What are the criteria for selecting a suitable transistor for high frequency VCO?

I want to design 20 GHz Colpitts or LC cross coupled oscillator after designing 4GHz cross coupled oscillator.

what is the criteria to select a suitable transistor for high freq VCO?
It is the ft( transition freq)parametre of the transistor which decides(HBT transistor)!
Or simple take any transistor,apply stability factor criteria to find K.
which resonator circuit (MOS capacitor,accumulated or diode connected,LC,)
is suitable for high frequency VCO;

Please answer and discuss in detail.If possible give references.

Thanks in advance.

about the transistor , it can be CMOS or HBT but u need to check the FT and Fmax of the transistor. u can get this frequency using current CMOS technology like 90 or 65 nm.

to reach this frequency the inductance will be very low i thin it can be a stripline on the IC which gives u this frequqncy.

the varactor could be MOS capacitor

about the topology u can use crosscoupled. or Colpits

check the recent papers of Ali Niknejad for 60 GHz front end

Khouly

I think you should use 0.13um or 90nm cmos process.
Ft should be more than 60GHz.
Mos capactor is better because it can provide larger tuning range than diode with same parasitic capacitance.
And to design so high frequency, the current is not improtant.
But whether it can work or not is detemined by the RF model.