Overview¶
A thorough treatment of MOSFETs can be found in Chapter 4 of the ELEC 2210 textbook,Microelectronics Circuit Design by R.C. Jaeger.
MOSFET Operation Regions Drain end inversion charge decreases to 'zero' or VGD is just equal to threshold voltage (VTN) VGD = VGS-VDS VGD = VTN So MOSFET is in linear operation region when 0 VDS. Initially you get an increasing current with increasing drain bias 0 VD VG VG When you reach VDsat = VG – VT, inversion is disabled at the drain end (pinch-off), but the source end is still inverted The charges still flow, just that you can’t draw more current with higher drain bias, and the current saturates Square law theory of MOSFETs I = meff ZCox(VG – VT )VD- VD2/2/L, VD.
Transistor Characteristics¶
In this lab, we will first measure the I-V characteristics of MOSFETs,including: Download stata for mac.
- Ids-Vgs in a saturation byconnection configuration, e.g.with gate tied to the drain, from which we candetermine threshold voltage.
- Ids-Vds curves for multiple gate-to-source voltages (Vgs), from whichwe can observe linear and saturation operation regions.
Using measured threshold voltage and Ids-Vds curves, we can thencheck how well first-order MOSFET theory holds up in real devices andget a practical feel of the limitation of first-order theoretical MOSFETequation.
The MOSFETs we will use in this experiment are from ALD1105, an ICcontaining two n-MOSFETs and two p-MOSFETs.A circuit symbol description of the two pairs of transistorsfrom the data sheet is shown below in figure 1.
Figure 1: Circuit Symbol and Pin Numbers for the Pairs of N-channel and P-channel MOSFETs.
Webex teams loginb. Note each transistor has four terminals: drain (D), source (S), gate (G), and substrate, which iscalled body (B) in our text.As we learned in class, all the n-MOSFETs on an IC share the same p-type body, whichneeds to be tied to the lowest voltage in a system to keep all the source/drain to body PN junctions zero or reverse biased.
Similarly, all the p-MOSFETs on an IC share the same n-type body, which needs tobe tied to the highest voltage in a system to keep all the source/drain to body PN junctionszero or reverse biased.
The pin diagram seen in figure 2 shows the package layout and various pin connections for ALD1105.
MOSFET as a Switch¶
A MOSFET is a natural voltage-controlled switch,as illustrated in figure 3.A high gate voltage turns on the MOSFET channel, allowingcurrent to flow between drain and source, thereby turninga load, which can be a LED, a speaker or a fan.
Mosfet Vdsat Calculation
The amount of current the MOSFET can provide depends onthe transistor physical properties such as width, length, oxide thickness, etc.,the gate voltage, and the load.
Mosfet Vdsat Equation
CMOS Inverter¶
MOSFETs are mostly used in CMOS circuits.There are many advantages of CMOS, with the biggest beingzero standby power consumption, at least ideally.
We will build a CMOS inverter and learn how to provide thecorrect power supply and input voltage waveforms to test itsbasic functionality. For a given supply VDD, your voltage low shouldbe zero, and voltage high should be VDD.
By default, the function generator gives an output that variesfrom -VPP/2 to +VPP/2, with VPP being peak-to-peak voltage.For a square wave, the voltage low is -VPP/2, voltage high is+VPP/2. You can set the DC offset to VPP/2 to make voltage low 0.
Music downloader for mac. We will build complexCMOS logic gates and sequential CMOS circuits from scratch usingtransistors in other labs.