HEMT damaged in oscillator configuration

[Lathas]

I have designed an oscillator at 2.4G using GaAs HEMT to provide the negative resistance( designed o/p power was around 1dbm). But when I connect the Vds and Vgs, the transistor stops working after sometime. What could be the possible reasons for this? In one of the several tries, I could observe a peak in power spectrum at the designed frequency with just Vds connected, but when Vgs was connected the HEMT started drawing more current and the part was damaged.

 

[KlausST]

Hi,

My first thought is bias current.

But without a schematic it's only guessing.

Klaus

 

[Lathas]

Thanks Klaus, the problem seems to be the bias indeed. I will modify the circuit and update the status here.

 

[dick_freebird]

The gate voltage sets drain current and why it works
"better" with the gate open, I guess says the gate
voltage is wrong or maybe poorly decoupled. If gate
connected causes current increase then Vgs has to
be wrong polarity / value.

 

[Lathas]

I am getting very high current( being drawn from the supply sources) values when simulating using the EMDS layout component in ADS.

But when I did a dc analysis on the schematic of the circuit in ads with microstrip components, it shows the correct biasing. So now I am again stuck as what could be wrong with my circuit that is killing the transistor. Could it be that the oscillations at the gate side of the HEMT is not being by passed by the biasing line that is causing this?

- - - Updated - - -

I am seeing very high current being drawn from the supply sources when simulating using the EMDS layout component in ADS. Someone suggested that this could be due to the non-convergence problem while simulating with EMDS layout component. Anybody has encountered similar problem before?

 

[afz23]

How can you connect drain voltage,before giving -ve gate voltage to a HEMT?
The test method is incorrect,it can damage the device.

 

[biff44]

it could be a lot of things. but the likely reason is there is a very low frequency oscillation (like at 2 MHz), and it drove the hemt into a region that blew it up or vaporized the gate with conduction current/breakdown. You can slowly ramp up the DC power, and watch the bias with an oscilloscope to see if the pooh hits the fan