MOSFET Characteristics

Depletion Type mosfet

A depletion type mosfet has a channel permanently fabricated at the time of its construction itself.

N-channel depletion-type MOSFET Characteristics

n-channel depletion MOSFET Characteristics

As shown in above diagram, gate-to-source voltage is set to zero by shorting gate and source and a voltage V_DS is applied between drain and source.

Since V_GS=0 therefore, no charging of capacitor therefore no change in channel. Due to V_DS , a current I_DSS flows between D and S.

Lets set V_GS=-1V now. This charges capacitor with gate plate negative and channel as positive plate.

Depending upon the magnitude of V_GS , a level of recombination of electrons and holes will occur , reducing the number of free electrons available for conduction. Recombination reduces channel width or increases channel resistance and ∴ I_DS decreases. As we keep increasing the negative V_GS , I_DS keep decreasing and finally becomes zero at particular V_GS.This V_GS is known as pinch-off voltage V_P .

If V_GS is set +ve say +1V , just opposite process occurs. +ve V_GS charges gate plate +ve and channel as –ve plate.

This increases the channel width or decreases channel resistance. Hence, I_DS increases. Transfer characteristics are extrapolated from drain characteristics.

As evident from characteristics curves, a MOSFET has three operating regions :-

(1) Cut-off region :- In cut-off region, MOSFET will be OFF i.e I_DS=0 and it behaves like an open-circuit between D and S terminals. This operation is obtained by keeping V_GS (negative) ≥ V_P .

(2) Ohmic or linear region :- In ohmic region, I_DS increases almost linearly with an increase in V_DS . MOSFETs operating in this region are used as amplifiers.

(3) Saturation region :- In saturation region, MOSFET has its I_DS constant inspite of an increase in V_DSand it behaves like a closed-switch between D and S terminals.

P-channel depletion-type MOSFET Characteristics

Characteristic explanation would be exactly similar as that for n-channel except that the voltage and current directions would be reversed.

p-channel depletion MOSFET Characteristics

Enhancement Type mosfet

An enhancement type mosfet does not have a channel permanently fabricated rather it is induced by applying a voltage between gate and source.

N-channel Enhancement-type MOSFET Characteristics

Lets set V_GS=0V and apply a voltage V_DS between drain and source.

Since there is no channel fabricated in an enhancement mosfet and no channel is induced as V_GS=0V therefore I_D=0A.

Let make V_GS some positive value say V_GS=+4V .This +4V charges capacitor with gate plate +ve and channel –ve. Thus a channel is created. This created channel enables conduction between D and S.

As we keep increasing V_GS , the channel penetrates more deeper into mosfet i.e channel width increases or channel resistance decreases therefore drain current increases as shown in below figure.

n-channel enhancement MOSFET Characteristics

You may have noted a term V_T in characteristics , called threshold voltage. It is the minimum gate-source voltage V_GS that must be applied so that drain current I_D just starts to flow.

P-channel Enhancement-type MOSFET Characteristics