very much for your valuable answers. To start a fairly correct model of the diodes in ADS, can I use the following model, considering parasitic elements? I'll try SDD2P block later in order to get better understanding about diode's behavior.
Dear Minh
I think, you can try your model, however you must get the simulated results fit with the measured ones in terms of I-V and C-V characteristics. Just in case you would like to build SDD2P block, I attached an example regarding Schottky didoe, which may help for proper model in ADS.
All the best
Dear Minh,
Your model is okay and you use it for the analysis of the circuit. What is important to say that:
The model of Rj, Cj and Rs are the small signal diode parameter. It is valid at certain DC operating point. The model parameters are fairly independent of frequency.
The parameters depend on the operating point. So, assuming the nonlinear mode of operation means that the dependence of these parameters on the operating point is taken into consideration.
This is what i meant by small and large signal analysis. Large signal analysis is achieved by letting the parameters of the diode vary with the its dynamic biasing.
Best wishes
Dear Mr. Muttlak,
Thank you very much for your examples. I'll follow these examples to verify my model.
Dear Mr. Zerky,
Thank you very much for your answer. Now I'm using LSSP + HB modes to simulate my rectifier as in another threat of mine. Could you please explain more about the operating point? In my understanding so far, with HB mode I can monitor the I, V characteristics of the diode, while with S-para mode I can get S11, S12 and so on. The difference between S-para and LSSP modes is that in LSSP I can observe the variation of S-parameters with varying input power. Is that the operating point you mentioned?
The most accurate way to model the diode into the ADS is using the SDD feature in ADS. It could describe the nonlinear operation of the diode based on its differential equation of both Cj and Rj.
You can find more information on how to use this capability in the following link:-
http://edadocs.software.keysight.com/display/ads2008U1/Custom+Modeling+with+Symbolically-Defined+Devices
Hello, I am trying to model a transistor on ADS using SDD feature but I do not know how to show or apply bias voltages like Vds and Vgs, if anyone could help me, that would be great!
Hello,
I have zero biased schottky diode power detctor circuit designed in hfss (with diode 3D model) and its s-parameter imported to ADS. When I do Harmonic Balance simulation, I am getting very less voltage (1 uV) compared to expected (1 mV). I am trying to find the issue since very long time, any suggestion might help. Thank you.
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