Ode to Lateral MOSFETs

[Lumba Ogir]

Is there anybody interested in this kind of contribution to the constant ongoing discussion, verticals vs laterals?

Once upon a time, maybe Anno Domini 1977, Hitachi announced its S series stating the lateral structure was used in order to achieve enhanced high frequency and gain performance and low gate-drain capacitance.
The predominant vertical MOSFET topology has due to huge industrial demand been subject of massive continuous research and development. However, the lateral topology encompasses features which can not be replicated by any means.
Linear transfer means, most importantly, low intermodulation distortion . High bandwidth, in subjective terms, signifies a soft, musical and less distorted sound, characterizing a large spectrum of the audible frequency range.

Parameters of MOSFETs regarding transfer and high frequency performance:
Idss = saturated drain (cut-off) current. High Idss values have the penalty of increased Crss and Ciss.
Rdson = static drain-source on resistance. Low Rdson brings about high Coss. Rdson increases with increasing temperature.
W = channel width.
L = channel length. A crucial structural difference. L is very short and more precisely defined for laterals offering several advantages like low Crss.
In the case of verticals, the lack of close control over the device characteristics in the vertical dimension results in longer channel lengths, increased leakage current, and increased parasitic capacitances. A large W/L ratio is necessary to achieve high Idss and low Rdson values being important for switching but that is not possible without increasing certain capacitances.
VGSth = the minimum gate bias voltage which enables the formation of the channel between drain and source.
Gm = forward transconductance. The ratio of a change in AC output current to a change in AC input voltage at other parameters fixed. Gm is proportional to W/L. Gm decreases with increasing temperature.
ft = maximum frequency of operation. ft is directly proportional to Gm and inversely proportional to Ciss and to L.
Capacitances supplied by manufactures in data sheets:
Ciss = input capacitance, the sum of Cgs and Cgd. Cgd is small compared to Cds.
Coss = output capacitance, the sum of Cgd and Cds. It is mainly a junction capacitance having its highest value at Vgs = 0, decreasing with increasing Vds, its minimum value specified at a given Vds. Coss is directly proportional to W/L.
Crss = reverse transfer or feedback Miller capacitance, same as Cdg, suffers from voltage gain multiplication. Despite being the lowest value compared to Ciss and Coss, it has the most devastating effect on high frequency performance, thus on the sound. Low Crss values reduce Idss.
For good high frequency performance a low ratio of Crss/Coss and even more importantly a high ratio of Gm/Crss is essential.
High Gm is required also for high power gain and linearity, obtained at low gate voltages and having flat characteristics over a wide range of gate voltage.
In conclusion: large power handling capability has an inevitable price. Because of its construction vertical MOSFETs can not challenge the linear transfer characteristics and high frequency performance of laterals.

[richie00boy]

Read the Bob Cordell MOSFET thread.

[jackinnj]

I think that what you wrote classicists would more correctly describe as a "paean" rather than an "ode".

[hihopes]

Very informative Lumba. I now feel much happier about my laterals.

[Lumba Ogir]

jackinnj ,
right, you are thoroughly conversant with Greek mythology...

[Workhorse]

Originally posted by Lumba Ogir
In conclusion: large power handling capability has an inevitable price. Because of its construction vertical MOSFETs can not challenge the linear transfer characteristics and high frequency performance of laterals.

Think once more while talking about the challenge for Verticals

[jackinnj]

Originally posted by Lumba Ogir
jackinnj ,
right, you are thoroughly conversant with Greek mythology...

8 years of Jesuits.

It is going to be really interesting to hear Mark Brasfield's presentation at the AES conference next Monday. Mark wrote the application note on the LM4702 -- which Troy Huebner updated to use lateral MOSFETs.

[mlloyd1]

yeah, wish i was going.
i hope National puts together one of their road shows after the conference. would be interesting ...

mlloyd1

Originally posted by jackinnj
... It is going to be really interesting to hear Mark Brasfield's presentation at the AES conference next Monday ...

[AndrewT]

Is anyone going to tell us how to take the apparent disadvantages of Verticals and use them to the benefit of us Audioistas?

Some have done it and done it very well, but what technically needs doing to achieve the desired end.
We can copy a successful design, but let's help us go further.

[peranders]

I like lateral mosfets but this is mostly because I have only used those in my designs. On thing is good: The gm is rather constant = low distortion