Hi,
I would like to gain more insights on HEMT figures of merit (cutoff frequency f_T, maximum oscillation frequency f_max, breakdown voltage BV, on-resistance R_on, maximum drain current I_dmax) to understand the relationship between the device performance (in terms of RF operation) and each of the figures of merit. However, I have not been able to find good resources on this and all the review papers seem to assume the readers already have this knowledge. Below are the some of the examples I have been pondering:
1. What is the benefit of having higher drain current I_dmax in RF operation and what are the factors that increase I_ds in designing a device?
2. What are the factors that increase f_T? For example, is it a shorter gate length, high electron mobility, etc? Does higher f_T mean higher gain?
3. What is the benefit of having lower R_on?
Thank you.
Dear Seohong Park ,
welcome, I will try to answer your questions one by one.
What is the benefit of having higher drain current I_dmax in RF operation and what are the factors that increase I_ds in designing a device?
The requirements on a device is determined by its intended applications.
It is the power handling capability of the device which is also called the power ratings. It is limited by its maximum operating drain voltage VDSmax and it maximum operating drain current IDS. As they increase the power handing capability increases.
The factors that limit the maximum current is the the maximum on gate to source voltage, the channel width/ channel length ratio and the mobility of channel.
What are the factors that increase f_T? For example, is it a shorter gate length, high electron mobility, etc? Does higher f_T mean higher gain?
fT is the gain bandwidth product of the device and it is basically controlled by the transit time in the channel T= L/ vd where L is the channel length and vd is the drift velocity. The drift velocity itself is eqaul to mu times the electric field.
So T will decrease by making the channel shorter and the mobility higher. and operating the transistor at higher voltage.
The gain of the transistor operating as an amplifier is limited by its transconductance gm which depends basically on drain current and W/L ratio in addition to the mobility. So the factors increasing the the drain current increases also gm. The other factor controlling the highest gain of the transistor is its drain to source dynamic resistance.
Higher fT means either higher gain or higher bandwidth or both together.
fT is the gin bandwidth product.
What is the benefit of having lower R_on?
If the transistor is used a as a switch its on resistance must be as small as possible ideally zero. This is required to make the on power loss as small as possible.
Best wishes
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